bluuebunny/crossref_metadata_2025_split huggingface dataset:
The electrochemical reduction of diazanaphthalenes; a spectro‐electrochemical investigation
Authors: Wolsink J. H. , Spaans A. , van den Ham D. M. W. , Feil D. 
Published: September 2010

AbstractThe electrochemical reduction of diazanaphthalenes in dimethylformamide at first‐ and second‐wave diffusion levels has been investigated by means of transmission spectro‐electrochemical methods. It has been found that the reaction of dianions with parent neutral molecules is an essential step in the reaction mechanism. Rate constants for all non‐electrochemical steps have been measured or estimated.Independent estimations of diffusion and absorbance coefficients are presented, as well as some principles of digital simulation.
Effects of γ‐hydroxyl groups in lithium‐ammonia reductions of α,β‐unsaturated carbonyl compounds via an inverse‐addition, low‐temperature method
Authors: de Groot Ae. , Jansen B. J. M. , Peterse A. G. J. M. , Wijnberg J. B. P. A. 
Published: September 2010

AbstractA new inverse‐addition, low‐temperature procedure for lithium‐ammonia reductions has been developed. Using this method it has been demonstrated that the reduction of ethyl 5,5‐(ethylenedioxy)‐8β‐hydroxy‐4aβ‐methyl‐2‐oxo‐2,3,4,4a,5,6,7,8‐octahydro‐1‐naphthalenecarboxylate (3) results in an unusual cis‐reduction product. The 8β‐hydroxyl group, low reaction temperature (–80°C) and addition of lithium‐ammonia to the substrate all proved to be necessary in order to achieve this cis reduction.
Nouvelle synthèse et étude de la stabilité de l'amino‐3 thiophène
Authors: Rault S. , de Sevricourt M. Cugnon , Robba M. 
Published: September 2010

AbstractA new synthesis of 3‐aminothiophene is proposed starting from 3‐thenoic acid via isopropyl 3‐thienylcarbamate as intermediate. Thermal decomposition of this latter in alkaline medium gives 3‐aminothiophene.NMR and mass spectral studies revealed that the instability of the amine is due to its polymerisation via the formation of the dimer di‐3‐thienylamine and the trimer tri‐3‐thienylamine.
Studies on polypeptides XXXVII. Synthesis of [isoleucine‐13, homoserine‐20]‐s‐peptide lactone and semisynthesis of [isoleucine‐13, homoserine‐20]‐RNase A
Authors: Hoogerhout P. , Kerling K. E. T. 
Published: September 2010

AbstractThe solid‐phase synthesis of [Ile13, Met20]‐S‐peptide‐(Gly‐Gly) on a Merrifield resin, using tert‐butoxycarbonyl‐ and benzyl‐type protecting groups, is reported. Deprotection was achieved by reaction with liquid hydrogen fluoride and purification by ion‐exchange chromatography and HPLC.The above peptide was converted into [Ile13, Hse20]‐S‐peptide lactone by reaction with cyanogen bromide. Reaction of the synthetic 20‐peptide lactone with “native” bovine S protein at pH 7.5 resulted in the formation of [Ile13, Hse20]‐RNase A in about 10% yield. The semisynthetic enzyme was purified by ion‐exchange chromatography and the kinetic parameters were determined from 2′,3′‐CMP hydrolysis.
1,3,4‐Trimethyl‐2,2′‐pyrromethen‐5[1H]‐one and its 1′‐methyl derivative; Synthesis, thermal and photochemical reactivity
Authors: de Groot J. A. , Lugtenburg J. 
Published: September 2010

AbstractCondensation of 1,3,4‐trimethyl‐1,5‐dihydro‐2H‐pyrrol‐2‐one, prepared in high yield from 3,4‐dimethylpyrrole, with pyrrole‐2‐carboxaldehyde and 1‐methylpyrrole‐2‐carboxaldehyde gives a mixture of Z‐ and E‐1,3,4‐trimethyl‐2,2′‐pyrromethen‐5[1H]‐one (7 and 8) and 1,1′,3,4‐tetramethyl‐2,2′‐pyrromethen‐5[1H]‐one (9 and 10), respectively. The Z and E forms both isomerise thermally upon heating in basic methanol to the same equilibrium state as is observed during the synthesis. Upon irradiation with light (λ > 360 nm), 7, 8, 9 and 10, similarly undergo Z‐E isomerisation.
Synthesis of meso‐ and (±)‐2,2′‐bis(carboxymethoxy)oxydiacetic acid via cis‐ and trans‐2,5‐(diallyloxy)‐2,5‐dihydrofuran: Model compounds for the calcium complexation of oxidized sucrose
Authors: Nieuwenhuizen M. S. , Kieboom A. P. G. , van Bekkum H. 
Published: September 2010

