bluuebunny/crossref_metadata_2025_split huggingface dataset:
Heterotricyclodecane. VII. 2,7‐Dithia‐isotwistan und Derivate
Authors: Ganter C. , Wigger N. 
Published: December 2004

AbstractThe synthesis of 2,7‐dithia‐isotwistane (18) and several of its derivatives (13–17) was achieved starting from endo‐2‐chloro‐9‐thiabicyclo[3.3.1]non‐6‐ene (3) and using the bicyclic mercaptan 7 as a key intermediate.The isotwistane halides 13 and 14 were treated under reaction conditions suitable for molecular rearrangements involving an epi‐sulfonium ion (a) by neighbouring group participation. However, only isotwistane derivatives were formed (13 → 16, 13 → 17, 14 → 16, 14 → 17) and no rearranged products with twistane structure could be isolated.
Über Pterinchemie. 40. Mitteilung [1]. Weiterer Beitrag zur Synthese des Biopterins
Authors: Viscontini M. , Provenzale R. , Frei W. F. 
Published: December 2004

AbstractEine neue Synthese des Biopterins ausgehend von L‐4‐Methyl‐erythrulose und 2,4,5‐Triamino‐6‐hydroxy‐pyrimidin wird beschrieben. Bei der Oxydation des intermediär gebildeten Tetrahydrobiopterins wird die Dihydroxypropyl‐Kette so leicht abgespalten, dass die Ausbeute an Biopterin auch bei dieser Synthese 15% nicht übersteigt. Eine mögliche Erklärung dieser Abspaltung wird vorgeschlagen.
On the Mechanism of Decarboxylation of Betanidine. A Contribution to the Interpretation of the Biosynthesis of Betalaines
Authors: Dunkelblum E. , Miller H. E. , Dreiding André S. 
Published: December 2004

AbstractBei der Biogenese des Betanins (2) aus Dopa (1) wird die Carboxylgruppe der Aminosäure in die Carboxylgruppe‐C(19) umgewandelt. Dieser Schluss beruht auf einem Deuterierungsversuch, bei dem gezeigt wird, dass die Monodecarboxylierung von Betanidin (3) zu einem Verlust von C(19) führt und dass dabei die Doppelbindung C(17)C(18) nach C(14)C(15) wandert. Wenn radioaktives Betanidin (3), erhalten aus Einbauexperimenten mit DL‐Dopa‐[1‐14C], in Äthanol decarboxyliert wird, erhält der danach isolierte Dimethylester 7 des Decarboxybetanidin (6)‐hydrochlorids nur noch 14% des 14C.Bei der alkali‐katalysierten Äquilibrierung von Betanidin (3) ⇆ Isobetanidin (9) tritt im Gegensatz zur Decarboxylierung keine Wanderung der Doppelbindung ein.
The HCL‐Catalyzed Stereomutation of Cyclic Sulfoxides
Authors: Sagramora L. , Garbesi A. , Fava A. 
Published: December 2004

AbstractThe critical step in the HCl‐catalyzed stereomutation of sulfoxides is a nucleophilic substitution at the sulfur atom [7]. The rate of this reaction has been determined for cyclic sulfoxides as a function of ring size. The trend closely matches that long since established for substitution at alicyclic carbon [11], which confirms the similarity of the geometries of the transition states for substitution at carbon and at sulfur. The results are compared to those for the analogous silicon and phosphorous compounds, and the question of formation of pentacovalent intermediates for substitutions at sulfur is briefly discussed.
Die 1,3‐dipolare Cycloaddition von Acetylendicarbonsäure‐estern an 2‐Methyl‐4‐phenyl‐chinazolin‐3‐oxid
Authors: Stauss U. , Härter H. P. , Neuenschwander M. , Schindler O. 
Published: December 2004

AbstractThe 1,3‐dipolar addition of acetylenedicarboxylic esters (IX and X) to 2‐methyl‐4‐phenyl‐quinazoline 3‐oxide (VIII) in benzene/methanol and benzene/ethanol, respectively, gives the esters XI and XII of 3‐amino‐3‐phenyl‐2‐(2‐acetamidophenyl)‐acrylic acid as main products and the esters XIII and XIV of 2‐methyl‐4‐phenyl‐5H‐benzo[d][1,3]diazepin‐5‐carboxylic acid as by‐products. The constitutions of XI and XII are elucidated by acid hydrolysis to the 2‐phenylindole‐3‐carboxylic esters VI and VII, respectively, and by ozonolysis of XII to give benzamide and ethyl o‐acetamido‐mandelate (IV). The alkaline hydrolysis of XI or XII gives the enamine derivative XVIII, which is hydrolysed by acid to oxindole and benzoic acid. The structure elucidation of XIII and XIV is based on spectroscopic data together with thc formation of XV by alkaline hydrolysis. Mechanisms arc proposed for the reaction paths.
The π‐Orbital Energies of the Acenes
Authors: Clark Patricia A. , Brogli F. , Heilbronner E. 
Published: December 2004

