Showing papers on "Conformational isomerism published in 1972"
TL;DR: Analyses of published NMR spectra for uridine, β-pseudouridine and their 3′-monophosphates, together with the deshielding effect of a phosphate group, have made possible specific assignments of the signals of the two 5′ protons, allowing of a more accurate evaluation of the relative populations of the three CH 2 OH conformers.
125 citations
TL;DR: In this article, the 13C resonances of the glycerol and choline groups of dipalmitoyl-lecithin in CD3OD, CDCl3, and D2O have been assigned from the 14N−13C and 31P-13C coupling constants, and proton decoupling studies on the assigned 1H spectra.
Abstract: The 13C resonances of the glycerol and choline groups of dipalmitoyl-lecithin in CD3OD, CDCl3, and D2O have been assigned from the 14N–13C and 31P–13C coupling constants, and proton decoupling studies on the assigned 1H spectra. Well-defined solvent effects are observed, but the chemical shifts for dipalmitoyl-lecithin in bilayers in D2O are very similar to the shifts of the corresponding resonances in biological membranes. From an analysis of the 1H spectrum of the CH2 multiplet in the CH–CH2OCOR fragment, the two vicinal coupling constants were extracted and used to calculate rotamer populations from a Karplus treatment. In a high proportion of the molecules of dipalmitoyl-lecithin in CD3OD and in CDCl3 the fatty acid chains are arranged gauche to each other, suggesting the presence of strong hydrophobic interactions. From an AA′BB′ analysis of the choline –CH2·CH2–fragment in dipalmitoyl-lecithin observed vicinal coupling constants indicate a similar conformation to that found in choline derivatives where an electrostatic interaction between the N+ and the O atoms leads to the conformation being exclusively in the gauche+N/O form.
117 citations
TL;DR: Chain dimensions are calculated for random coil poly rA chains in which stacking is introduced by both noncooperative and cooperative processes and the effects of partially stacked conformations in polyribonucleotides are investigated using the virtual bond treatment.
Abstract: Conformational energies are calculated for pairs of successive bond rotations within an internal residue of a polyribonucleotide chain. Contributions to these energies include bond torsional strain, van der Waals repulsions, London attractions, electrostatic interactions, and inductive interactions between nonbonded atoms in the nucleotide repeat unit. The average dimensions of unperturbed random-coil polyribonucleotide chains are then evaluated on the basis of energies thus estimated, using for this purpose the previously developed virtual bond treatment. The characteristic ratio of the mean-square end-to-end distance calculated for polyribonucleotide chains in which all pentose rings are fixed in a C3′-endo conformation is ≈9; for chains consisting exclusively of C2′-endo units it is ≈25. These values are considerably greater than those obtained by giving equal weight to all conformations judged to be sterically allowed. Satisfactory agreement between the calculations here and experimental values from viscosity and light-scattering studies is achieved by treating the chain as a random copolymer of C3′-endo and C2′-endo conformational isomers. The critical dependence of the characteristic ratio on the rotation about bond C3′–O3′ in the C2′-endo chain, however, obscures the interpretation of chain dimensions. The chain is also treated in higher approximation as a sequence of independent repeat units, each of which consists of six chemical bonds. The characteristic ratio obtained in this manner is 6.5 for the C3′-endo chain and 18.5 for the C2′-endo chain. Finally, the effects of partially stacked conformations in polyribonucleotides are investigated using the virtual bond treatment. Chain dimensions are calculated for random coil poly rA chains in which stacking is introduced by both noncooperative and cooperative processes.
117 citations
TL;DR: In this article, the equilibrium between the cis and trans isomers of furfuraldehyde has been studied by measuring the stereospecific coupling constants between the aldehyde and ring protons in various solvents.
70 citations
69 citations
TL;DR: The electron spin resonance spectra of the NO derivatives of horse and human haemoglobin indicate the existence of two Conformational states, the distribution of which depends on pH.