AbstractCis‐ and trans‐2,5‐diallyloxy‐2,5‐dihydrofuran have been prepared by acid catalyzed transacetalization of 2,5‐dimethoxy‐2,5‐dihydrofuran with allyl alcohol. As a by‐product, 1,1,3‐triallyloxypropane was formed from 2‐allyloxy‐2,5‐dihydrofuran intermediates. The cis/trans configuration has been determined by NMR (chiral lanthanide shift reagent) and MS (elimination of allyl alcohol). Oxidation of the cis and trans compounds by potassium permanganate yielded meso‐ and (±)‐2,2′‐bis(carboxymethoxy)oxydiacetic acid, respectively. The calcium sequestering properties of the latter compounds were superior to those of the acetal polycarboxylate obtained by sucrose oxidation. This is due to the extra steric hindrance encountered for the latter upon calcium complexation.
N‐nitroso‐1,3‐oxazolidine, a carcinogenic compound detected as an impurity in a commercial cutting fluid
Authors: Ellen G. , Freudenthal J. , Schuller P. L. 
Published: September 2010

AbstractThe title N‐nitrosamine, a potent liver carcinogen, was detected in a mineral oil cutting fluid at concentrations up to 60 mg/kg. Identification was performed by gas chromatography/mass spectrometry and by synthesis. The compound is thought to have been formed from formaldehyde, sodium nitrite and an amino alcohol present in the cutting fluid.
Synthesis of some new pyrazole‐containing chelating agents
Authors: Driessen Willem L. 
Published: September 2010

AbstractProcedures are described involving the condensation of l‐(hydroxymethyl)pyrazole (1a) and l‐(hydroxymethyl)‐3,5‐dimethylpyrazole (1b) with the following amines (products given in parentheses): ethylamine (2a and 2b), aniline (3a and 3b), N,N′‐dimethyl‐ l,2‐ethanediamine (4a and 4b), 1,2‐ethanediamine (6a and 6b), diethylenetriamine (7a and 7b), triethylenetetramine (8a and 8b) and ammonia (9a and 9b). All amine hydrogens are completely substituted by pyrazol‐1‐ylmethyl groups (compounds 2a‐9a) or (3,5‐dimethylpyrazol‐1‐yl)methyl groups (compounds 2b‐9b with yields being better than 90%.
Synthesis of linear and cyclic peptides related to Met‐ and Leu‐enkephalin
Authors: van Nispen J. W. , Greven H. M. 
Published: September 2010

AbstractThe synthesis is described of several peptides containing the sequence Phe‐Met. In order to limit enzymatic degradation and to restrict the number of possible conformations they were cyclized with variable ring size. Some unexpected side‐reactions were encountered during the synthesis of the linear precursors: partial cleavage of the active ester during removal of the Bocprotecting group, partial cleavage of the Gly‐Gly peptide bond during hydrazinolysis of an ester function and failure to obtain the p‐nitrophenyl ester of Boc‐Phe‐Met‐Gly‐Gly‐OH via the DCC method. Diphenyl phosphoroazidate was finally used for the cyclization of several linear peptides. Some results of behavioural activity and metabolic stability are mentioned.
On the luminescence and photochemistry of the lower excited electronic states of some acyclic 1,2‐diketones
Authors: Sarphatie Louis A. , Verheijdt Paul L. , Cerfontain Hans 
Published: September 2010

AbstractThe absorption and luminescence spectra of a number of acyclic 1,2‐diketones have been determined,viz. of 2,3‐butanedione, 2,2,5,5‐tetramethyl‐3,4‐hexanedione, some dicycloalkyl and of cyclopropyl‐ and phenyl‐substituted 1,2‐diketones. For the 1,2‐diketones without conjugating substituents, the time‐averaged ground‐state dihedral angles between the two carbonyl group (magnified image) have been estimated from the positions of the long‐wavelength UV absorption band. The variation in the fluorescence transition energy with the ground‐state dihedral anglemagnified imageis only small,viz. 1500 cm−1, on varyingmagnified imagefrom 0 to 180°. The difference between the fluorescence and phophorescence transition energies is independent of the ground statemagnified image. The results show that the 1,2‐diketones emit from excited states with coplanar diketo geometry.The λ 403 nm photochemistry of 2,2,5,5‐tetramethyl‐3,4‐hexanedione (2) and dicyclopentyl‐ (6) and dicyclohexyl‐ethanedione (7) has been studied in some detail. The yields of the 2‐hydroxycyclobutanone products (14), formed by aNorrishtype II process of the triplet‐excited 1,2‐diketone, decrease in the substrate order2>6>7an explanation for this order is proposed.
Réactivité de cyclodi‐ et tri‐germazanes: Mise en évidence de la participation de germylèneamines dans des réactions d'insertion‐élimination
Authors: Lacrampe G. , Lavayssière H. , Riviére‐Baudet M. , Satgé J. 
Published: September 2010

