Showing papers on "Conformational isomerism published in 1985"
TL;DR: An evaluation of the potential of nuclear magnetic resonance as a means of determining polypeptide conformation in solution is performed with the aid of a new distance geometry program which is capable of computing complete spatial structures for small proteins from n. m.m.r. data.
362 citations
TL;DR: These analyses indicate: C5=C6 of dT6p- is saturated and the -pd4T base is more aromatic, and the dT 6p- base possesses a configuration of 5R, 6S, which indicates it has anti-type glycosidic conformations.
Abstract: Irradiation of dTpdT with 300 kJ/m2 of 254 nm produces numerous photo-products, one of which labeled dT6pd4T[1] was purified by HPLC. dT6pd4T has a UV spectrum (H20, pH 7) with lambda max = 326 nm and lambda min = 265 nm, and a P-31 NMR resonance at -3.46 ppm (normal dTpdT occurs at -4.01 ppm; TMP, 30 degrees C). 2-D COSY NMR spectra facilitated proton resonance assignments and 2-D NOESY spectra aided analysis of spatial orientation. Carbon-13 and proton-coupled P-31 NMR spectra of dT6pd4T were also obtained. These analyses indicate: C5=C6 of dT6p- is saturated and the -pd4T base is more aromatic; the dT6p- base possesses a configuration of 5R, 6S; dT6p- and -pd4T have anti-type glycosidic conformations; furanose conformation of dT6p- is mainly C3'-endo and that of -pd4T exists in a C3'-endo in equilibrium C3'-exo; exocyclic bonds gamma (C5'-C4'), beta (05'-C5') and epsilon (C3'-03') are non-classical rotamers; dihedral angle about epsilon (C3'-03') is smaller relative to dTpdT.
102 citations
TL;DR: In this paper, the conformational properties of the C5-C6 bond in α- and β(1-6)-linked disaccharides in solution were clarified based on the 1 H-NMR spectra of the (6R)- and (6S)-deuterated derivatives.
88 citations
69 citations
TL;DR: In this paper, the geometry and energy of styrene have been calculated using the 6-31G basis set as a function of the C β C 2 C 1 C 2 dihedral angle.
52 citations
TL;DR: In this article, it has been found that 1 exists, in solution, as an equilibrium mixture of slowly interconverting, structured conformational isomers, 1a and 1b, which correspond to a B-type DNA duplex and a single-stranded, hairpin-loop structure.
Abstract: Nuclear magnetic resonance (NMR) and circular dichroism (CD) studies have been carried out with the oligodeoxyribonucleotide mismatch sequence, d(CGCGATTCGCG), 1. It has been found that 1 exists, in solution, as an equilibrium mixture of slowly interconverting, structured conformational isomers, 1a and 1b. On the basis of the concentration dependence of the 1a-1b equilibrium, the 1H NMR spectrum of the imino protons of the nucleotide bases, and the individual CD spectra of 1a and 1b, it is suggested that the two species correspond to a B-type DNA duplex and a single-stranded, hairpin-loop structure; the portion of the single-stranded species not involved in the loop appears to have a B-type DNA structure (on the basis of the CD measurements). To facilitate 1H NMR resonance assignments, the two possible des-methyl thymidine derivatives of 1 were synthesized; the effect of this substitution on the physical chemical properties of 1 was explored. The 1H NMR spectra of 1, as a function of temperature, showed that, under conditions wherein both species were present to a significant extent, the duplex form melted at a lower temperature than the single-stranded, hairpin loop structure.
51 citations
TL;DR: The cis-and trans-2-amino-4-cyclohexene-1-carboxylic acids 1 and 3 react with imidates to give the condensed-skeleton, bicyclic cis- and trans-pyrimidin 4-ones as discussed by the authors.
47 citations
TL;DR: The application of ab initio molecular orbital theory to the study of molecular conformational analysis is discussed in this paper, where some examples include methyl rotational barriers, internal rotation in 1,2-dihalogenoethanes, cis-trans isomerism in 1-2-dimethylene and conformational preferences in substituted hydrazines.