AbstractThe ‘observed’ π‐orbital energies ϵv,j = −Iv,j derived from the vertical ionization potentials obtained by a photoelectron spectroscopic investigation of the acenes benzene A(1), naphthalene A(2), anthracene A(3), naphthacene A(4) and pentacene A(5) have been compared with π‐orbital energies calculated by three different approximations: (a) the standard Hückel HMO model; (b) a first‐order perturbation treatment, based on (a), that takes into account bond length changes which follow the ionization process; (c) a SCF π‐electron model of the type proposed by Pople and by Pariser & Parr. In agreement with previous experience it is found that model (b) yields the most satisfactory parametrization of the experimental data.
Decomposition of Aryldiazonium Ions in Homogenous Solutions Part II: Arylations with benzenediazonium ions and p‐nitrobenzenediazonium ions in dimethyl sulfoxide
Authors: Gloor B. , Kaul B. L. , Zollinger H. 
Published: December 2004

AbstractWhen p‐nitrobenzenediazonium tetrafluoroborate is decomposed in the presence of dimethyl sulfoxide/benzene or dimethyl sulfoxide/nitrobenzene systems under nitrogen, the respective biphenyl derivatives are obtained in good yield. With benzenediazonium tetrafluoroborate in the same systems the yields are low. The influence of oxygen, iodine, iodobenzene and N, N‐diphenylhydroxylamine on the products of the reactions were determined. The by‐products, isomer distribution with nitrobenzene as a substrate, and the partial and total rate factors were also determined. From the above data it can be shown that the reaction with p‐nitrobenzenediazonium salt is a homolytic process and that with the unsubstituted benzenediazonium salt is essentially a heterolytic reaction. The problem of the mechanism of the radical forming steps (electron transfer or adduct formation) and the mechanistic relationship to other modes of arylation (aroylperoxide, classical two‐phase Gomberg‐Bachmann reaction, catalysis by transition metal ions, nitrite ions, etc. electron transfer properties of DMSO) are discussed.
The N1s ESCA Spectrum of Protonated 1,8‐Bis‐(Dimethylamino) Naphthalene (“proton sponge”)
Authors: Haselbach E. , Henriksson Anita , Jachimowicz F. , Wirz J. 
Published: December 2004

AbstractThe ESCA spectrum of protonated 1,8‐bis‐(dimethylamino) naphthalene (“proton sponge”) (I) has been recorded in the region of N1s binding energies. The results indicate that IH+ possesses an unsymmetrical NH…︁N hydrogen bridge.
Zur Reaktionsweise von Enaminen mit Cyclopropenonen IV. Einsatz von Ketenacetalen
Authors: Steinfels M. A. , Krapf H. W. , Riedl P. , Sauer J. , Dreiding A. S. 
Published: December 2004

AbstractA new interpretation – based on a reevaluation of the spectroscopic properties of products 16 to 27 – is proposed for the reaction of diphenyl‐cyclopropen‐one 14 and ‐thione 15 with ketene‐A, N‐diacetals 8 to 13 (A  R2N, RO and RS) originally reported by Sauer & Krapf. It is concluded that the previous structural assignments (see the a‐structures), made on the assumption of a prevailing “C,C‐insertion” reaction, must be rcplaced as follows: (1) All the “secondary adducts” are, in fact, derivatives (amides and lactams) of 2,3‐diphenyl‐penta‐2, 4‐dienoic acid and thioacid (structures 16b to 24b); (2) the “isomerization products”, differ from the latter only in the configuration of the α,β‐double bond (structures 25b and 26b); (3) the common “hydrolysisproduct” is α,β‐diphenyl‐γ‐methyl‐γ‐hydroxy‐Δα‐butenolide (27b), The above cyclopropenone‐ketcneacetal reactions represent, therefore, cases of “C, N‐insertion”.This is rationalized with a reaction scheme, in which the “acylide” structure of the “primary adducts” plays a role.
Membranelektrode zur selektiven, potentiometrischen Erfassung organischer Kationen
Authors: Scholer R. , Simon W. 
Published: December 2004