AbstractLes cyclogermazanes (R2GeNR′)n (n = 2, 3), formes polycondensées des germyléneamines R2Ge=NR′, sont obtenus par aminolyse ou ammonolyse des liaisons Ge‐Cl, par transamination intermoléculaire, ou par action des chlorures germaniés sur les aminodilithiens. Leur degré de condensation (n = 2, 3) est fortement lié à la nature des substituants portés par l'azote ou le germanium.Les réactions de clivage par divers réactifs protoniques bifonctionnels tels que les diols, dithiols, aminoalcools, aminothiols, aminoacides ont été étudiées et conduisent aux hétérocycles germaniés à 5 chaînons correspondants, par clivage de 2 liaisons Ge‐N consécutives.Les réactions d'insertion d'hétérocumulènes ou de dipǒles insaturés tels que RNCO, RNCS, CO2, CS2, PhCHO, et l'oxirane, conduisent à des réactions d'extension de cycle.Les dérivés de diaddition des isocyanates et isothiocyanates aux cyclogermazanes se décomposent avec formation transitoire de germylèneamines mises en évidence dans leur réaction de cycloaddition 1‐2 avec les iso‐ ou isothio‐cyanates et formation de diazagermétidinones (ou thiones) instables.De même, les dérivés d'addition des cyclogermazanes aux aldéhydes, CO2, CS2 conduisent aux cyclogermoxanes ou germathianes via les germanones et germathiones correspondantes.Les dipǒles 1–3 tels que les nitrones régissent avec les cyclogermazanes à des températures voisines de 150°C avec formation transitoire de 1,3,5,2‐oxadiazagermolidines instables.La réactivité des cyclodigermazanes est exaltée en présence de triéthylamine, de HMPT. La possibilité d'une participation de la forme monomère germylèneamine R2Ge=NR' dans ces dernières réactions, découlant d'un équilibre cyclodigermazane ⇋ germylèneamine est discutée.
The kinetics and primary isotope effect for reduction of acridinium ions by a NAD(P)H “model”; facts and fallacy
Authors: van Laar A. , van Ramesdonk H. J. , Verhoeven J. W. 
Published: September 2010

AbstractA kinetic study of the reduction of the 10‐methyl‐9‐phenylacridinium ions (AcPh⊕) by 1‐benzyl‐1,4‐dihydronicotinamide (PyH2) is presented. The primary kinetic isotope effect (kH/kD = 4.5) of this reaction equals the product isotope partitioning observed upon reduction with mono C‐4 deuterated 1‐benzyl‐1,4‐dihydronicotinamide (PyHD). Furthermore, the rate constant for the reduction decreases upon increasing the solvent polarity. This behaviour contrasts with that reported for the reduction of the 10‐methylacridinium ion (AcH⊕) where acceleration due to increased solvent polarity and significant discrepancies between the kinetic isotope effect and the product isotope partitioning have been observed. Reinvestigation of the latter process by means of kinetic measurements and via a competition experiment, involving reduction with equal amounts of PyH2 and the dideuterated species (PyD2), shows that the kinetic data and especially the kinetic isotope effect data are easily subject to experimental error. Thus it is shown that an intolerable inaccuracy is introduced if the value of kH/kD is determined by comparing the rate of reduction with PyH2 with that of PyHD rather than by comparing those of PyH2 and PyD2. Furthermore, the kinetic data in protic solvents for AcH⊕, but not for AcPh⊕, are found to be disturbed by a reversible side reaction. This reaction is thought to involve formation of a covalent adduct by attack of PyH2 at the electrophilic C‐9 position of AcH⊕. The less electrophilic nature of this position in AcPh⊕ prevents such adduct formation and leads to uncomplicated kinetic behaviour in all the solvent systems investigated.
The conformational equilibria of 1‐ and 2‐phenyl‐1,2‐dihydronaphthalene
Authors: Lamberts J. J. M. , Laarhoven W. H. 
Published: September 2010