44 citations
IBM1
TL;DR: In this paper, the structural data for the two rotamers of ground-state HOCO are determined in a series of CI calculations which use a localized orbital representation, based on a partitioning of the Cl matrix according to a well-defined subspaces and type of excitations.
Abstract: Accurate structural data for the two rotamers of ground-state HOCO are determined in a series of CI calculations which use a localized orbital representation. The trans rotamer is found to be 3.3±0.5 kcal/mole lower in energy than the cis. In contrast, the Hatree-Fock approximation does not correctly predict the relative stability of these rotamers, the cis isomer more stable than the trans by 0.15±0.1 kcal/mole. A general method which permits a complete and economical analysis of correlation contributions to any desired molecular property is presented. It is based on a partitioning of the Cl matrix according to a well-defined subspaces and type of excitations. Its application to the energy difference between cis- and trans-HOCO shows that the predicted stability of the trans isomer is due to correlation effects uhich shift charge into the open-shell orbital localized on carbon. Electrons residing in lone-pair orbitals are more mobile than electrons in bonding orbitals. This mobility is enhanced when the unfilled and lone-pair orbitals are oriented trans to one another. It is this enhancement which is responsible for stabilization of trans-HOCO. Dipole moments and a population analysis of the CI wavefunction are also used to assist in the identification of correlation effects which make different contributions in the two isomers. Transition states for the rotational and in-plane barriers for interconversion are examined. The predicted rotational barrier is 10.1 kcal/mol above the trans isomer and 6.7 kcal/mol above the cis. The lowest planar interconversion barrier is estimated 34 kcal/mol above the trans isometric energy.
44 citations
TL;DR: In this paper, the authors showed that local geometry maps (of conformationally dependent structural trends) are as important as Local Energy Maps for the characterization of peptide systems, and the geometrical parameters which should be treated as variables in both empirical energy calculations and, possibly, in the fitting of polypeptide chains in proteins by X-ray methods, are identified.
Abstract: Energy pathways between the α R , β′, C 7 eq and β-regions of the conformational energy surface of N -acetyl N ′-methyl glycine amide were obtained by SCF ab initio calculations on the 4–21G level with gradient geometry optimization at each point. The calculations point to the possibility that no barrier exists at this computational level between α R and β′. The variation of geometry (bond distances and bond angles) with conformation is analyzed in detail and the geometrical parameters which should be treated as variables in both empirical energy calculations and, possibly, in the fitting of polypeptide chains in proteins by X-ray methods, are identified. The study shows that, in general, Local Geometry Maps (of conformationally dependent structural trends) are as important as Local Energy Maps for the characterization of peptide systems.
42 citations
TL;DR: Two conformational states of the coat protein of the filamentous bacteriophage M13 have been detected in detergent solution by using magnetic resonance techniques and appear to be consistent with a more general effect of the overall micelle structure on the conformational state of the protein.
Abstract: Two conformational states of the coat protein of the filamentous bacteriophage M13 have been detected in detergent solution by using magnetic resonance techniques. When 3-fluorotyrosine is incorporated in place of the two tyrosine residues in the protein, four 19F nuclear magnetic resonance signals are observed, two for each conformer of the protein. The equilibrium between the two forms can be modulated by pH, temperature, and detergent structure. The rate of interconversion of the isomers is rapid on the minutes time scale but is slow relative to the T1 relaxation time of the fluorine resonances of approximately 50 ms. The conformational change between the conformers results in the perturbation of a basic residue in the protein such that this group has a pKa of approximately 9.5 in one state which shifts to 10.5 or more in the other conformational state. The temperature dependence of the equilibrium suggests an enthalpy difference of about 10 kcal/mol which is offset by entropy to give nearly zero free energy difference between the states at pH 8.3 in deoxycholate solution at room temperature. This suggests a substantial reorganization of the noncovalent interactions defining the two conformational states. The conformational equilibrium is strongly dependent on detergent structure and the presence of phospholipid in the detergent micelle. The results are not consistent with a strong, specific lipid binding to the protein but appear to be consistent with a more general effect of the overall micelle structure on the conformational state of the protein.