AbstractA liquid ion exchanger membrane electrode responding preferentially to large onium ions is described. Selectivities of the sensor for different cations vary by factors of up to 1014 and the selectivity constants correlate closely with the corresponding exchange constants of the extracation system involved. For most cations the electrode shows theoretical response at activities >10−6M and has a life time of at least 4 months.
Heterotricyclodecane X. 2‐Oxa‐7‐thia‐isotwistan und 2‐Oxa‐7‐thia‐twistan sowie Derivate
Authors: Wigger N. , Ganter C. 
Published: December 2004

AbstractThe synthesis of 2‐oxa‐7‐thia‐isotwistane (11) and 2‐oxa‐7‐thia‐twistane (22) as well as of several of their derivatives is described starting from endo‐2‐hydroxy‐9‐thiabi cyclo[3.3.1]non‐6‐ene (4).The 10O(2)‐isotwistane bromide 13, iodide 14 and tosylate 30 as well as the 10S(7)‐isotwistane iodide 15, tosylate 20, and the 10S(7)‐twistane tosylate 21 were treated under reaction conditions suitable for molecular rearrangements involving the epi‐sulfonium ion i and the oxonium ion g. respectively, by neighbouring group participation.
The Use of Intermediates with Preformed Disulfide Bridge for the Synthesis of Oxytocin and Deamino‐oxytocin
Authors: Mühlemann M. , Titov M. I. , Schwyzer R. , Rudinger J. 
Published: December 2004

AbstractThe disulfide‐bridged hexapeptides 6a and 6b have been prepared from benzyl‐protected intermediates. Coupling of 6a and 6b with prolyl‐leucyl‐glycine amide afforded deamino‐oxytocin (7a) and the protected oxytocin derivative 7b, respectively; the latter was converted to oxytocin (7c) by removal of the Boc protecting group.
Thermische Zerfallsreaktionen von Nitroverbindungen I: Dissoziation von Nitromethan
Authors: Glänzer K. , Troe J. 
Published: December 2004

AbstractThe thermal decomposition of CH3NO2 highly diluted in Ar has been studied in shock waves at 900 < T < 1500 K and 1.5 · 10−5 < [Ar] < 3.5 · 10−4 mol/cm3. Concentration profiles of CH3NO2 and NO2 were recorded. The unimolecular reaction was found to be in its fall‐off range. Limiting low pressure rate constants of k0 = [Ar] · 1017.1 exp(−42(kcal/mol)/RT) cm3/ mol sec in the range 900 < T < 1400 K and limiting high pressure rate constants of k∞ = 1016.25 exp (−(58.5 ± 0.5 kcal/mol)/RT) sec−1 have been derived. A rate constant of 1.3 · 1013 cm3/mol sec was found for the first subsequent reaction CH3+NO2 → CH3O+NO.
Die Kristallstrukturen von zwei mononuklearen Peroxotitan (IV)‐dipicolinaten
Authors: Schwarzenbach D. 
Published: December 2004

AbstractThe crystal structures of two mononuclear peroxotitanium(IV) chelates, the triclinic, deep red, pleochroitic diaquoperoxotitanium dipicolinate [TiO2(C7H3O4N) (H2O)2]2H2O, and the monoclinic, orange difluoroperoxotitanium dipicolinate K2[TiO2(C7H3O4N)F2]2H2O, have been determined from X‐ray diffractometer data, and refined to R = 2.7% (3488 reflections) and R = 5.1% (2324 reflections) respectively. Analogous to a yellow‐orange dinuclear peroxotitanium dipicolinate described earlier, the utanium atoms are coordinated approximately pentagonal bipyramidally, with the peroxo group and the chelate ligand occupying the equatorial sites and with H2O or F‐ forming the apices. The OO bond length in the peroxo group is the same in all structures, but there is a very slight variation of the Ti‐peroxide distances apparently connected with the colours of the compounds. The more basic the apical ligands are (H2O → F− → μ‐oxygen), the higher is the frequency of the absorption band, the longer are the Ti‐peroxide distances, and the shorter are the apical bond lengths. Difference Fourier maps based on the final structures agree with this. The red diaquo complex shows highest residual peaks between titanium and the peroxo group, whereas the orange difluoro complex shows them near the apical TiF bonds. The packing of the complexes and hydrogen bonding are discussed. HO distances seem to indicate that the very acidic diaquo complex is near a transitional state towards a hydroxonium salt.
Photochemisches Verhalten von 1‐ und 2‐alkylierten 1, 2‐Dihydronaphthalinen bei tiefen Temperaturen
Authors: Sieber W. , Heimgartner H. , Hansen H.‐J. , Schmid H. 
Published: December 2004