Abstract: The electron spin resonance spectra of the nitric oxide derivatives of horse and human haemoglobin indicate the existence of two Conformational states, the distribution of which depends on pH. The pK-value of this allosteric transformation is 5.35. This pK is shifted by 0.3 units to 5.65 in the presence of a 50-fold molar excess of 2,3-bisphosphoglycerate. The ESR spectrum of nitrosyl-myoglobin corresponds exactly to that of the alkaline species of nitrosyl-haemoglobin, but no pH dependent change was observed. These findings might suggest a relation between the observed Conformational transition and the acid Bohr effect.
At 77 K the spectrum of the acid as well as that of the alkaline species of NO-haemoglobin indicates rhombic symmetry of the inner molecular electric field. Raising the temperature to 298 K the ESR spectrum of the alkaline species shows a transition to axial symmetry. The spectrum of the acid species exhibits hyperfine structures in the direction of the x- and z-axis, that of the alkaline species in the direction of the z-axis only. Substitution of 14NO by 15NO reveals that these hyperfine lines are caused by an interaction of the unpaired electron with the N-nucleus of the 6th ligand. The hyperfine coupling tensor of the acid species (15N: ΔH3, 3= 23 G; 14 N: ΔH3,3,3= 17 G) is smaller than in case of the alkaline species (15N:Δ H3,3= 32 G; 14N:ΔH3,3,3= 23 G).
65 citations
TL;DR: The potential for application of 13C chemical shift data to conformational analysis in membranes has been investigated and can be interpreted in terms of populations of gauche and trans conformers at various points along a hydrocarbon chain.
59 citations
TL;DR: The most probable conformational arrangements of isomeric bipyrroles, bifurans, bithiophenes, thienylfurans and biselenophenes have been investigated using the Extended Huckel MO approach as discussed by the authors.
50 citations
TL;DR: The vicinal31P-O-C-1H couplings of a series of acyclic phosphate esters have been measured and rationalized in terms of varying populations of individual rotamers as mentioned in this paper.
Abstract: The vicinal31P—O—C—1H couplings of a series of acyclic phosphate esters have been measured and rationalized in terms of varying populations of individual rotamers. Variable temperature measurements have been interpreted in terms of the enthalpy and entropy differences between gauche and trans rotational isomers.
48 citations
TL;DR: The presence of only small ring-current shielding contributions to the Phe aromatic protons from the aromatic ring in Trp can be taken to indicate that the aromatic rings are well separated (>5 A) in the most populated conformers of gastrin pentapeptide in aqueous solution.
Abstract: The proton n.m.r. spectra of Asp, Phe, Asp-Phe-NH2, Met-Asp-Phe-NH2, Trp-Met-Asp-Phe-NH2, and Boc-βAla-Trp-Met-Asp-Phe-NH2 have been studied in aqueous and dimethyl sulphoxide solutions. From an analysis of the 1H spectra of the Asp and Phe C(α)H·C(β)H2 side-chains the vicinal coupling constants were obtained and used to calculate the rotational populations. There are no significant differences in the Phe side-chain rotamer populations between the tripeptide Met-Asp-Phe-NH2 and gastrin pentapeptide βAla-Trp-Met-Asp-Phe-NH2, but the Asp residue shows some small changes.The presence of only small ring-current shielding contributions to the Phe aromatic protons from the aromatic ring in Trp can be taken to indicate that the aromatic rings are well separated (>5 A) in the most populated conformers of gastrin pentapeptide in aqueous solution.For dimethyl sulphoxide solutions, NH doublets could be observed for the peptides studied; the large values of the vicinal JNC coupling constants for Met, Asp, and Phe in the tetrapeptide Trp-Met-Asp-Phe-NH2 are shown to be consistent with the most populated conformations, having a ϕ angle of ca. –95°, from considerations of potential energy diagrams and the Bystrov-modified Karplus curve which relates dihedral angles to vicinal coupling constants.
36 citations
TL;DR: In this paper, the Nuclear Overhauser Effect was used to assign unequivocally the cis and trans configurations of 3,5-dialkyl-1,2,4-trithiolanes.
TL;DR: Satisfactory agreement is achieved between the theoretical values and experimental results on apurinic acid by treating the poly dA chain as a random copolymer of C3′‐endo and C2‐endo conformational isomers present in a ratio of ∼1 to 9.
