bluuebunny/crossref_metadata_2025 huggingface dataset:
Powerful solvent systems useful for synthesis of sparingly‐soluble peptides in solution
Authors: KURODA HISAYA , CHEN YUN‐NENG , KIMURA TERUTOSHI , SAKAKIBARA SHUMPEI
Published: August 2010

Our maximum protection strategy for the synthesis of human parathyroid hormone(1–84) indicates that fully protected peptide segments in the form of Boc‐peptide phenacyl (Pac) ester are relatively soluble in ordinary organic solvents such as DMF, NMP or DMSO, which are suitable for coupling segments. However, about 1% of such segments synthesized were found to be insoluble even in the most polar solvent, DMSO. Thus, a more powerful solvent which can be used for their peptide synthesis was pursued. Among the solvent systems tested, a mixture of trifluoroethanol (TFE) or hexafluoroisopropanol (HFIP) and trichloromethane (TCM) or dichloromethane (DCM) was found to be most powerful for dissolving such sparingly‐soluble protected peptides. These solvent systems were confirmed to be useful for the removal reaction of the carboxy‐terminal Pac esters from the sparingly‐soluble segments. They were then tested for the coupling reactions of fully protected Boc‐peptides with other sparingly‐soluble peptide esters. The TFE/TCM or TFE/DCM system was extremely useful for coupling segments without danger of racemization and of trifluoroester formation, if WSCI was used as the coupling reagent in the presence of 3,4‐dihydro‐3‐hydroxy‐4‐oxo‐1,2,3‐benzotriazine (HOOBt).
Application of carboxylic‐phospholanic mixed anhydrides to fragment coupling in peptide synthesis
Authors: POULOS CONSTANTINE , ASHTON CHRIS P. , GREEN JEREMY , OGUNJOBI OLUYINKA M. , RAMAGE ROBERT , TSEGENIDIS THEODOROS
Published: August 2010

The application of 1‐oxo‐l‐chlorophospholane as a novel reagent for the in situ activation of peptide fragments for use in peptide bond forming reactions, either in liquid or solid phase, has been examined. 24.1 MHz 31P NMR spectroscopy has been employed to follow the formation, stability and reactivity of the intermediate phospholanic‐carboxylic mixed anhydride
Conformation of cyclosporin A in polar solvents
Authors: KO SOO YOUNG , DALVIT CLAUDIO
Published: August 2010

A major conformation of cyclosporin A in methanol and in aqueous methanol was revealed by some simple NMR experiments. Thus, a stepwise transition of cyclosporin A conformation from 100% CDCl3 to 100% CD3OD was followed by 1H NMR, which showed that the chloroform conformation of cyclosporin A was still the major one in methanol. Employing the same technique, it was also shown that the chloroform conformation of cyclosporin A was one of the major conformations in 50% aqueous methanol. This may be the first experimental determination of a major conformation of cyclosporin A in polar solvents.
Activation of carboxylic acids by pyrocarbonates
Authors: POZDNEV V.F.
Published: August 2010

Activation of carboxylic acids was achieved via dialkyl pyrocarbonates (ROCO2O, R= C2H5, i‐C3H7, sec‐C4H9, tert.‐C4H9) in aprotic solvents in the presence tertiary amines. A convenient procedure for the preparation of carboxylic acid anhydrides from carboxylic acids and di‐tert.‐butyl pyrocarbonate in the presence of pyridine is reported. Analogously, di‐isopropyl‐ or diethyl pyrocarbonate may be used in the presence of N‐methylmorpholine (triethylamine). With pyridine, di‐isopropyl‐ or diethyl pyrocarbonate carboxylic acids form isopropyl‐ or ethyl esters, respectively. A wide variety of esters were prepared in good yields in a one‐pot procedure from carboxylic acids, including N‐protected amino acids, and alcohols or from phenols by means of di‐tert.‐butyl pyrocarbonate in the presence of pyridine (Boc2O‐pyridine system). t‐Butyl esters of carboxylic acids were obtained by the same procedure with 4‐dimethylarninopyridine. In the absence of carboxylic acid, with 4‐dimethylaminopyridine Boc2O and alcohols generate alkyl tert.‐butyl carbonates.
Synthesis and characterization of defensin NP‐1
Authors: RAO A. GURU RAJ , ROOD TRACY , MADDOX JOYCE , DUVICK JONATHAN
Published: August 2010

Defensins are a group of small, cationic, antimicrobial proteins found in the cytoplasmic granules of neutrophils and macrophages of a variety of mammalian species. One such defensin, NP‐1, isolated from rabbit neutrophils, has been shown to consist of 33 amino acids rich in arginine and cysteine residues (6). We have synthesized NP‐1 on an Applied Biosystems Model 431A peptide synthesizer using FastMoc™ chemistry involving HBtu [2‐1H‐benzotriazol‐1‐y1)‐1,1,3,3‐tetramethyluronium hexafluorophosphate] activation for coupling amino acids. The linear peptide was folded by air oxidation to the biologically active form containing three disulfide bonds and purified by reverse phase chromatography. The amino acid sequence of the synthetic peptide was confirmed by Edman degradation. Molecular weight determination by plasma desorption mass spectroscopy (PDMS) gave a value of 3898.6, in agreement with the expected molecular weight of 3898. The biological activity of the synthetic peptide, as measured by its antifungal activity against several pathogenic fungi, was indistinguishable from that of the natural NP‐1. Also, the CD spectrum was equivalent to that of natural NP‐1, indicating conformational identity of the two species.
Solution conformation of endothelin and point mutants by nuclear magnetic resonance spectroscopy
Authors: DALGARNO DAVID C. , SLATER LEO , CHACKALAMANNIL SAMUEL , SENIOR MARY M.
Published: August 2010

Two‐dimensional NMR techniques were utilized to determine the secondary structural elements of endothelin‐1 (ET‐I), a potent vasoconstrictor peptide, and two of its point mutants, Met‐7 to Ala‐7 (ETM7A), and Asp‐8 to Ala‐8 (ETD8A) in acetic acid‐d3/water solution. Sequence specific NMR assignments were determined for all three peptides, as well as chemical shifts and NOE connectivity patterns. The chemical shifts of ET‐1 and ETM7A are identical (± 0.05 ppm) except for the site of substitution, whereas marked shift changes were detected between ET‐1 and ETD8A. These chemical shift differences imply that the Asp‐8 to Ala‐8 mutation has induced a conformational change relative to the parent conformation. All three molecules show the same basic nuclear Overhauser effect (NOE) pattern, which suggests that the gross conformation of all three molecules is the same. Small changes in sequential NOE intensities and changes in medium‐range NOE patterns indicate that there are subtle conformational differences between ET‐1 and ETD8A.
Conformational investigation of αβ‐dehydropeptides
Authors: PIETRZYŃSKI GRZEGORZ , RZESZOTARSKA BARBARA , KUBICA ZBIGNIEW
Published: August 2010