AbstractThe irradiations of 1, 1‐dimethyl‐ (8), 1, 1‐di‐(tri‐deuteriomethyl)‐ (d6–8), 1, 1, 2, 2‐tetramethyl‐ (9) and cis‐ and trans‐1, 2‐dimethyl‐1, 2‐dihydronaphthalenes (cis‐ and trans‐10) were investigated in 2, 2‐dimethylbutane/pentane at −100° using a mercury high‐pressure lamp, and with mercury high‐ and low‐pressure lamps at room temperature. The results were compared with one another, and those of the individual compounds are collected in schemes 2 and 4–7. The most important results are the following:1. The 1, 2‐dihydronaphthalenes undergo a conrotatory ring opening to the o‐quinodimethanes on irradiation with high‐ or low‐pressure lamps at room temperature or at −100°. Thermal reactions ([1, 7a]H‐shifts, electrocyclisations) are suppressed at −100°. The o‐quinodimethanes formed from 8 (scheme 2), 9 (scheme 5) or cis‐10 (scheme 6) undergo on irradiation with the high‐pressure lamp, [1, 5]H‐shifts or photochemical Diels‐Alder reactions after renewed photochemical excitation, to yield the benzobicyclo[3.1.0]hex‐2‐ene derivatives. These Diels‐Alder reactions do not proceed stereospecifically, and therefore are not orbital symmetry controlled reactions.2. If the 1, 2‐dihydronaphthalenes are irradiated at room temperature with either a high‐ or a low‐pressure lamp, then the initially formed o‐quinodimethanes undergo thermal [1, 7a]H‐shifts, in preference to all other reactions, as long as this is sterically possible; the resulting products can undergo secondary photochemical transformations. Such o‐quinodimethanes are formed on irradiation of 8, 9 and cis‐10. From trans‐10, an o‐quinodimethane mixture is formed, of which one component (cis, cis‐22) undergoes thermal [1, 7a] H‐shifts, while the other (trans, trans‐22) suffers a thermal disrotatory electrocyclisation to give cis‐10. If a high‐pressure lamp is used in the last experiment, then the competing photochemical Diels‐Alder cyclisation to bicyclic compounds of the type 23 (scheme 7) can result in the trans, trans‐22.As was shown by Salisbury [3], and confirmed by ourselves in other cases [2], photochemical Diels‐Alder reactions or [1, 5]H‐shifts in the o‐quinodimethanes require light of wavelength ⩾ 400 nm (high‐pressure lamp).The present photochemical investigations amplify and confirm our earlier conclusions concerning the photochemistry of the 1, 2‐dihydronaphthalenes [2].
Hyperfine Shifts of the 13C‐NMR. in low spin iron(III) porphyrin complexes
Authors: Wüthrich Kurt , Baumann Rudolf 
Published: December 2004

AbstractThe 13C‐NMR. in Zn(II)(Porphin), Fe(III)(Porphin)(CN2), Zn(II)(Tetraphenyl‐porphin), and Fe(III)(Tetraphenylporphin)(CN2) have been identified, and the 13C hyperfine shifts in the iron complexes evaluated. It was found that dipole‐dipole coupling with the electron spin localized in the π‐orbitals of the aromatic carbon atoms makes an important contribution, to the 13C hyperfine shifts. In a preliminary analysis the experimental spin density distribution obtained from the combined 1H‐ and 13C‐NMR.‐data is compared with theoretical models of the iron porphyrin complexes.
NMR.‐Study of Nitrogen Inversion and Conformation of 1,5‐Dihydro‐isoalloxazines (‘Reduced Flavin’). Studies in the flavin series, XIX. Communication
Authors: Tauscher Ludwig , Ghisla Sandro , Hemmerich Peter 
Published: December 2004

