A theoretical analysis of the conformation of Ac-L-Ala-L-Pro-L-Ala-NHMe with intramolecular hydrogen bonds

Summary 1. The optimum conformations of Ac-L-Ala-L-Pro-L-Ala-NHMe with eight possible systems of intramolecular hydrogen bonds have been calculated by the method of theoretical conformational analysis.2. The structures found serve as canonical forms for the study of the conformational states of sections of peptide-protein systems with proline residues.3. It has been shown that the interaction of adjacent residues in the structure of the compound mentioned play the dominating role.M. M. Shemyakin Institute of Bioorganic Chemistry, Moscow. Translated from Khimiya Prirodynykh Soedinenii, No. 2, pp. 233–238, March–April, 1976.     

Theoretical conformational analysis of the methylamides of N-acetyl-L-alanyl-L-proline and N-acetyl-L-prolyl-L-alanine. I

Summary 1. The most preferred forms of the compounds Ac-L-Ala-L-Pro-NHMe participating in a conformational equilibrium are characterized by both the trans and the cis configurations of the tertiary amide group.2. The formation of intramolecular hydrogen bonds in the conformations of Ac-L-Ala-L-Pro-NHMe realized is unlikely.3. The conformational states of the compounds Ac-L-Pro-L-Ala-NHMe with the trans configurations of the tertiary amide group are most probable. In nonpolar media a probability of the formation of structures with hydrogen bonds is high.4. The conformational distribution of the fragments -X-Pro- and -Pro-X- in proteins agrees satisfactorily with the calculated values of the energy of the optimum forms of the corresponding dipeptides, Ac-L-Ala-L-Pro-NHMe and Ac-L-Pro-L-Ala-NHMe.M. M. Shemyakin Institute of Bioorganic Chemistry, Academy of Sciences of the USSR. Translated from Khimiya Prirodnykh Soedinenii, No. 5, pp. 618–626, September–October, 1975.     

Theoretical conformational analysis of tripeptides with a glycine residue

Summary 1. The conformational states of glycine residues — R, B, L, P — in the low-energy forms of the tripeptides (I–III) are found with equal frequency.2. The conformations of the main chain in sections of protein with glycine residues correspond to the preferred forms of the tripeptide fragments (I–III).3. The close-, medium-, and long-range interactions in proteins do not contradict one another and exist at the favorable forms of the main chain.M. M. Shemyakin Institute of Bioorganic Chemistry, Academy of Sciences of the USSR. Translated from Khimiya Prirodnykh Soedinenii, No. 5, pp. 631–637, September–October, 1975.     

Conformation of the rings and relative stabilities of isomers of trans,trans- and trans,cis-eudesm-6,12-olides

An investigation has been made by the method of molecular mechanics of a number of isomers of trans,trans- and trans,cis-eudesm-6,12-olides. Questions of the energetic advantageousness of particular isomers, the influence of the orientation of the substituents in the mechanism on their conformation, and the relative stability of the conformers with different methods of linking the rings are considered.A. M. Nesmeyanov Institute of Organometallic Compounds, USSR Academy of Sciences, Moscow. Translated from Khimiya Prirodnykh Soedinenii, No. 2, pp. 193–197, March–April, 1990.     

Vibrational spectra and conformations of chloromethyldimethyl- and tris(chloromethyl)phosphine oxides

A study was carried out on the IR spectra of (CH₃)₂P(O)CH₂Cl and (CH₂Cl)₂PO. The frequencies and forms of the normal modes were calculated. The molecular mechanics method was used to calculate the energy of the various conformations of these molecules. (CH₃)₂P(O)CH₂Cl in the liquid and solution is a mixture of trans and gauche conformations, while (CH₂Cl)₃PO is a mixture of trans,gauche,gauche, trans,gauche,gauche, and gauche,gauche,gauche conformations.S. M. Kirov Kazan Chemical Engineering Institute and A. E. Arbuzov Institute of Organic and Physical Chemistry, Kazan Branch, Academy of Sciences of the USSR. Translated from Zhurnal Strukturnoi Khimii, Vol. 30, No. 6, pp. 38–46, November–December, 1989.     

A study of the conformational states of cyclopeptide systems V. NMR spectra of cyclohexapeptides constructed from alanine and glycine residues, the spin-spin coupling constants of the NH-CH protons, and the "pleated-sheet" structure

Summary 1. On the basis of an analysis of the experimental and calculated data on peptide systems containing IMHBs of the 4 1 type, it has been shown that positions 2 and 5 in the preferred conformations of the cyclohexapeptides must be characterized by low values of the³J_NH-CH constants (1–5 Hz), and positions 3 and 6 by large values of these constants (7–10.5 Hz).2. The positions of the IMHBs in cyclohexapeptides including L-alanine and glycine residues have been refined, and the signals of the NH groups in the NMR spectra have been assigned to the individual amino acid residues.3. In the dominating conformations of the cyclohexapeptides, the lateral methyl groups of the alanine residues generally occupy the pseudoaxial orientation in positions 1, 3, 4, and 6 and the pseudoequatorial orientation in positions 2 and 5.For the preceding communication, see [1].M. M. Shemyakin Institute of the Chemistry of Natural Compounds of the Academy of Sciences of the USSR. Translated from Khimiya Prirodnykh Soedinenii, No. 3, pp. 339–346, May–June, 1971.     