AbstractThe conformational equilibria of 1‐ and 2‐phenyl‐1,2‐dihydronaphthalene (1 and 2) have been studied using 500 MHz 1H NMR spectroscopy.The signals due to the protons at C(1) and C(2) of compound 2 and of a deuterated derivative of 1 (3,4‐d2‐1) were analysed as ABX patterns. The fraction x of the conformer, having the phenyl group in a pseudo‐axial (pa) position, was found to be 0.44 for 3,4‐d2‐1, in contrast to the marked preference shown by non‐aromatic substituents at C(1) of 1,2‐dihydronaphthalenes for the pa position. This difference is most probably due to π‐overlap between a pseudo‐equatorial phenyl substituent and the aromatic moiety of the 1,2‐dihydronaphthalene counterbalancing the peri‐interaction with H(8).For 2, x = 0.31, indicating a slight preference of the phenyl group for the pseudo‐equatorial (pe) position which preference is also observed for other substituents at C(2) of 1,2‐dihydronaphthalenes.
Exchange processes in spirophosphoranes with a dioxaphosphorinane ring. A 13C and 1H NMR study
Authors: van Ool Peter J. J. M. , Buck Henk M. 
Published: September 2010

AbstractVariable temperature 13C and 1H NMR studies have revealed the equatorial‐apical preference of dioxaphosphorinane rings in pentacoordinated trigonal bipyramidal spirophosphoranes.
Photo‐oxidation of 4‐aminobiphenyl
Authors: Spee Ton , van Dijk‐Knepper Joke , Cornelisse Jan 
Published: September 2010

AbstractUpon eosin‐sensitized irradiation (λ > 500 nm) of 4‐aminobiphenyl, 4‐nitrobiphenyl is produced in 50% yield. 3‐Aminobiphenyl is converted into 3‐nitrobiphenyl upon sensitized as well as upon direct irradiation. At sufficiently high concentrations of 4‐aminobiphenyl (≥9 × 10−5 M), the quantum yield of its disappearance becomes greater than unity. Addition of the singlet‐oxygen acceptor, 1,4‐diazabicyclo[2.2.2]octane, surprisingly causes a considerable increase in the quantum yield of the disappearance of 4‐aminobiphenyl. Upon addition of 4‐aminobiphenyl to a solution of the singlet‐oxygen acceptor bilirubin in aqueous 0.2 M NaOH containing eosin, the rate of photobleaching of bilirubin does not decrease, but rather increases sixfold. A possible explanation for these remarkable phenomena is discussed.There are strong indications that attack of singlet oxygen is the primary step in the photo‐oxidation of 4‐aminobiphenyl.
Synthesis of 10,10‐dimethylprostaglandin F1 and F2 analogues
Authors: Plantema O. G. , de Koning H. , Huisman H. O. 
Published: September 2010

AbstractThe synthesis of the 10,10‐dimethyl analogues of PGF1α (21c), PGF1β (22c), 11‐epi‐PGF1β (23c), PGF2α (30b) methyl ester and their C(15)‐epimers (21d, 22d, 23d and 30c, respectively) is described. Some biological results obtained using these analogues are also discussed.
The composition of methylcopper(I) species in solution. A 1H NMR study
Authors: Westmijze H. , George A. V. E. , Vermeer P. 
Published: September 2010

AbstractThe composition of methylcopper(I) species, prepared from cuprous halides and MeM (M = Li, MgX or Mg1/2) in various ratios, has been studied by means of 1H NMR spectroscopy. Tetrahydrofuran (THF) was used as solvent. Evidence is presented that in the presence of hexamethylphosphoric triamide (HMPT), methylhalocuprates, [MeCuX]M, can be obtained stoichiometrically pure. By means of 1H NMR analysis it could be shown that the complex MeCu[P(OMe)3]3 also exists in solution. The trimethyldicuprate [Me3Cu2]2Mg has only been obtained stoichiometrically pure on starting from MeMgCl and cuprous chloride; it was formed in equilibrium with [Me2Cu]2Mg and [MeCuBr]2Mg when the starting Grignard reagent or the cuprous halide contained bromide. Homocuprates [Me2Cu]M were always obtained in mixtures containing, in addition to the homocuprate, [Me3Cu2]M and MeM when the starting reagent MeM was a Grignard reagent. Attempts to prepare species with another stoichiometry failed when Grignard reagents MeM were used.
Photochemistry of dienones. Part 13. Photochemistry of the valeryl analogues of (E)‐α‐ and (E)‐β‐ionone; absence of Norrish type II processes
Authors: Visser Cornelis P. , van der Wel Hans , Cerfontain Hans 
Published: September 2010