Solution conformations of three series of model peptides, homochiral Ac‐Pro‐L‐Xaa‐NHCH3 and heterochiral Ac‐Pro‐D‐Xaa‐NHcH3 (Xaa = Val, Phe, Leu, Abu. Ah) as well as αβ‐unsaturated Ac‐Pro‐ΔXaa‐NHCH3 [Δ Xaa =ΔVal, (Z)‐ΔPhe, (Z)‐ΔLeu, (Z)‐ΔAbu] were investigated in CDCl3 and CH2Cl2 by 1H‐, 13C‐NMR, and FTIR spectroscopy. NH stretching absorption spectra, solvent shifts Δδ for NH (Xaa) and NHCH3 on going from CDCl3 to (CD3)2SO, diagnostic interresidue proton NOEs, and trans‐cis isomer ratios were examined. These studies performed showed the essential difference in conformational propensities between homochiral peptides (L‐Xaa) on the one hand and heterochiral (D‐Xaa) and αβ‐dehydropeptides (ΔXaa) on the other. Former compounds are conformationally flexible with an inverse γ‐bend, a β‐turn, and open forms in an equilibrium depending on the nature of the Xaa side chain. Conformational preferences of heterochiral and αβ‐dehydropeptides are very similar, with the type‐II β‐turn as the dominating structure. There is no apparent correlation between conformational properties and the nature of the Xaa side chain within the two groups. The β‐turn formation propensity seems to be somewhat greater in αβ‐unsaturated than in heterochiral peptides, but an estimation of β‐folded conformers is risky.
HPLC and PDMS analysis of cleavage products aid in the design of small, stable ANP analogues
Authors: DEVINE EDWARD M. , BUKO ALEXANDER M. , CEPA STEVEN P. , DAVIDSEN STEVEN K. , HOLLEMAN WILLIAM H. , MARSELLE CAROL A. , OPGENORTH TERRY J. , ROCKWAY TODD W. , GELDERN THOMAS W. VON
Published: August 2010

The degradation of a prototypical small analogue of atrial natriuretic peptide (ANP) has been studied using HPLC and mass spectrometric techniques. These studies revealed that removal of theN‐terminal amino acid as the primary catabolic eventin vitro.Based on this information theN‐terminus was remanufactured to provide a family of more stable analogues. Additional stabilization was provided through modification of theC‐terminal tripeptide. Though dramatically more stablein vitro, these new analogues do not appear to have longerin vivohalf‐lives.
Conformational study of a peptide epitope shows large preferences for β‐turn conformations
Authors: RIPOLL DANIEL R.
Published: August 2010

The conformational preferences of the 7‐residue peptide Glu‐Val‐Val‐Pro‐His‐Lys‐Lys was investigated using a global search algorithm, namely the Electrostatically Driven Monte Carlo (EDMC) method, and the ECEPP/2 potential energy function. This particular sequence corresponds to the N‐terminal portion of a 19‐residue peptide antigen whose three dimensional structure, when complexed to a cognate antibody, was reported recently. As a result of this study a series of low‐energy conformations were identified showing a common folding pattern with residues Val‐3, Pro‐4, His‐5 and Lys‐6 forming a β turn. A comparison of the computed conformations with the one determined by X‐ray crystallography in the antibody‐antigen complex reveals marked similarities. In most of the cases rms deviations smaller than 1.1 Å were found for the backbone atoms of the four residues forming the turn. These results suggest that the recognition process is accomplished in this case through the interaction of the antibody with relatively stable conformers of the antigenic peptide.
NMR study on solution structure of the site‐specific mutant Leu48→Ala transforming growth factor alpha§
Authors: KLINE TAMMY PAGE , MUELLER LUCIANO
Published: August 2010

The NMR spectra of the Leu48 → Ala mutant of human transforming growth factor α were compared to that of the wild‐type. All chemical shift changes are 0.02 ppm with the exception of resonances associated with residues 47, 48 and 50 (all 0.07 ppm). Minimal changes were observed for NOEs associated with residues Vall to His45. The weakening of some NOEs associated with the region Ala46‐Ala50 may suggest a slightly increased flexibility for this region. Refinement of the previously calculated wild‐type structures using distance constraints derived from the L48A mutant had little overall effect. Leu48‐Ala50 is ill‐defined for both wild‐type and mutant proteins. These results suggest that Leu48 has no structural role and thus must be an important factor in the protein‐receptor interface.
Synthesis and biological activities of linear and cyclic enkephalin analogues containing a Ψ (E,CH=CH) or Ψ (CH2CH2) isosteric replacement
Authors: TOURWÉ D , COUDER J. , CEUSTERS M. , MEERT D. , BURKS T. F. , KRAMER T. H. , DAVIS P. , KNAPP R. , YAMAMURA H. I. , LEYSEN J. E. , VAN BINST G.
Published: August 2010

The peptide CO–NH function was replaced by atranscarbon‐carbon double bond or by a CH2–CH2isostere in enkephalin analogues of DADLE, DCDCE–NH2or DPDPE. In DADLE the 2‐3 and the 3‐4 peptide bond was modified, whereas in the cyclic analogues the Gly3‐Phe4bond was replaced by the isosteres GlyΨ (E,CH=CH)Phe [5‐amino‐2‐(phenylmethyl)‐3(E)‐pentenoic acid] or GlyΨ(CH2CH2)Phe [5‐amino‐2‐(phenylmethyl)pentanoic acid]. In general, the modification results in a drop in potency which is the largest for the flexible CH2–CH2replacement. The Gly3Ψ (E,CH=CH)Phe4DCDCE–NH2analogue retains considerable potency. These results confirm the importance of the peptide function at the 2‐3 and 3‐4 position in enkephalin analogues for biological potency.
Search for peptide immunogens of the β‐subunit of human chorionic gonadotropin (hCG) capable of eliciting hormone specific and neutralizing antisera
Authors: IYER K.S.N. , UPADHYE S. , KADAM L.R. , MAHALE S.D. , DHANASEKHARAN S. , NATRAJ U. , NANDEDKAR T.D.
Published: August 2010