TL;DR: Based on X-ray crystal structure results, a four-state conformational model for the glycosidic bond is introduced in this article consisting of an equilibrium between two syn and two anti conformations.
Abstract: Measurements have been made of 3J(C-6,H-1′) and 3J(C-2,H-1′) magnitudes of pyrimidine nucleosides and nucleotides in solution by natural abundance, 13C n.m.r. spectroscopy and the results have been interpreted in terms of the glycosidic bond conformations of the nucleic acid derivatives. Based on X-ray crystal structure results a four-state conformational model for the glycosidic bond is introduced consisting of an equilibrium between two syn and two anti conformations; each syn or anti conformation is symmetrically related to the C-1′–H-1′ bond direction and appropriate syn and anti conformations are symmetrically related by 180°. It is shown that a quantitative estimate of the glycosidic bond conformation may be determined from the sum of observed coupling constants [i.e., ΣJ=J(C-6,H-1′)+J(C-2,H-1′)] and that the equilibrium composition of syn and anti conformers may be determined from the difference in observed coupling constants, i.e., ΔJ=J(C-6,H-1′)–J(C-2,H-1′). Results for uridine and cytidine derivatives are discussed.
TL;DR: The most plausible source of the solid-state 13 C-shift dispersions of the resonances of C-1 and C-4 is the diversity of conformations about the glycosidic linkage as mentioned in this paper.
TL;DR: Barriers to methyl internal rotation have been investigated through ab initio MO calculations with geometry optimization on toluene, o -xylene, and hexamethylbenzene as discussed by the authors.
Abstract: Barriers to methyl internal rotation have been investigated through ab initio MO calculations with geometry optimization on toluene (STO-3G and 4-31G), o -xylene (STO-3G), and hexamethylbenzene (STO-3G), and compared with experimental results. Toluene is found to prefer a conformation in which one methyl CH bond makes a dihedral angle of 90° with the ring plane. However, the sixfold barrier to internal rotation is only 17 J mol −1 at the 4-31G level. The potential energy surface for coupled methyl group rotation in o -xylene is displayed and the preferred conformer is that known from microwave spectroscopy. Neglect of coupling and the sixfold component are partially responsible for over-estimates of the barrier from experiments. The most stable conformer of hexamethylbenzene has D 3d symmetry with slight puckering of the ring and tilting of the methyl groups to reduce steric crowding. The barrier to geared internal rotation is 2.42 kJ mol −1 .
TL;DR: In this article, a general synthesis of the 7-azabicyclo[2.2.1]heptane system is described, using pressures of up to 1.4 GPa and the properties of Diels-Alder adducts of some N-acylated pyrroles.
Abstract: The syntheses, using pressures of up to 1.4 GPa, and the properties of Diels–Alder adducts of some N-acylated pyrroles are described. Both exo and endo adducts may be obtained and purified and are, in general, unstable with respect to the retro reaction. Progress towards a general synthesis of the 7-azabicyclo[2.2.1]heptane system is described. Structures of two of the adducts, those between N-phenylmaleimide (2b) and N-benzoylpyrrole (1a) and the dihydro adduct of (2b) with N-acetylpyrrole (1b), have been determined by X-ray crystallographic analysis. Examination of these structures by means of 1H n.m.r. temperature-dependent spectra and nuclear Overhauser effect differences confirm that conformational isomerism is due to restricted rotation about the amide C–N bond, the energy barrier, 70 kJ mol–1, being similar to that in acyclic amides.
TL;DR: In this paper, two sets of triplet-state ESR spectra corresponding to the s-cis and s-trans conformers of 2,2′-bipyridine have been observed in mixtures of water with various alcohols at 77K.