AbstractThe photochemistry of 1b and 2b, i.e. the valeryl analogues of (E)‐α‐ (1a) and (E)‐β‐ionone (2a), respectively, has been studied.Upon irradiation with λ 254 and 300 nm, the α‐isomer 1b exhibits the same reactions as the parent ionone 1a, viz. (E)‐(Z) isomerization, followed by photo‐deconjugation and subsequent α‐cleavage with formation of the 1,3‐ and 1,5‐acyl shift isomers. Upon irradiation of the β‐isomer 2b with λ 254 nm, only the α‐pyran isomer 15 is formed, although extensive polymerization does occur. Neither the α‐ nor the β‐isomer yields any detectable product resulting from a Norrish type II reaction in which the γ‐hydrogen of the butyl chain is involved.
An approach to the synthesis of 7β‐amino estrogens
Authors: Loozen H. J. J. , van Dam W. , de Winter M. S. 
Published: September 2010

AbstractThe synthesis of 7β‐amino‐1,3,5(10)‐estratriene‐3,17β‐diol (1) has been accomplished using a sequence of transformations proceeding through the 6α,7α‐epoxy‐1,3,5(10)‐estratriene‐3.17β‐diol dibenzyl ether (6). This latter compound is reduced by LiAlH4to give the 7α‐hydroxy compound 7, which, upon subsequent treatment with tosyl chloride (to give8), followed by reaction with NaN3, provided the 7β‐azido‐1,3,5(10)‐estratriene‐3,17β‐diol dibenzyl ether (9). Reduction of9with LiAlH4, followed by catalytic removal of the benzyl ethers, gave1.Attempts to obtain an epoxide from 1,3,5(10),6‐estratetraene‐2,3–17β‐triol tribenzyl ether (17) failed, and led to the isolation of the corresponding 6α,7α‐diol (19) and 6β,7α‐diol (20). A novel transformation,viz.subsequent treatment of either19or20with SOCl2and LiAlH4, selectively provided 1,3,5(10)‐estratriene‐2,3,7α,17β‐tetraol 2,3,17‐tribenzyl ether (21). Further treatment of21, as delineated for7, provided the required 7β‐amino‐1,3,5(10)‐estratriene‐2,3,17β‐triol (2).
Chemical studies on carbohydrates (III). The ring conversion of hydrazinoaldoses into pseudo‐N‐nucleosides by ring transformation
Authors: Smit P. , Stork G. A. , van der Plas H. C. 
Published: September 2010

AbstractOn reacting 3‐deoxy‐3‐hydrazino‐1,2: 5,6‐di‐O‐isopropylidene‐α‐D‐allofuranose (2) or 3‐deoxy‐3‐hydrazino‐1,2: 5,6‐di‐O‐isopropylidene‐α‐D‐glucofuranose (8) with N‐methylpyrimidinium methyl sulfate (4) at room temperature, formation of 3‐deoxy‐3‐N‐pyrazolyl‐1,2: 5,6‐di‐O‐isopropylidene‐α‐D‐allofuranose (3) and 3‐deoxy‐3‐N‐pyrazolyl‐1,2: 5,6‐di‐O‐isopropylidene‐α‐D‐glucofuranose (9), respectively, takes place. This ring transformation has been explained by an ANRORC mechanism. Similarly, 6‐deoxy‐6‐hydrazino‐1,2‐O‐isopropylidene‐α‐D‐glucofuranose (10) and methyl‐4,6‐O‐benzylidene‐2‐deoxy‐2‐hydrazino‐α‐D‐mannopyranoside (11) react with the pyrimidinium salt to give the corresponding pyrazolylaldoses.Starting from 3,6‐anhydro‐5‐deoxy‐5‐hydrazino‐1,2‐O‐isopropylidene‐β‐L‐idofuranose (16), in addition to the expected 3,6‐anhydro‐5‐deoxy‐5‐N‐pyrazolyl‐1,2‐O‐isopropylidene‐β‐L‐idofuranose (17a), 3,6‐anhydro‐5‐deoxy‐5‐(1,2,4‐triazol‐1‐yl)‐1,2‐O‐isopropylidene‐β‐L‐idofuranose (17b) is formed. The same conversion of substituted hydrazines into pyrazolyl and triazolyl derivatives is observed in the reaction of 1‐deoxy‐1‐hydrazino‐2,3‐O‐isopopylidene‐D‐ribofuranose (12) with the pyrimidinium salt. This pyrimidine‐to‐triazole ring transformation can also be explained by an ANRORC mechanism.Further investigation of the reactivity of hydrazinoaldoses towards other positively charged azines shows the general scope of this reaction.
(S‐Butyldithiocarbanilato)copper(I), Cu[BuSC(S)NPh]. Preparation and crystal structure
Authors: Willemse J. , Bosman W. P. , Noordik J. H. , Cras J. A. 
Published: September 2010