The work reported herein describes our attempts to identify peptide immunogens of the β‐subunit of the pregnancy‐specific placental hormone, human chorionic gonadotropin (hCG), capable of eliciting hormone specific and neutralizing antisera. Hydrophilicity profiles of the β‐subunits of hCG and the homologous pituitary hormone, human luteinizing hormone (hLH) were compared and two sequences which are hydrophilic but unique to hCG‐β were identified. They are 6‐12 and 109‐119 of hCG‐β. Results of the studies with the undecapeptide 1009‐119 of hCG‐β are reported in this paper. The undecapeptide amide was synthesized using the p‐(acyloxy) benzhydrylamine resin and antisera to the peptide were elicited in rabbits using the peptide‐diphtheria toxoid conjugate. The peptide is highly immunogenic as both the rabbits responded with high titers of antibodies to the peptide. The antipeptide antibodies bound to hCG but not to hLH showing thereby that the region 109‐119 of hCG‐β is a unique determinant of hCG. However, the antibodies were found not to neutralize the biological activity of hCG.
Orthogonal solid‐phase synthesis of a monobiotinylated analog of Neuropeptide Y
Authors: BAEZA CLAUDIO RIVERA , UNDÉN ANDERS
Published: August 2010

Analogs of Neuropeptide Y (NPY) were synthesized with conventional Boc/benzyl protective group strategy. Instead of Asn7 in the native scquence, Boc‐Lys(Alloc)‐OH was incorporated. At the end of the synthesis the Alloc group was selectively removed by palladium‐catalyzed hydrostannolysis and biotin coupled to the e‐amino group of Lys7. After cleavage and characterization with plasma desorption mass spectrometry the Ne‐7‐biotinyl‐[Lys7]‐NPY and the nonbiotinylated analog [Lys7]‐NPY were investigated as ligands to the NPY receptor from rat cerebral cortex. Both analogs were found to be high affinity ligands to the NPY receptor and bound with essentially the same affinity as unmodified NPY.
Effect of electrophilic and nucleophilic substituents on the protonation microequilibria of tyrosine derivatives
Authors: SIPOS PÁL , PEINTLER GABOR , TÓTH GÉZA
Published: August 2010

The microscopic protonation constants of 10 tyrosine‐like, unusual amino acids used in the syntheses of opioid peptides have been determined by using a combined pH‐metric‐spectrophotometric method, at 0.10 mol dm −3 (NaCl) ionic strength and 25.0°. The role of the different electrophilic and nucleophilic substituents on the individual basicity of the aliphatic amine and phenolic hydroxylate basic centers is discussed in detail. The interactivity parameters between these two groups correlate fairly well with the structure of the skeleton and the distance between the two basic centers, but they were found to be substituent‐independent. This finding made it possible to extend the calculations to compounds having non‐overlapping protonation equilibria.
Potent agonists of growth hormone‐releasing hormone
Authors: ZARANDI MARTA , SERFOZO PETER , ZSIGO JOZSEF , BOKSER LAZARO , JANAKY TAMAS , OLSEN DONALD B. , BAJUSZ SANDOR , SCHALLY ANDREW V.
Published: August 2010

Analogs of the 29 amino acid sequence of growth hormone‐releasing hormone (GH‐RH) with agmatine (Agm) in position 29 have been synthesized by the solid phase method, purified, and tested in vitro and in vivo. The majority of the analogs contained desaminotyrosine (Dat) in position 1, but a few of them had Tyr1, or N‐MeTyr1. Some peptides contained one or more additional l‐ or d‐amino acid substitutions in positions 2, 12, 15, 21, 27, and/or 28. Compared to the natural sequence of GH‐RH(1‐29)NH2, [Dat′,Ala15]GH‐RH(1‐28)Agm (MZ‐3‐191) and [d‐Ala2,Ala15]GH‐RH(l‐28)Agm (MZ‐3‐201) were 8.2 and 7.1 times more potent in vitro, respectively. These two peptides contained Met27. Their Nle27 analogs, [DatI,Ala15,Nle27]GH‐RH(1‐28)Agm(MZ‐2‐51), prepared previously (9), and [D‐Ala2,Ala15,Nle28]GH‐RH(l‐28)Agm(MZ‐3‐195) showed relative in vitro potencies of 10.5 and 2.4, respectively. These data indicate that replacement of Met27 by Nle27 enhanced the GH‐releasing activity of the analog when the molecule contained Dat1‐Ala2 residues at the N‐terminus, but peptides containing Tyr1‐D‐Ala2 in addition to Nle27 showed decreased potencies. Replacement of Ser28 with Asp in multi‐substituted analogs of GH‐RH(l‐28)Agm resulted in a decrease in in vitro potencies compared to the parent compound. Thus, the Ser28‐containing MZ‐2‐51, and [Dat1,Ala15,d‐Lys21,Nle27]GH‐RH(l‐28)Agm, its Asp28 homolog (MZ‐3‐149), possessed relative activities of 10.5 and 5.6, respectively. In vivo after the iv injection, the analogs [Dat1,Ala15,Nle27,Asp28]GH‐RH(l‐28)Agm (MZ‐3‐149), [Dat1, Ala15]GH‐RH(l‐28)Agm, (MZ‐3‐191) and [d‐Ala2,Ala,5]GH‐RH(l‐28)Agm (MZ‐3‐201) showed a potency equivalent to 7.6, 4.9 and 3.3 times that of GH‐RH(1‐29)NH2, respectively, at 5 min and 20.3, 4.3 and 1.7 times higher, respectively, at 15 min. After sc administration, analogs MZ‐3‐149, MZ‐3‐191, and MZ‐3‐201 were shown to be 63.7, 55.2 and 56.8 times more potent than the parent hormone at 15 min and 57.6, 60.6, and 42.6 times more active, respectively, at 30 min. In addition, MZ‐3‐149 had prolonged GH‐releasing activity as compared to the standard, and proved to be more potent than MZ‐2‐51, the most active member of our previous series (8, 9). Our studies indicate that very potent GH‐RH analogs can result from the combination of agmatine in position 29 with other substitutions.
Effect of copper(II) chloride on suppression of racemization in peptide synthesis by the carbodiimide method
Authors: MIYAZAWA TOSHIFUMI , OTOMATSU TOSHIHIKO , FUKUI YOSHIMASA , YAMADA TAKASHI , KUWATA SHIGERU
Published: August 2010