Study of N-aldonoyl amino acids by IR spectroscopy

Conclusions IR spectroscopy is suggested as a convenient method for determining the difference in the cyclic (dioxomorpholine) and acyclic forms of N-aldonoyl amino acid derivatives. It has been shown that in the acyclic forms, unlike the cyclic, there are strong absorption bands at 1570 cm^–1 and 690–720 cm^–1 corresponding to the amide II and V vibrations.M. M. Shemyakin Institute of the Chemistry of Natural Compounds of the Academy of Sciences of the USSR. Translated from Khimiya Prirodnykh Soedinenii, No. 3, pp. 381–384, May–June, 1974.     

Asymmetric nitrogen. 57. Investigation of the stereochemistry of aziridinecarboxylic acid derivatives by NMR

Conclusions The preferred trans configuration of 1,2-disubstituted aziridines and cis configuration of NH-aziridine-2-carboxylic esters were established by means of the¹H and¹³C NMR spectra and the two-dimensional NOESY and COSY-45 spectra.For previous communications see [1].Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 5, pp. 1110–1121, May, 1988.     

An investigation of the conformational states of the methylamide of N-acetyl-L-histidine

Summary 1. All the conformational states of the methylamide of N-acetyl-L-histidine have been calculated.2. It has been shown that in the case of the tautomer of the imidazole ring with a hydrogen atom on the atom, form R of the main chain is preferred, and in the tautomer with the proton on and also in the protonated state of the side chain B forms are preferred.3. The conformational equilibrium is displaced in the direction of form I.M. M. Shemyakin Institute of Biochemistry, Academy of Sciences of the USSR, Moscow. Translated from Khimiya Prirodnykh Soedinenii, No. 3, pp. 371–378, May–June, 1976.     

1H and13C NMR spectra,stereoisomerism, and conformational states of 3-phenyl-5-isopropoxytetrahydro-2-furanones

The¹H and¹³C NMR spectra of the diastereomers of 3-phenyl-5-isopropoxytetrahydro-2-furanone were obtained. The stereoisomerism and preferred conformational states of these diastereomers were established by means of the spectroscopic criteria found for 3,4-disubstituted sulfolanes and 1,1,3,4-tetrasubstituted silacyclopentanes. It is shown that for the cis isomer the equilibrium is shifted to favor pseudoequatorial conformers, whereas for the trans isomer it shifted to favor ae conformers with a preferred pseudoequatorial orientation of phenyl substituent.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 593–597, May, 1988.     

Photoisomerization of sterically hindered polymethine dyes

Conclusions In sterically hindered polymethine dyes the photoisomerization process, with the formation of sterically hindered trans and di-cis forms, successfully competes with the nonradiative degradation of the energy on the higher vibrational sublevels of the ground state.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1926–1930, August, 1977.     

Stereochemistry of heterocycles

Twenty-eight previously unreported compounds of the 4,5-dialkyl-, 4,5,5-trialkyl-, 2,2,4-trimethyl-5-alkyl, and 2,2,4-trimethyl-5,5-dialkyl-1,3-dioxane series, including 24 individual cis and trans isomers, obtained with a purity of the order of 85–99% by fractionation of the starting mixtures of stereoisomers with total condensation columns, were synthesized. The applicability of the Cram rule, modified for compounds with complexforming groups, for the estimation of the possible configurations and conformations of the stereoisomers of the examined series was demonstrated. The cis configuration of the lowboiling stereoisomers and the trans configuration of the high-boiling stereoisomers were proved by NMR spectroscopy; the investigated compounds exist predominantly in the chair conformation. The 5-C-axial position is preferred over the 4-C-axial conformation in the examined series of compounds. The synthesis and hydrolysis of the examined 1,3-dioxanes proceed without involvement of the steric centers of the molecules, and the configurations of the corresponding diastereomeric 1,3-diols can be established on the basis of data on the configurations of the 1,3-dioxanes. The IR spectra of the stereoisomers of the examined series of compounds were studied, and the spectral characteristics of the trans and cis isomers, respectively, were found.See [30] for communication XII.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 7, pp. 893–904, July, 1971.     

Conformational analysis of the complexes of copper(ii) with two identical α-amino acid ligands,relative stability of the cis and trans isomers

Conclusions The energy closeness of the cis and trans isomers of the (Ala)₂Cu complexes, and consequently the possibility of the presence of the cis isomers in the equilibrium mixture, was shown. The differences in the energies of the cis and trans isomers increase in the complexes with the N-mono- and N,N-dimethyl-substituted alanine. The coexistence of both isomers is thereby possible for the monosubstituted compounds, but the disubstituted cis isomers are virtually unrealized. According to the conformational calculations, the formation of the optically active compounds containing ligands of one configurational series is more preferred among the complexes considered.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, pp. 2484–2487, November, 1987.     