Abstract: The virtual bond scheme set forth in preceding papers for treating the average properties of polyriboadenylic acid (poly rA) is here applied to the calculation of the unperturbed mean-square end-to-end distance of polydeoxyriboadenylic acid (poly dA). The modifications in structure and in charge distribution resulting from the replacement of the hydroxyl group at C2′ in the ribose residue by hydrogen in deoxyribose produce only minor modifications in the conformational energies associated with the poly dA chain as compared to those found for poly rA. The main difference is manifested in the energy associated with rotations about the C3′–O3′ bond of the deoxyribose residue in the C2′-endo conformation; accessible rotations are confined to the range between 0° and 30° relative to the trans conformation, whereas in the ribose unit the accessible regions comprise two ranges centered at approximately 35° and 85°. The characteristic ratio 〈r2〉0/nl2 calculated on the basis of the conformational energy estimates is ≈9 for the poly dA chain with all deoxyribose residues in the C3′-endo conformation and ≈21 with all residues in the C2′-endo form. Satisfactory agreement is achieved between the theoretical values and experimental results on apurinic acid by treating the poly dA chain as a random copolymer of C3′-endo and C2′-endo conformational isomers present in a ratio of ∼1 to 9.
TL;DR: Nuclear magnetic resonance data suggest that the peptide in methylene chloride solution takes up a conformation containing three cis' Pro C(alpha)-C=O bonds and three cis Gly-Pro peptide bonds, and that water and dimethylsulfoxide stabilize a conformer in which one (or two) sets of such bonds of a given Pro-Gly unit have undergone interconversion to trans'/trans forms.
Abstract: A cyclic hexapeptide, cyclo(Pro-Gly-Pro-Gly-Pro-Gly), has been synthesized; its solution conformations were examined by 220-MHz nuclear magnetic resonance spectroscopy. The solution structures have been deduced, and shown to vary as a function of solvent polarity. In addition, it has been found that this cyclic peptide binds alkali metal cations. While the predominant conformation of this cyclic peptide is 3-fold symmetric in the apolar solvent methylene chloride, an asymmetric structure is preferred in some polar solvents (water, dimethylsulfoxide). However, addition of alkali metal salts, such as sodium thiocyanate, to dimethylsulfoxide solutions of the peptide shifts the conformational equilibrium in favor of a second type of C(3)-symmetric structure, presumably the result of the formation of a stable peptide-metal ion complex. Nuclear magnetic resonance data suggest that the peptide in methylene chloride solution takes up a conformation containing three cis' Pro C(alpha)-C=O bonds and three cis Gly-Pro peptide bonds; that water and dimethylsulfoxide stabilize a conformer in which one (or two) sets of such bonds of a given Pro-Gly unit have undergone interconversion to trans'/trans forms; and that alkali metal cations complex the cyclic peptide in a C(3)-symmetric all-trans'/trans structure.
TL;DR: In this article, it was shown that the tetrakis(pyrazolyl)borate ligand is bidentate, while the cyclopentadienyl group is pentahapto.
TL;DR: In this paper, the electronic structures of methyl vinyl ether (MVE) and of 3-fluoropropene (3FP) were investigated through ab initio SCF calculations using minimal basis sets of AOs contracted from 7 s, 3 p(for C, O and F) and 3 s(for H) Gaussian functions.
Abstract: The electronic structures of methyl vinyl ether (MVE) and of 3-fluoropropene (3FP) are investigated through ab initio SCF calculations using minimal basis sets of AOs contracted from 7 s, 3 p(for C, O and F) and 3 s(for H) Gaussian functions. The predicted structures and dipole moments of the two conformers of 3FP and the computed barrier heights hindering the internal rotation in 3FP agree quite well with experiment. The most stable conformer of MVE is correctly found to be s-cis-staggered, while the second conformer is predicted to be s-trans, which is at variance with a gauche form suggested by electron diffraction studies. A barrier height in the range 5.7–6.6 kcal/mol is found by assuming C—O bond distances ranging from 1.36 to 1.42 A. The conjugation in MVE is studied with reference to 3FP using the Mulliken population analysis and the bond dipole moments calculated over localized MOs. The computed energy maxima between the two stable forms of 3FP and MVE are interpreted in terms of steric and conjugation contributions.