Copper(II) chloride was found to be an extremely efficient racemization‐suppressing additive in the DCC method as compared with the hitherto known ones, by employing the model coupling Z‐Gly‐l‐Val‐OH +h‐l‐Val‐OMe in DMF. Although some other copper salts also had a profound effect, copper(II) chloride was the best from the viewpoint of both racemization suppression and coupling efficiency. The effectiveness of copper(II) chloride was further confirmed by employing the EDC‐mediated couplings of Z‐Gly‐containing dipeptides with amino acid esters or dipeptide esters, and those of Z‐l‐Ala (or l‐Val)‐l‐Val‐OH with amino acid esters or dipeptide esters. In almost all the cases studied, no detectable amount (< 0.1 %) of epimer was observed by the HPLC analysis in the presence of copper(II) chloride. This was also the case even with an extremely stringent coupling system Z‐l‐Pro‐l‐Val‐OH + H‐l‐Pro‐OMe. With reference to the mechanism of racemization suppression, it was found that copper(II) chloride has a strong ability to suppress the racemization of the 5(4H)‐oxazolone, which may be formed from an activated carboxyl component during the coupling.
Assessment of δ‐opioid receptor activation by a series of peptides in cultured cells
Authors: COSTA ELIZABETH M. , HOFFMAN BRIAN B. , LOEW GILDA H.
Published: August 2010

This study was designed to investigate the relative ability of a series of cyclic opioid peptides to initiate the first activation steps following their binding of δ‐opioid receptors. The extent of stimulation of low Km guanosine‐triphosphatase (GTPase activity) and inhibition of hormonally‐stimulated cAMP accumulation in the NG108‐15 (neuroblastoma‐glioma) hybrid cell line were determined and compared for six closely related peptides. In addition, their binding affinity was assessed by competition with ‐3H‐[D‐Pen2D‐Pen5]‐enkephalin (3H‐DPDPE) in membranes from these cells. All peptides tested elicited comparable maximal effects for both functional responses. Different potencies in stimulating the low Km GTPase was observed at sub‐maximal agonist concentrations, although the shallow dose‐response behavior did not allow accurate determination of ED50s. Estimation of ED50s for inhibition of c AMP accumulation could be made by curve fitting and were similar for four of these peptides, while DCDPE and 3R‐methylDCDPE, the highest affinity analogs, were considerably more potent. In general, the observed differences in hormonal activity somewhat parallel the rank order of binding affinities, but no strict relationship was found between receptor binding and activation.
Reduced peptide bond cyclic somatostatin based opioid octapeptides Synthesis, conformational properties and pharmacological characterization
Authors: KAZMIERSKI WIESLAW M. , FERGUSON RON D. , KNAPP RICHARD J. , LUI GEORGE K. , YAMAMURA HENRY I. , HRUBY VICTOR J.
Published: August 2010

The conformational and pharmacological properties that result from peptide bond reduction as well as the use of secondary amino acids in a series of cyclic peptides related to the μ opioid receptor selective antagonist d‐Phe1‐Cys2‐Tyr3‐d‐Trp4‐Orn5‐Thr6‐Pen7‐Thr8‐NH2 (IV), have been investigated. Peptide analogues that contain [CH2NH] and [CH2N] pseudo‐peptide bonds (in primary and secondary amino acids, respectively) were synthesized on a solid support. Substitution of Tyr3 in IV by the cyclic, secondary amino acid 1,2,3,4‐tetrahydroisoquinoline carboxylate (Tic) and of d‐Trp4 with d‐1,2,3,4‐tetrahydro‐β‐carboline(d‐Tca4), gave peptides 4 and 1, respectively. Both analogues displayed reduced affinities for μ opioid receptors. Conformational analysis based on extensive NMR investigations demonstrated that the backbone conformations of 1 and 4 are similar to those of the potent and selective Analogue d‐Phe‐Cys‐Tyr‐d‐Trp‐Lys‐Thr‐Pen‐Thr‐NH2 (I), while the conformational properties of the side chains of Tic3 (4) and d‐Tca4 (1) resulted in topographical properties that were not well recognized by the μ opioid receptor. Peptide bond modifications were made including (Tyr3‐ψ[CH2NH]‐d‐Trp4), 3; (Tyr3‐ψ[CH2N]‐d‐Tca4), 2; and (Cys2‐ψ[CH2N]‐Tic3), 6. These analogues showed decreases in their μ opioid receptor affinities relative to the parent compounds IV, 1, and 4, respectively. 1H NMR based conformational analysis in conjunction with receptor binding data led to the conclusion that the reduced peptide bonds in 2, 3, 5, and 6 do not contribute to the process of discrimination between μ and δ opioid receptors, and in spite of their different dynamic behaviors (relative to 1 and 4), they are still capable of attaining similar receptor bound conformations, possibly due to their increased flexibility.
Pepsin as a catalyst of peptide synthesis Enzyme co‐precipitation with emerging peptide products
Authors: MALAK CAMELIA ADLY ABDEL , FILIPPOVA IRINA Y. , LYSOGORSKAYA ELENA N. , ANISIMOVA VERONIKA V. , LAVRENOVAI GALINA I. , STEPANOV VALENTIN M.
Published: August 2010

Pepsin successfully catalyzed the synthesis of several peptide derivatives from N‐protected di‐ or tripeptides and amino acid or peptide esters or p‐nitroanilides in dimethylformamide‐water solutions at pH 4.6. An optimal substrates:pepsin ratio depended on the structure of starting peptides, especially their fit to the substrate binding sites of the enzyme. For hexapeptide Z‐Ala‐Ala‐Phe‐Leu‐Ala‐Ala‐OCH3 formation, an equilibrium yield was attained at 1:3. 105 enzyme‐substrates ratio that indicated high efficiency of pepsin in synthesis reactions. In the course of the equilibrium peptide synthesis, pepsin gradually disappeared from the liquid phase due to its entrapment within a gel, formed by the hexapeptide product, while retaining its activity. The inclusion into the precipitate was not specific for pepsin, so far as inert proteins, lysozyme, ribonuclease A and carbonic anhydrase, when added to the reaction mixture, became also co‐precipitated with the hexapeptide formed. It appears that co‐precipitation of pepsin, an important factor limiting the enzyme efficiency, might be operative as well for other proteinases used to catalyze peptide synthesis.
Efficient approach to synthesis of two‐chain asymmetric cysteine analogs of receptor‐binding region of transforming growth factor‐α
Authors: TAM JAMES P. , SHEN ZHI‐YI
Published: August 2010