AbstractThe pyramidal inversion of the N(5)‐centre of several reduced flavins was measured by NMR. The inversion barrier was found to be ∼10 kcal/mol in acetone solutions and to be independent of the size of the N(5) substituent. An increase of the inversion barrier of ∼5 kcal/mol was observed in the case where the N(5) substituent could only be in axial position, and an increase of ∼3.5 kcal/mol was observed for an acyl‐like N(5) substituent. In aqueous solution the inversion barrier increases by ∼3 kcal/mol. The stereochemistry of reduced flavin and its potential relevance in flavin‐dependent biological dehydrogenations is discussed.
Über den Einfluss nichtpolarer Lösungsmittel auf Lage und Intensität von Absorptionsbanden in den Elektronenspektren apolarer Molekeln: I. Modell
Authors: Christen Heinz , Straub Peter A. 
Published: December 2004

AbstractBased on the assumption that the influence of the solvent on the wavelength and intensity of the absorption spectrum of non‐polar molecules is due to Coulombic interaction of the electronic transition moments, three models are presented to calculate the wavelength and intensity changes using perturbation theory. In addition a computer program which allows to calculate solvent effects by simulation of the molecular solvent shell is developped. Using known data from solution spectra of polyenes, the experimental wavelength and intensity changes are compared with the calculated values.
Über die Biogenese des Rifamycins S
Authors: Brufani Mario , Kluepfel Dieter , Lancini Gian Carlo , Leitich Johannes , Mesentsev Albert Sergeevitch , Prelog Vladimir , Schmook Fritz Peter , Sensi Piero 
Published: December 2004

AbstractPropionic acids labelled either with 14C in positions 1, 2, or 3, or with 14C in position 1 and 3H in position 3 have been used as precursors in biosynthesis of rifamycins by Streptomyces mediterranei. The resulting distribution of radioactivity in rifamycins S – as determined by systematic degradation – shows that 23 of the 37 carbon atoms in rifamycin S originate from propionic acid. This result and the distribution pattern of radioactivity are in agreement with those obtained recently by 13C‐NMR. spectroscopy [7].The S‐methyl groups of methionin, labelled with 14C, are incorporated in rifamycins by Streptomyces mediterranei only in the methoxy group.The consequences of these findings for the biogenesis of other ansamycins, e.g. streptovaricins, are discussed. The similarities in the constitution and configuration of ansamycins and macrolides (cf. [9]) indicate that all these microbial metabolites are formed according to the same biogenetic pattern.
Triasteranetrione
Authors: McDonald Ian A. , Dreiding André S. 
Published: December 2004

AbstractEine Synthese von Bicyclo[3.3.1]nonan‐3,7,9‐trion‐9‐(äthylen)acetal (6) durch doppelte Michael‐Addition von Aceton‐dicarbonsäure‐dimethylester (9) an Benzochinonmono(äthylen)acetal (8) und anschliessende Verseifung und Decarboxylierung des intermediären 2,4‐Dicarbomethoxy‐bicyclo[3.3.1]nonan‐3,7,9‐trion‐9‐(äthylen)acetals (10) wird beschrieben. Bromierung des Triketon‐monoacetals 6 mit Brom in Eisessig liefert 2α,4β,6α,8β‐Tetrabrombicyclo [3.3.1]nonan‐3,7,9‐trion‐9‐(äthylen)acetal (12), dessen Konfiguration sich aus dem NMR.‐Spektrum mit Hilfe von Symmetrieargumenten und aus Vergleichen von chemischen Verschiebungen ergibt. Durch Behandlung des rohen Tetrabromids (12) mit Triäthylamin entsteht ein Gemisch von 2,4‐Dibrom‐ (13) und 2,6‐Dibrom‐triasteran‐3,7,9‐trion‐9‐(äthylen)acetal (14), welches sich einerseits mit Säure zu einem 1:2‐Gemisch von 2,4‐Dibrom‐ (15) und 2,6‐Dibrom‐ triasteran‐3,7,9‐trion (16) hydrolysieren und andererseits mit Tri‐n‐butyl‐zinnhydrid zu Triaste‐ran‐3,7,9‐trion‐9‐(äthylen)acetal (17) reduktiv debromieren lässt. Hydrolyse des Triketon‐mono‐acetals (17) oder reduktive Debromierung des Triketodibromid‐Gemisches (15 und 16) führt zum hochschmelzenden, in den meisten Lösungsmitteln wenig löslichen Triasteran‐3,7,9‐trion (4). Die Spektraleigenschaften der erwähnten Substanzen werden diskutiert und zur Bestätigung der Strukturen zugezogen.
Nucleosides and Nucleotides. Part 7. Four dithymidine monophosphates with different anomeric configurations, their synthesis and behaviour towards phosphodiesterases
Authors: Séquin Urs 
Published: December 2004