TL;DR: In this article, the preparation and N.M.R. spectra of five title compounds, none of which were substituted at the aryl 2 or 6 positions, are described.
Abstract: The preparation and N.M.R. spectra of five title compounds, none of which were substituted at the aryl 2 or 6 positions, is described. Appreciable (ΔG* c ± 1 kcal/mol) barriers to rotation about the sp2-sp3 carbon-carbon bonds exist in this series and the hydroxyl oxygen atom is in the plane of the aryl ring in the energetically favoured conformers. Two unequally populated low energy conformers were identified in the title compounds carrying different substituents at the 3 and 5 positions of the phenyl ring. Some general aspects of the determination of activation parameters from variable-temperature N.M.R. spectra are discussed.
TL;DR: In this paper, the s-cis ⇌ s-trans equilibrium of enamino ketones and aldehydes was evaluated based on the results of aromatic solvent induced shift measurements and of protonation of the title compounds.
Abstract: The s-cis ⇌ s-trans equilibrium of several enamino ketones and aldehydes,
has been evaluated based on the results of aromatic solvent induced shift measurements and of protonation of the title compounds. In contrast to α,β-unsaturated ketones bearing no heteroatom, the Δδ3,cis value but not the Δδ3,trans, proved to be useful in conformational assignments. Protonation, which occurs mainly on oxygen, enhances the rotational barrier, thus enabling the observation of both rotamers at room temperature. Steric hindrance to conjugation enhances the rate of protonation at the carbon C2.
TL;DR: In this paper, the i.r. spectra of 2-chloro, 2-bromo and 2-iodobutane in the crystalline and glassy states have been obtained.
TL;DR: In this paper, the influence of substituents upon the Amide I and νNH frequencies has been analyzed for ten amides substituted by aliphatic side chains, and the presence of an equilibrium between two conformers is shown and a semiquantitative estimation of the energies involved.
Abstract: The influence of the substituents upon the Amide I and νNH frequencies has been analyzed for ten amides substituted by aliphatic side chains. By considering the aliphatic chain field effect, some data are obtained as to the conformation of the NCα bond. In two amides, the presence of an equilibrium between two conformers is shown and a semiquantitative estimation of the energies involved is obtained. The consequences for polypeptides conformation, especially for poly-L-valine, are discussed.
TL;DR: Vibrational spectra of tetramethoxymethane (TMM), TEM, TMS, and TES have been recorded and assigned as mentioned in this paper, and the S4 form is about 1 kcal/mole more stable than the D2d form of TMM in the liquid.
Abstract: Vibrational spectra of tetramethoxymethane (TMM), tetraethoxymethane (TEM), tetramethoxysilane (TMS) and tetraethoxysilane (TES) have been recorded and assigned. DTA curves and temperature vs. time curves have been recorded in the region −150 to + 30°. No transition points have been observed, melting ranges are −1 to +2 (TMM); −16 to −12 (TEM); +10 to +16 (TMS) and −71 to −68° (TES).
The liquid state of tetramethoxymethane (TMM) contains two rotamers; the skeleton C(OC)4 may have S4 or D2d symmetry. The S4 form is about 1 kcal/mole more stable than the D2d form of TMM in the liquid. Solid TMM consists probably of the D2d configuration only.
There is no distinction between either configuration in tetramethoxysilane (TMS) as the rotation of methoxy groups around their SiO axes is apparently free for practical purposes.
The methyl group of an ethoxy group in X(OC2H5)4 may occupy a gauche or a trans position relative to its XO bond. Rotational isomerism of this kind is probably present in tetraethoxymethane (TEM).
Rotational isomerism is probably absent in tetraethoxysilane (TES).
TL;DR: In this article, the NMR spectra of oriented crystals of toluene, ethylbenzene, cumene, tert -butyl benzene and 3-phenylpentane have been measured and conformers present in the liquid and crystalline state have been determined.