The putative receptor‐binding region of human transforming growth factor‐α (TGF α) has been shown to be contributed by two fragments: an A‐chain (residue 12‐18) and a 17‐residue carboxyl fragment (residue 34‐50) that includes a disulfide‐containing C‐loop (residue 34‐43). An approach to the synthesis of two‐chain analogs containing an intermolecular disulfide linked A‐chain and the 17‐residue carboxyl fragment (C‐fragment) possessing receptor‐binding activity is described. The synthesis was achieved by the solid‐phase method using the Boc‐benzyl protecting group strategy. The single Cys of the A‐chain was activated as a mixed disulfide with 2‐thiopyridine to form the intermolecular disulfide bond with Cys41 or Cys46 of the C‐fragment on the resin support. Prior to this reaction, the acetamido (Acm) protecting group of Cys41 or Cys46 was removed by Hg(OAc)2 on the resin support. The peptide and side chain protecting groups including the S‐methylbenzyl moiety of the Cys34 and Cys43 were concomitantly cleaved by high HF. The intramolecular disulfide with two unprotected Cys was formed in the presence of an intermolecular disulfide. This intramolecular disulfide bond formation was usually not feasible under the traditionally‐held scheme at basic pH since disulfide interchange would occur faster than intramolecular oxidation. To prevent the disulfide interchange, a new method was devised. The intramolecular disulfide bond oxidation was mediated by dimethylsulfoxide at an acidic pH, at which the disulfide interchange reaction was suppressed. The desired product was obtained with a 60‐70%, yield. In contrast, the conventional scheme of using I2 to form the intermolecular disulfide between the Cys(Acm) of the A‐chain and C‐fragment with the preformed intramolecular disulfide bond in solution phase did not result in any product. The purified two‐chain analogs were found to be unstable and rearranged to the homo‐dimers. This reaction was greatly accelerated in I2, which explained the difficulty associated with the conventional scheme. When assayed against A431 and NRK clone 49F cells, both the A‐chain and the C‐fragment did not exhibit any biological activity independently, but the two‐chain analogs showed low receptor‐binding activity with an IC50 at 0.3 mM level. Unexpectedly, dimeric C‐fragment, which resulted from the rearrangement reaction, also showed receptor‐binding activity. Our results demonstrate that the two‐chain analogs exhibit low but distinct biological activity and provide evidence that the putative TGFα receptor binding region may be discontinuous. In addition, we also provide an efficient approach to further explore the two‐chain receptor‐binding analogs of TGFα.
Synthesis and opioid activity of 2–substituted dynorphin A‐(1‐13) amide analogues1
Authors: STORY SANDRA C. , MURRAY THOMAS F. , DELANDER GARY E. , ALDRICH JANE V.
Published: August 2010

A series of 2‐substituted dynorphin A‐(1‐13) amide (Dyn A‐(1‐13)NH2) analogues was prepared by solid phase peptide synthesis and evaluated for opioid receptor affinities in radioligand binding assays and for opioid activity in the guinea pig ileum (GPI) assay. Amino acid substitution at the 2 position produced marked differences in both opioid receptor affinities and potency in the GPI assay; Ki values for the analogues in the radioligand binding assays and IC50 values in the GPI assay varied over three to four orders of magnitude. The parent peptide, Dyn A‐(1‐13)NH2, exhibited the greatest affinity and selectivity for kappa receptors and was the most potent peptide examined in the GPI assay. The most important determinant of opioid receptor selectivity and opioid potency for the synthetic analogues was the stereochemistry of the amino acid at the 2 position. Except for [D‐Lys2]Dyn A‐(1‐13)NH2 in the kappa receptor binding assay, the analogues containing a D‐amino acid at position 2 were much more potent in all of the assays than their corresponding isomers containing an L‐amino acid at this position. The L‐amino acid‐substituted analogues generally retained some selectivity for kappa opioid receptors. The more potent derivatives with a D‐amino acid in position 2, however, preferentially interacted with mu opioid receptors. Introduction of a positively charged amino acid into the 2 position generally decreased opioid receptor affinities and potency in the GPI assay.
Hydrolysis of peptide esters by different enzymes
Authors: REISSMANN SIEGMUND , GREINER GEORG
Published: August 2010

The combined use in peptide synthesis of the Fmoc‐group with methyl, benzyl or p‐nitro benzyl esters is not practical because of the elimination of the Fmoc‐group under basic conditions and by catalytic hydrogenation. Nevertheless the solution synthesis of peptides requires those combinations in some cases. For this purpose we have investigated enzymatic hydrolysis of some tri and tetrapeptide esters. The hydrolysis were carried out under pH‐control. We measured deprotection of the carboxyl group by thermitase, porcine liver esterase, carboxypeptidase A and x‐chymotrypsin. The main problems are to suppress proteolytic degradation of the peptide bond and to bring the protected peptides into solution. To solve both problems we used dimethylformamide and dimethylsulfoxide as cosolvents. The ratios between esterolytic and proteolytic activity were estimated under various cosolvent concentrations. Advantages of this method are to avoid side reactions of alkaline instable side chains (e.g. asparagine, glutamine), cleavage of base labile protecting groups and racemization by alkaline saponification. The enzymatic deprotection was followed by HPLC, HPTLC and titration. On a preparative scale this method gives good yields and sufficently pure products.
Predicted secondary structure of bovine prothrombin fragment 1 and related proteins in different environments by circular dichroism spectroscopy
Authors: BALBES LISA M , PEDERSEN LEE G. , HISKEY RICHARD G.
Published: August 2010

Circular dichroism spectroscopy was used to investigate the structure of bovine prothrombin fragment 1 (BF1) and related proteins in several environments. The conformational change induced in BF1 by the addition of Mg[II] ions was found to be different from that induced by Ca[II] or Sr[II]. The Ca[II] and Sr[II] conformations appear to differ only slightly from the apo‐metal conformation. The conformation of the 1–45 fragment of prothrombin, however, is markedly different than the conformation of the same fragment in the presence of either Ca[II] of Mg[II]; both of the latter structures differ substantially from one another. The presence of phospholipids has almost no effect on the structure of either BF1 or the 1–45 fragment; in the presence of both phospholipids and Ca[II] a structural change is seen for the 1–45 fragment but not BF1 (relative to the protein alone). The addition of phospholipids to the Mg[II]/BFl structure did not induce a CD‐detectable conformational change, while the addition of phospholipids to the Ca[II]/BFl or Sr[II]/BFl structures induced a change to a conformation similar in secondary structure composition to the relative apometal structures.
Efficient solid phase synthesis of mixed Thr(P)‐, Ser(P)‐ and Tyr(P)‐containing phosphopeptides by “global”“phosphite‐triester” phosphorylation*
Authors: PERICH JOHN W.
Published: August 2010