AbstractThe four isomeric dithymidine monophosphates βTd‐βTd (1), αTd‐βTd (2), βTd‐αTd (3) and αTd‐αTd (4), differing only by the anomeric configurations of the nucleoside units, were prepared from the suitably protected nucleosides and nucleotides. The four dinucleoside monophosphates were tested as substrates of snake venom phosphodiesterase and spleen phosphodiesterase. Both enzymes accept all four compounds as substrates, however, the rate of the hydrolysis is considerably smaller if the enzymic attack takes place at the α‐nucleoside moiety of the dinucleoside monophosphate.
Sur la composition de l'arǒme de the noir V
Authors: Cazenave Paul , Horman Ian 
Published: December 2004

AbstractMethyl anthranilate, geranylacetone and five lactones: γ‐valerolactone, γ‐heptalactone, γ‐décalactone, δ‐décalactone, 5‐hydroxy dec‐7‐enoic acid‐lactone have recently been identified in black tea aroma by means of GC./MS. and by sniffing the effluent from a gas chromatograph. Their intrinsic character, rather fruity and flowery, gives a certain freshness and an agreable note to the aroma.
Synthese von β‐Keto‐amidinen durch Sulfidkontraktion von Acylmethyl‐isothioharnstoffen
Authors: Heffe Wilhelm , Balsiger Rudolf W. , Thoma Klaus 
Published: December 2004

AbstractN‐Substituted β‐ketoamidines are obtained readily by condensing N‐substituted thioureas 6 with α‐halogenoketones 5 and extruding the sulfur from the resulting acylmethylisothiourea 7 with triphenylphosphine (sulfide contraction).
Etude de la réactivité de la fonction carbonyle avec le cétène en présence d'un alcoxyde de titane. 1ère communication. Nouvelle méthode de synthèse de β‐hydroxyesters
Authors: Vuitel Laurent , Jacot‐Guillarmod André 
Published: December 2004

AbstractA new synthesis of β‐hydroxyesters involving a reaction between a carbonyl compound, ketene and an alkyl‐orthotitanate is described.The following carbonyl compounds have been studied: aldehydes, ketones, α‐diketones, α‐ or γ‐ketoesters. A reaction mechanism is proposed.
Etude de la réactivité de la fonction carbonyle avec le cétène en présence d'un alcoxyde de titane. 2e communication. A propos de l'interprétation de quelques spectres RMN de β‐hydroxyesters formés
Authors: Vuitel Laurent , Tabacchi Raffaele , Jacot‐Guillarmod André 
Published: December 2004

AbstractThe NMR. spectra of a serie of β‐hydroxyesters have been studied. It has been found that the methylene protons are magnetically nonequivalent only when the substituents on the center of asymmetry of I or II are very different. The magnetically non‐equivalence of the isopropylmethylprotons arise when the β‐hydroxyesters contain an aromatic or aromatic conjugated group directly bonded to the asymmetric carbon. The interpretation of this finding is proposed.
A Thermodynamic and Kinetic Study on the Stability of the Conformational States of N‐Acetyl‐proline‐methylamide by IR. spectroscopy and chemical relaxation
Authors: Hopmann Rudolf 
Published: December 2004

AbstractN‐Acetyl‐proline‐methylamide (APMA) was synthesized by the mixed anhydride method and investigated by IR. spectroscopy and chemical relaxation measurements. The temperature‐induced variation of the IR. absorption bands of the internally hydrogen bonded (b) and of the extended, unbonded (e) species at 3330 and 3450 cm−1 respectively, were used to evaluate the molar absorptivities, a(b) = 280 and a(e) = 50 l/mol · cm, the equilibrium constant K = 0.70, and the molar enthalpy of reaction ΔH = − 2280 ± 60 cal/mol. The entropy was estimated to be in the range − 8 to − 9 e.u. The reaction rates of this conformational transition were measured by the chemical dipole field effect. The relaxation time of the rate process is τ = 2.7 · 10−9s, the rate constant for the formation of the hydrogen bond k(b) is 2.2 · 108 s−1, and that for the unfolding accompanied by the breakage of the amide hydrogen bond k(e) is 1.5 · 108s−1.