Study of acidity of various zeolites by thermal desorption of ammonia

Using the method of thermal desorption of ammonia, we have studied the acidity of hydrogen forms of a number of zeolites: type Y, ultrahigh silica TsVK, synthetic and natural mordenite (M). The hydrogen forms of the TsVK and M zeolites have similar acidity spectra. But for HTsVK, the overall concentration of acidic centers is sufficiently lower than for HM, including weak centers (2.5–3 times) and strong centers (1.5 times). The concentration and strength distribution of acidic centers are very close in samples of synthetic and natural mordenite.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 25, No. 2, pp. 244–247, March–April, 1989.     

Isomerism of isatin-α-aroylhydrazones

The reaction of isatin O-methyl ether with hydrazides of aromatic acids gives the cis and trans forms of -aroylhydrazones.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1050–1052, August, 1971.     

Conformational stability,barriers to internal rotation,structural parameters,ab initio calculations,and vibrational assignment of cyclopropanecarboxaldehyde

The asymmetric torsional potential function, conformational energy difference, vibrational frequencies, and structural parameters of Cyclopropane-carboxaldehyde have been obtained from ab initio calculations at the 3–21G and/or 6-31G^* baiss set levels. These results have allowed for a reinterpretation or clarification of some of the corresponding results obtained from experiment. The conformations that have the oxygen atom oriented cis and trans to the three-membered ring are observed and calculated to be the most stable and high energy forms in the gaseous phase, respectively. From the ab initio calculations using the 6–31 G^* basis set, the energy difference between the two conformers is 114 cm^–1. For the liquid, the trans conformer is more stable and is the only rotamer present in the annealed solid. Based on a combination of results obtained from ab initio calculations, microwave spectroscopy, and the electron diffraction technique,r _o structural parameters have been obtained for both conformations.     

Octa-1,3,5,7-tetraene: Ab initio geometries and vibrational spectra of some stable structures

Complete geometry optimizations of trans, Trans, trans, Trans, trans-, gauche, Trans, trans, Trans, trans-, trans, Trans, trans, Cis, trans-, and gauche, Trans, trans, Cis, trans-octa-1,3,5,7-te-traenes were carried out at the RHF/6-31G level. Characteristic changes in the geometry are found in going from the planar conformers of octatetraene to the corresponding higher-energy stable forms. The harmonic force constants were computed for the above conformers at the RHF/6-31G//RHF/6-31G level using analytical second derivatives. The computed force fields of these four molecules were then corrected using empirical scale factors transferred fromtrans-buta-1,3-diene. To account for the vibronic coupling effect, which may be a characteristic of oligoenes, a special scale factor was introduced for the two internal coordinates which correspond to stretching the central C=C double bonds. A complete assignment of the experimental spectra oft, T, t, T, t-octatetraene is also given.On leave from Laboratory of Molecular Spectroscopy, Department of Chemistry, Moscow State University, Moscow 119899, U.S.S.R.Preliminary results were reported at the Thirteenth Austin Symposium on Molecular Structure, Austin, TX, USA March 12–14, 1990, S 5, p. 91. and at the Second World Congress of the Theoretical Organic Chemists, Toronto, Canada July 8–14, 1990, BP-35 (Canada).     

Absorption spectra of 2,3-dihydrothionaphthen-3-one and its derivatives

A number of new thioindigoid dyes have been synthesized. They have been subjected to photochemical isomerization. The absorption spectra of the dyes synthesized have been compared with the absorption spectra of analogous dyes not containing bromine. The spectra of the cis forms are less sensitive to the introduction of a bromine atom than those of the trans forms.For Communication XI, see [1].Translated from Khimiya Geterotsiklicheskikh Soedinenii, Vol. 6, No. 8, pp. 1031–1033, August, 1970.     

Determination of uronic acids in natural materials by the decarboxylation method

Summary A preferred variant of the decarboxylation method for the determination of uronic acids is proposed which uses 57% hydriodic acid and the direct nonaqueous titration of the CO₂ formed. The range of concentrations of uronic acids that can be determined is from 2 to 100 µM. The sensitivity of the method is 1.5 µM, its reproducibility 0.6%, and the time of one determination 40–50 min. The proposed variant of the method is applicable to the study of various polysaccharides and other carbohy drate-containing biopolymers.Pacific Ocean Institute of Bioorganic Chemistry, Far Eastern Scientific Center of the Academy of Sciences of the USSR, Vladivostok. Translated from Khimiya Prirodnykh Soedinenii, No. 1, pp. 10–13, January–February, 1976.     

PMR studies based on lanthanoid shift reagents: Aurone and thioaurone steric structures

The proton resonance spectra have been used along with the interactions with the lanthanoid shift reagent tris-1,1,1,2,2,3,3-heptofluoro-7,7-dimethyloctane-2,6-dione-Eu(III) to establish the isomeric forms and preferred conformations of substituted aurones and thioaurones. Calculations have been performed on the geometrical structures of the LSR-substrate adducts for certain compounds.Translated from Teoreticheskaya i Éxperimental'naya Khimiya, Vol. 21, No. 1, pp. 108–111, January–February, 1985.