The synthesis of the mixed Thr(P), Tyr(P)‐containing peptide, Ala‐Thr(P)‐Tyr(P)‐Ser‐Ala, was accomplished by “phosphite‐triester” phosphorylation of the resin‐bound Thr Tyr‐containing peptide using di‐t‐butyl N,N‐diethylphosphoramidite as the phosphitylation reagent. The pentapeptide‐resin was assembled by Fmoc/ solid‐phase peptide synthesis with the use of PyBOP® as coupling reagent and the hydroxy‐amino acids incorporated as side‐chain free Fmoc‐Tyr‐OH and Fmoc‐Thr‐OH. “Global” bis‐phosphorylation of the peptide‐resin was accomplished by treatment with di‐t‐butyl N,N‐diethylphosphoramidite/1H‐tetrazole followed by m‐chloroperoxybenzoic acid oxidation of the intermediate di‐t‐butylphosphite triester. Simultaneous peptide‐resin cleavage and peptide deprotection was effected by treatment of the peptide‐resin with 5% anisole/TFA and gave the Thr(P) Tyr(P)‐containing phosphopeptide in high yield and purity. In addition, the tyrosyl residue was found to be phosphitylated in preference to the threonyl residue since the phosphitylation of the pentapeptide‐resin using only 1.1 equiv. of di‐t‐butyl N,N‐diethylphosphoramidite gave Ala‐Thr‐Tyr(P)‐Ser‐Ala as the major product and both Ala‐Thr(P)‐Tyr(P)‐Ser‐Ala and Ala‐Thr‐Tyr‐Scr‐Ala as minor products.
Resolution of proline acylation problem for thiol capture strategy by use of a chloro‐dibenzofuran template
Authors: FOTOUHI NADER , BOWEN BENJAMIN R. , KEMP DANIEL S.
Published: August 2010

The acyl transfer rate for proline, in the prior thiol capture strategy, was enhanced by changing the electronic character of the dibenzofuran template. The rate of amide bond formation between proline and cysteine by the 1‐chloro‐4‐hydroxy‐6‐mercaptodibenzofuran was measured to be 0.012 min‐1, which translates to a half‐life of 53 min. Further enhancement of the reaction rate was accomplished by the use of a 1,3‐dichloro‐dibenzofuran template. The k1 for the reaction was measured to be 0.093 min‐1, and the half‐life was calculated to be 7 min. To test the applicability of the activated template, 1‐chloro‐4‐hydroxy‐6‐mercaptodibenzofuran, in peptide synthesis, the 34 amino acid long peptide, H‐RPDFCLEPPYTGPCRKARNNFKSADECMRTCGGA‐OH, was synthesized. This peptide represents the condensation of the N‐terminal 13‐mer and the C‐terminal 21‐mer of the basic pancreatic trypsin inhibitor.
Side reactions in solid phase synthesis of histidine‐containing peptides
Authors: PESSI ANTONELLO , MANCINI VINCENZO , FILTRI PAOLO , CHIAPPINELLI LORELLA
Published: August 2010

The flow‐polyamide synthesis of a histidine‐containing sequence (Ac‐YDNVLDHLTGR) produced two major impurities which were isolated through Sample Displacement Chromatography and characterized by Fast Atom Bombardment‐mass analysis. The impurities correspond to the sequence Ac‐YDNVLDH, and to a peptide with the Asp residue cyclized to a succinimide. The latter side‐reaction took place during the acetylation procedure, and demands attention where capping procedures are planned in the synthesis of Asp‐His sequences.
C9 conformation of N‐(Nα‐[(tert.‐butyloxy)‐carbonyl]‐L‐alanyl]‐N,N'‐dicyclohexylurea in solid and solution
Authors: SUDARSANAKUMAR C. , SRINIVASAN S. , JAYAKUMAR R.
Published: August 2010

An X‐ray diffraction study was carried out on a single crystal of N‐(Nα‐[(tert.‐butyloxy)‐carbonyl)‐l‐alanyl)‐N,Nα‐dicyclohexylurea belonging to the tetragonal space group P41212, having cell dimensions a = b= 10.102(3) Å, c = 46.067(7) Å, V = 4701.2 Å3, Z = 8. The crystal structure was solved by direct methods and refined to an R value of 0.056 for 1602 unique reflections with I<2.5 σ(I). Crystal structure analysis shows the presence of an intramolecular N–H O=C H‐bond stabilizing the molecule in a folded form similar to that of a β turn, forming a nine‐membered ring. IR and 1H‐NMR studies in CDCI3 solution confirm the stable folded conformation found in the crystalline state, as well as the existence of N–H O=C H‐bonds in the title compound, as in peptides.
Non coded Cα,α‐disubstituted amino acids
Authors: PAVONE V. , DI BLASIO B. , LOMBARDI A. , MAGLIO O. , ISERNIAI C. , PEDONE C. , BENEDETTI E. , ALTMANN E. , MUTTER M.
Published: August 2010

The crystal and molecular structure of the fully protected dipeptide Boc‐Val‐(S)‐α‐MeSer‐OMe has been determined by X‐ray diffraction techniques. Crystals grown from ethyl acetate/n‐pentane mixtures are tetragonal, space group 141, with cell parameters at 295 K of a= 15.307(2), c= 18.937(10)Å, V = 4437.1 Å3, M.W. = 332.40, Z = 8, Dm= 0.99 g/cm3 and Dx= 0.995 g/cm3. The structure was solved by application of direct methods and refined to an R value of 0.028 for 1773 reflections with I≥3σ(I) collected on a CAD‐4 diffractometer. Both chiral centers have the (S) configuration. The dipeptide assumes in the solid state an S shape. The urethane moiety is in the cis conformation, while the amide bond is in the common trans conformation. The conformational angles φ1, ψ1 of the Val and φ2, and ψ2 of the (S)‐αMeSer fall in the F region of the φ‐ψ map. The isopropyl side chain of the Val residue has the (t, g−) conformation, while the Ser side chain has a g+ conformation. The hydrogen bond donor groups are all involved in intermolecular H‐bond interactions. Along the quaternary axis the dipeptide molecules are linked to each other with the formation of infinite rows.
Synthesis and biological activity of [L‐hydroxyproline]3‐tuftsin analogue and its α‐ or β‐O‐D‐glucosylated derivatives
Authors: BIONDI L. , FILIRA F. , ROCCHI R. , TZEHOVAL E. , FRIDKIN M.
Published: August 2010