AbstractThe preparation and the crystal structure of a copper(I) complex with the deprotoned S‐Butyldithiocarbanilic ester is described. The crystals of Cu[BuSC(S)NPh] are rhombohedral [space group R3, Z = 6, a = 14.684(3) Å, α = 112.96(4)°] with one hexameric unit in the unit cell.
Thermolytic reactions of esters. Part XIII The effect of electron‐attracting α‐substituents in alkyl acetates
Authors: Louw Robert , Tinkelenberg Arie , Werner Erik S. E. 
Published: September 2010

Abstractα‐Alkylation of esters, e.g. AcO‐iPr → AcO‐tBu, results in a large rate increase for β‐elimination of acetic acid, indicating that in the transition state Cα is positively charged. Replacement of α‐H by electron‐attracting groups Z, such as COOMe, also enhances the rate of formation of alkene. Neither differences in product stability nor steric acceleration appear to play a significant role. A major factor is dipolar destabilization of starting compounds containing two electronegative groups, such as AcOCMe2Z compared with AcO‐tBu. The free energies of the transition states are found to parallel those of the fully ionized species, AcO(‐) + R(+).At elevated temperatures, esters such as AcOCMe(CN)Ph will give rise to O‐C bond homolysis rather than to molecular elimination of acetic acid.
The electrochemical preparation of [Au9(PPh3)8]+. A comparative study of the structures and properties of [Au9(PPh3)8]+ and [Au9(PPh3)8]3+
Authors: van der Velden J. W. A. , Bour J. J. , Bosman W. P. , Noordik J. H. , Beurskens Paul T. 
Published: September 2010

Abstract[Au9L8]PF6 (L = PPh3) deposits on the cathode during the coulometric two‐electron reduction of [Au9L8](PF6)3 in acetonitrile at constant potential (−0.60 V relative to an Ag/AgCl electrode). X‐ray crystal structure determination shows a centred cubic arrangement of the 9 Au atoms in the cluster. Physical data, including (solid state) 31P[1H]NMR and 197Au Mössbauer data, are provided and compared with those of [Au9L8]3+.
Studies on polypeptides XL. The role of histidine‐119 in non‐covalent semisynthetic ribonuclease; its replacement by 3‐(3‐pyrazolyl)‐L‐alanine, Nπ‐methyl‐L‐histidine and Nτ‐methyl‐L‐histidine
Authors: Serdijn J. , Bloemhoff W. , Kerling K. E. T. , Havinga E. 
Published: September 2010

AbstractNon‐covalent semisynthetic ribonuclease analogues are used to study the role of histidine‐119 in the active site of RNase A. The solid‐phase synthesis of three RNase 111‐124 peptides, in which histidine‐119 is replaced by 3‐(3‐pyrazolyl)‐L‐alanine, Nπ‐methyl‐L‐histidine and Nτ‐methyl‐L‐histidine, respectively, is described. The binding of these peptides to RNase 1‐118 protein is examined. The enzymatic activities of the resulting complexes towards first and second step substrates and their binding to the inhibitor 3′‐cytidine monophosphate are determined.The Nπ‐methylhistidine‐ and the 3‐(3‐pyrazolyl)alanine RNase analogues are devoid of catalytic activity. The Nτ‐methylhistidine analogue is found to be enzymatically active, using yeast ribonucleic acid and 2′,3′‐CMP as substrates. With the latter substrate, kinetic parameters at pH 6.0 have been determined. The results provide strong evidence to suggest that the pros‐nitrogen atom of His‐119 plays an essential role as acid/base catalyst in the enzymatic reaction of RNase. There are indications that the tele‐nitrogen atom of histidine‐119 in the natural enzyme – besides effecting the right pK value at the pros‐nitrogen centre ‐ may also contribute by favouring the optimal steric orientation of the imidazole nucleus and of the pros‐nitrogen in particular.