Abstract: Two sets of triplet-state ESR spectra corresponding to the s-cis and s-trans conformers of 2,2′-bipyridine have been observed in mixtures of water with various alcohols at 77K. The intensity ratio of the ESR signal for the s-cis conformer to that for the s-trans conformer depends on the composition of the mixture and on the sample-cooling rate. From the analysis of the phosphorescence spectra, the lowest triplet state of the s-cis conformer is estimated to be close to that of the s-trans conformer. An anomalous ESR signal which cannot be ascribed to either the s-cis conformer or the s-trans conformer has been observed in a 1-propanol-water mixture. The geometries of these conformers are discussed from their observed zero-field splitting parameters, together with their potential energy diagrams, as obtained by an ab initio calculation.
TL;DR: An inversion barrier between enantiomorphic saddle conformers of nickel(II)ccccc-octaethyl-pyrrocorphinate in solution was observed by 1H n.m.r. spectroscopy.
Abstract: An inversion barrier between enantiomorphic saddle conformers of nickel(II)ccccc-octaethyl-pyrrocorphinate in solution is observed by 1H n.m.r. spectroscopy.
TL;DR: In this article, a potential function for internal rotation around the CS bond was analyzed to give a potential model for rotation of stable rotamers of ethene thiols, and extensive relaxation of the CCS angle and CS bond length during internal rotation was shown.
TL;DR: In this paper, a semi-empirical conformational analysis of a homologous series of saturated aliphatic n-alkanols with dipalmitoylphosphatidylcholine was performed.
TL;DR: The techniques have now reached the point where it is possible to use them to describe many detailed structural features of small globular proteins in solution and to detect and to describe conformational changes in such proteins.
Abstract: By the use of examples, mainly of rather rigid proteins, we hope to have shown that conformational analysis of proteins is a problem that is not simply related to the conformational analysis of small molecules. The primary difficulties with proteins are (1) the multitude of possible conformers, (2) the complex dynamical behaviour and (3) the degree of co-operativity within the molecules. Any experimentally derived structural description of a protein is an attempt to represent some average of a complex time dependence. N.m.r. techniques have now reached the point where it is possible to use them to describe many detailed structural features of small globular proteins in solution and to detect and to describe conformational changes in such proteins. In addition, analysis is becoming possible of much less ordered regions of polypeptides, such as are found in less compact proteins, of for example myosin, histones and virus coat proteins, or in denatured states. The limits to the detailed conformational analysis of such proteins are likely to be ones of reality rather than method but the description of the properties shown in Table 1 is by its very nature an extremely important problem in conformational analysis of dynamic macromolecules.
TL;DR: In this article, the electronic absorption spectra of 2,2′-bithienyl and a number of its derivatives were investigated and the predominant conformer as well as the polarity of the molecule were predicted from the spectrum.
Abstract: The electronic absorption spectra of 2,2′-bithienyl and a number of its derivatives have been investigated. The predominant conformer as well as the polarity of the molecule were predicted from the spectrum. Molecular orbital calculations were performed on different conformers of 5-formyl-2,2′-bithienyl and the s-trans geometry proved to be the predominant conformer of the molecule. A good agreement is observed between calculated and observed transition energy and intensity.
TL;DR: In this article, a ground state conformational analysis has been carried out on p-, m-, and o-terphenyl using a recently developed semi-empirical calculation method (CS-INDO).
Abstract: A ground state conformational analysis has been carried out on p-, m-, and o-terphenyl using a recently developed semiempirical calculation method (CS-INDO). The potential energy surface has been calculated as a function of the torsion angles of the two terminal phenyls relative to the central one. Two nearly isoenergetic rotamers are found to exist for both p- and m-terphenyl, at geometries corresponding to D2, and C2h symmetries for p-terphenyl, C2, and Cs symmetries for m-terphenyl. The torsion angles are ca. 35° in all cases. The barriers separating the two minima of each potential surface are low enough to allow a rapid interconversion of the rotamers at room temperature in the gas phase (they are in “dynamic” equilibrium). o-Terphenyl presents only one equilibrium geometry. These findings are compared with the results of previous experimental and theoretical studies, which are extensively reviewed.
TL;DR: Using proton NMR, the solution conformation of the A ring of vitamin D3 and its analogs has been studied by application of the Karplus relation to the observed coupling constants, and the A-ring conformations were found to be solvent dependent.