Syntheses are described of the Hyp3‐tuftsin analogue and of its derivatives α‐ or β‐O‐glycosylated at the side chain function of the hydroxyproline residue. The carbohydrate‐free tetrapeptide was prepared by reacting Z‐Thr‐Lys(Z)‐OH with H‐Hyp‐Arg(NO2)‐OBzl by the mixed anhydride procedure. In the synthesis of the α‐glycosylated analogue the O‐glycosyl amino acid was incorporated by reacting Boc‐(Glcα+β)Hyp‐OH with H‐Arg(NO2)‐OBzl through the same procedure. The α‐glucosylated dipeptide was isolated from the diastereomeric mixture, selectively deblocked, and acylated with Z‐Thr‐Lys(Z)‐OH by the mixed anhydride procedure. In the preparation of the β‐glucosylated analogue the BOP procedure was used for reacting Boc‐[Glc(Ac)4β]Hyp‐OH with H‐Arg(NO)2‐OBzl was well as for the final coupling to tetrapeptide. Removal of protecting groups from crude tetrapeptides was achieved by catalytic hydrogenation. Deacetylation of the sugar moiety of the β‐glucosylated tetrapeptide was achieved by treatment with sodium methoxide in methanol. The synthetic compounds were isolated by ion exchange chromatography, and characterized by elemental analysis, amino acid analysis, optical rotation and proton NMR. Their capacity to evoke the release of interleukin 1 from mouse peritoneal macrophages and to modulate immunogenic activity of antigen‐fed cells was evaluated, in comparison with tuftsin and rigin. All of the analogues were found to possess tuftsin‐like activity.
Side reaction during the deprotection of (S‐acetamidomethyl)cysteine in a peptide with a high serine and threonine content
Authors: LAMTHANH HUNG , ROUMESTAND CHRISTIAN , DEPRUN CLAUDE , MÉNEZ ANDRÉ
Published: August 2010

The acetamidomethyl (Acm) group is a widely used protecting group for the thiol of cysteine during the SPPS process. We prepared the amino terminal loop of the snake α‐neurotoxin, [Cys3, Cys23, Ser17(1–24) amide, from the linear peptide [Cys(Acm)3,23,Ser17](1–24) amide obtained by SPPS. Three different methods of deprotection of Cys(Acm) and disulfide bond formation were used: iodine, thallium(III) trifluoroacetate and mercuric acetate/potassium ferricyanide. The iodine method failed to yield the expected peptide, and gave instead the mono‐iodinated tyrosine analog. The disulfide cyclized peptide obtained by thallium(III) or Hg(II) procedures displayed a MW value observed by mass spectrometry that was higher than the calculated value. The difference (MWobs ‐ MWcalc) corresponded to a multiple of the Acm moiety, which is shifted intra‐and/or intermoleculary. Furthermore, we observed, in addition to the Acm shift in the disulfide cyclized decapeptide with a high Ser and Thr content (model peptide II), the dimerization phenomenon in the Tl(TFA)3 process. Therefore we conclude that a side reaction, a S–O(Ser,Thr) Acm shift, occured during the Cys(Acm) deprotection. This shift was supported by the demonstration of Ser(O‐Acm) formation in the reaction of Boc‐(L)‐Cys(Acm) with Tl(TFA)3 in the presence of an equimolar amount of (L)Ser. We report here the efficiency of a trivalent alcohol, glycerol, as scavenger in the both Tl(TFA)3 and mercuric/ferricyanide methods, in an attempt to circumvent this side‐reaction during the disulfide bond formation step starting from a bis‐Cys(Acm) peptide with a high Ser and Thr content, such as the N‐terminal loop of neurotoxin, model peptide II or a similar peptide.
Synthesis of tetrapeptide p‐nitroanilides catalyzed by pepsin
Authors: MALAK CAMELIA A. ABDEL , LAVRENOVA GALINA I. , LYSOGORSKAYA ELENA N. , FILIPPOVA IRINA YU. , YUTERENTEVA ELENA , STEPANOV VALENTIN M.
Published: August 2010

Swine pepsin at pH 5 efficiently catalyzes a condensation between Z‐Ala‐Ala‐Phe‐OH and p‐nitroanilides of Leu, Phe, Val, Ala and Arg that leads to formation of corresponding benzyloxycarbonyl‐tetrapeptide p‐nitroanilides with yields of 70–90%. These reactions are complicated by co‐precipitation of pepsin and the reaction products that necessitates the use of a relatively high concentration of pepsin.
Comparative study of the stability of the folding intermediates of the calcium‐binding lysozymes
Authors: NITTA KATSUTOSHI , TSUGE HIDEAKI , IWAMOTO HIROSHI
Published: August 2010

Unfolding profiles of two calcium‐binding lysozymes, equine milk lysozyme and pigeon egg‐white lysozyme, were obtained by circular dichroism and proton NMR measurements. Equine lysozyme unfolds through a stable molten globule intermediate. The molten globule of equine lysozyme was characterized as more ordered than that of bovine α‐lactalbumin. On the other hand, pigeon lysozyme unfolds by a two‐state mechanism and the intermediate could not be observed in guanidine or thermal unfolding, the same as with conventional non‐calcium‐binding lysozymes. Thus, from the point of view of the unfolding profile, equine lysozyme belongs to the group of α‐lactalbumin, but pigeon lysozyme belongs to the conventional lysozyme group.
Linear tripeptide conformation
Authors: KRAUSE JEANETTE A. , EGGLESTON DRAKE S.
Published: August 2010

The structures of two tripeptides, Cbz‐glycylglycyltyrosine methyl ester (ZGGYOMe) and Cbz‐glycyl‐(D,L)tyrosylglycine ethyl ester (ZGYGOEt) have been determined from single‐crystal X‐ray diffraction data. Crystals of ZGGYOMe are monoclinic, space group P21, with a= 12.427(3), b= 4.999(3), c= 17.401(6) Å, β= 99.98(2)° and Z= 2. The final R‐index is 0.049 for 1698 reflections with I≥2 σ(I). Crystals of ZGYGOEt are monoclinic, space group P21/n with a= 12.134(8), b= 14.614(3), c= 26.154(9) Å, β= 98.78(4)°, Z= 8. The final R‐index is 0.067 for 4457 reflections with I≥2 σ(I). Both peptides adopt highly extended structures; principal torsion angles are ω0= 175.0(4)°, φ1= 69.2(5)°, ψ1=− 154.9(4)°, ω1=−175.8(4)°, φ2= 165.4(4)°, ψ2= 154.2(3)°, ω2= 169.6(3)°, φ3=−94.8(5)°, ψ3=−47.6(5)° for ZGGYOMe and, for the two independent molecules of ZGYGOEt, ω0= 177.9(4)°, 178.9(4)°, φ1=−172.0(4)°, 169.7(4)°ψ1= 174.4(4)°, −162.5(4)°; ω1= ‐170.1(4)°, 176.7(4)°; φ2=−130.8(4)°, 130.3(5)°; ψ2= 162.8(4)°, −163.3(4)°; ω2=−177.6(4)°, 176.2(4)°; φ3=− 169.9(4)°, 172.9(4)°; ψ3=− 168.2(4)°, 160.9(4)°. The structures are of interest since the first one adopts a conformation unlike those of related GGX sequences and the latter shows an antiparallel hydrogen‐bonding pattern.
Synthesis of substrate analogues for trypanothione reductase
Authors: EL‐WAER ABDUSSALAM F. , BENSON TIMOTHY , DOUGLAS KENNETH T.
Published: August 2010

The synthesis and chemical characterisation of a range of substrate analogues for trypanothione reductase are described, with the spermidine portion of trypanothione replaced by the 3‐dimethylaminopropylamide moiety. Using 1‐hydroxybenzotriazole/N‐hydroxysuccinimide coupling, products were obtained which had a range of replacements of the γ‐glutamyl groups of the enzyme substrate. The materials were characterised by FPLC, 1H/13C NMR spectroscopy and FAB mass spectroscopy.
Unusual cleavage of peptidic hormones generated by trypanosome enzymes released in infested rat serum
Authors: TETAERT DANIEL , SOUDAN BENOI̊T , HUET‐DUVILLIER GUILLEMETTE , DEGAND PIERRE , BOERSMA ARNOLD
Published: August 2010

Gonad and thyroid dysfunctions are often observed in human and experimental models during african trypanosomiasis. The enzymatic activity of components released by the trypanosomes towards peptide hormones (e.g. GnRH, TRH) have consequently been studied. The incubation products of GnRH by (i) healthy or infested rat serum: (ii) trypanosomal components released by using a specific procedure; (iii) infested and normal rat brain extracts have been analysed by RP‐HPLC fractionation. The peptide cleavage has been assessed by determination of either the amino acid compositions or relative molecular weight (by FAB mass spectrometry) of the different resolved HPLC fractions. Different protease inhibitors and a reducing agent have also been tested and a serine, cation‐sensitive, thiol‐dependent endopeptidase activity has been predominantly identified to be released by the trypanosomes in host circulation. It has been shown that the peptidase activity(ies) is(are) able to (i) degrade the peptide hormones (GnRH, TRH) considered as important neuromodulators and neurotransmitters; (ii) generate an unusual N‐terminal tetrapeptide (GnRH1‐4) appearing to be still active towards the gonadal hypothalamo‐pituitary axis.
Peptide amidation by enzymatic transacylation and photolysis
Authors: HENRIKSEN D.B. , BREDDAM K. , BUCHARDT O.
Published: August 2010

A series of model peptides with a C‐terminal protected amide group were prepared by enzymatic transacylation. The protection groups were removed by photolysis to give the warranted peptide amides in high yields. Furthermore, fragments of human calcitonin were prepared. Various protective groups were employed, and the pH, solvent and concentration dependency of the enzymatic transacylation were examined. The photo‐cleavage reaction was examined for wavelength, concentration and pH dependency. It was shown that the optimal yields required addition of a chemical scavenger for the photolysis byproducts.
Multiple release of equimolar amounts of peptides from a polymeric carrier using orthogonal linkage‐cleavage chemistry
Authors: LEBL MICHAL , PÁTEK MARCEL , KOČIŠ PETR , KRCHŇÁK VIKTOR , HRUBY VICTOR J. , SALMON SYDNEY E. , LAM KITS.
Published: August 2010

Peptides generated on polymeric beads that are attached to the support via cleavable linkers are released into solution in equimolar amounts in several independent steps and screening of the “peptide libraries” can be performed in the usual manner used for testing peptides in solution.
Peptide mimics for structural features in proteins
Authors: KARLE ISABELLA L , FLIPPEN‐ANDERSON JUDITH L , UMA K , BALARAM P
Published: August 2010

The crystal structure determination of three heptapeptides containing α‐aminoisobutyryl (Aib) residues as a means of helix stabilization provides a high‐resolution characterization of 6→1 hydrogen‐bonded conformations, reminiscent of helix‐terminating structural features in proteins. The crystal parameters for the three peptides, Boc‐Val‐Aib‐X‐Aib‐Ala‐Aib‐Y‐OMe, where X and Y are Phe, Leu (I), Leu, Phe (II) and Leu, Leu (III) are: (I) space group P1, Z= 1, a= 9.903 Å, b 10.709 Å, c= 11.969 Å, α= 102.94°, β=103.41°, γ= 92.72°, R= 4.55%; (II) space group P21, Z= 2, a=10.052 Å, b=17.653 Å, c= 13.510 Å, β=108.45°. R= 4.49%; (III) space group P1, Z= 2 (two independent molecules IIIa and IIIb in the asymmetric unit). a=10.833 Å, b=13.850 Å, c=16.928 Å, α=99.77°, β=105.90°, γ= 90.64°, R= 8.54%. In all cases the helices form 310/α‐helical (or 3l10‐helical) structures, with helical columns formed by head‐to‐tail hydrogen bonding. The helices assemble in an all‐parallel motif in crystals I and III and in an antiparallel motif in II. In the four crystallographically characterized molecules, I, II, IIIa and IIIb, Aib(6) adopts a left‐handed helical (ht) conformation with positive φψ values, resulting in 6→1 hydrogen‐bond formation between Aib(2) CO and Leu(7)/Phe(7) NH groups. In addition a 4→1 hydrogen bond is seen between Aib(3) CO and Aib(6) NH groups. This pattern of hydrogen bonding is often observed at the C‐terminus of helices in proteins, with the terminal π‐type turn being formed by four residues adopting the hRhRhRhL conformation.