TL;DR: In this paper, the rotational spectrum of CH3CH(OH)CN has been investigated and the potential function and structural relaxation associated with the torsion have been determined from the level spacings and the variations of the rotation.
Abstract: The rotational spectrum of lactonitrile, CH3CH(OH)CN, has been reinvestigated. The analysis has been extended to the CH3CH(OD)CN species and to several excited states of the hydroxyl and methyl internal rotations. The molecule exhibits two conformers with the hydroxyl hydrogen gauche with respect to the cyano group. The conformer A with the hydroxyl hydrogen adjacent to the methinic hydrogen is 118±30 cm−1 (75±30 cm−1 for the OD species) more stable than the conformer B with the hydroxyl hydrogen adjacent to the methyl group. The barrier height to the interconversion is about 400 cm−1. Rotational A–E splittings due to methyl internal rotation have been observed in the first excited state of both conformers, yielding effective V3 barriers of 3.7 kcal/mol for conformer A and 4.3 kcal/mol for conformer B. By using a one‐dimensional flexible model treatment the potential function and the structural relaxation associated with the OH torsion have been determined from the level spacings and the variations of the...
TL;DR: In this article, the interconversion barrier of O,O-syn-rotamers of 2-acylfurans with the following alkyl groups bonded to carbonyl was investigated.
Abstract: Low-temperature 13C n.m.r. spectra allowed the observation of two different groups of signals, corresponding to the O,O-anti- and O,O-syn-rotamers, of a number of 2-acylfurans with the following alkyl groups bonded to carbonyl: Me, Et, Pri, But, pentan-3-yl. The ratios of these rotamers in dimethyl ether were thus obtained, and the barriers for the syn-anti interconversion were found to decrease with increasing bulkiness of the alkyl groups. Although separate 13C signals could not be detected for the rotamers of the corresponding 2-acylthiophenes, line-broadening effects nonetheless allowed measurement of the interconversion barriers. Lanthanide-induced shift experiments carried out on the latter derivatives (at both 1H and 13C frequencies) showed that in all the 2-acylthiophenes investigated the S,O-syn rotamers are predominant.
TL;DR: 1H-Nmr study and force field calculation of conformational Isomerism in (±)-indolactam-V which is a biologically active fragment of tumor promoter, teleocidin B, are described.
TL;DR: Two conformers for the cationic rhodium(I) complexes of the title 14-membered cyclic tetrasulphides have been prepared and the conformational effect upon the nucleophilicity of the Rhodium ion is shown by the oxidative addition reaction of CH2Cl2; {Rh[14]aneS4}Cl (1), in which the Rh atom lies 0.1332(2)A above the plane of four S atoms, forms a dimer through a Rh ⋯ Rh [3.313(1)
Abstract: Two conformers for the cationic rhodium(I) complexes of the title 14-membered cyclic tetrasulphides have been prepared and the conformational effect upon the nucleophilicity of rhodium(I) ion is shown by the oxidative addition reaction of CH2Cl2; {Rh[14]aneS4}Cl (1), in which the Rh atom lies 0.1332(2)A above the plane of four S atoms, forms a dimer through a Rh ⋯ Rh [3.313(1)A] and four Rh ···S [3.697(9)–3.822(3)A] intermolecular non-bonded interactions as established by an X-ray crystal structure determination.
TL;DR: In this article, the potential function for internal rotation of the asymmetric vinyl top has been determined and the following potential constants have been evaluated: V2 = 462 ± 29, V3 = 954 ± 12, V4 = 57 ± 8 and V6 = −58 ± 5cm−1.
TL;DR: In this article, the conformational composition of 1,2-dichloropropane at 26°C was 74% of the anti (A) conformer and 26% of gauche (G −1 with error limits ± 7%).
TL;DR: In this paper, the synthesis and conformational analysis of 2-fluoro, 2-hydroxy and 2-methoxy thioderivatives (thioethers, sulphoxides, sulphones and sulphonium salts) are reported.