TL;DR: The CD of five-and six-membered sultam derivatives of several l -amino acids and their intermediates has been measured in this article, and the data can be best interpreted by assuming an equilibrium of three conformers ( A, B, and C of Fig 1)
TL;DR: In this paper, a vibrational assignment of conformers for each conformer is presented for each of the conformers, and the fine structure of the OH stretching absorption of the C conformer in matrices displays irreversible changes on temperature cycling.
Abstract: Infra-red spectra are reported of 1,1,1,3,3,3-hexafluoropropan-2-ol (HFP) and its deuterated derivatives in argon, nitrogen, and carbon monoxide matrices at 20 K. Two main conformers (Cs and Cl) are observed in nitrogen and carbon monoxide matrices, as in the vapour phase and carbon tetrachloride solution at room temperature, but only the Cs conformer is found in an argon matrix. The fine structure of the OH stretching absorption of the Cs conformer in matrices displays irreversible changes on temperature cycling, suggesting that there are two species with closely similar structures. A vibrational assignment is presented for each of the conformers.
TL;DR: In this article, the solvent dependence of conformational equilibrium in mono- and di-chloro- and -bromo-cyclohexanes is given and compared with experiment, using a fixed cyclohexane geometry and constant C-X bond dipole moment.
Abstract: The calculation of the solvent dependence of the conformational equilibrium in mono- and di-chloro- and -bromo-cyclohexanes is given and compared with experiment. The calculations are based on an electrostatic theory of solvation and the model uses a fixed cyclohexane geometry and constant C–X bond dipole moment. The model gives generally good agreement with the observed dipole moments of all the chloro- and bromo-cyclohexanes. There is no solvent dependence in cyclohexyl halides but a large solvent dependence in trans-1,2-, cis-1,3-, and trans-1,4-dichloro- and -dibromo-cyclohexanes. The model provides a quantitative explanation of all the observed effects. This agreement allows the prediction of the vapour state energy differences in these systems. The diaxial conformer of trans-1,2-dichloro-(and dibromo-) cyclohexane is the preferred conformer in the vapour phase (ΔEv= 0·7 and 1·3 kcal mol–1 respectively) and this can be rationalised in terms of halogen–halogen repulsions. However, the diaxial conformer of trans-1,4-dichloro-(or dibromo-) cyclohexane is also the preferred conformer in the vapour phase (ΔEv 0·8 and 0·7 kcal mol–1 respectively), and this has no simple rationalisation. The energy differences for benzene as solvent are anomalous in these, as in all other related equilibria, and a method of allowing for this anomaly is given.
TL;DR: In this article, the authors obtained the 100 MHz n.m. spectrum of the title compound partially oriented in the nematic phase and interpreted the values of the direct dipolar couplings in terms of models which take into account the possibilities of internal rotation rates either faster or slower than the molecular reorientation.
Abstract: The 100 MHz n.m.r. spectrum of the title compound partially oriented in the nematic phase has been obtained and interpreted. The values of the direct dipolar couplings have been discussed in terms of models which take into account the possibilities of internal rotation rates either faster or slower than the molecular reorientation in the liquid crystals. It is concluded that the molecule exists in the conformations O-cis–cis(70–80%) and O-cis–trans(20–30%), whilst the proportion of the O-trans-trans rotamer is negligible.
TL;DR: In this paper, the conformations of eleven oxoalkylidenetriphenylphosphoranes have been assigned from their 1H n.m. spectra, rationalised in terms of steric and electrostatic interactions.
Abstract: The conformations of eleven oxoalkylidenetriphenylphosphoranes have been assigned from their 1H n.m.r. spectra. Mixtures of (Z)cisoid and (E)transoid conformers were detected in four compounds. The Z-geometry of β-oxomethylenetriphenylphosphoranes is favoured by the presence of β-substituents and the E-geometry is favoured by the presence of α-substituents. The conformational preferences are rationalised in terms of steric and electrostatic interactions. The absence of peak coalescence in the p.m.r. spectra up to 150° shows that interconversion of the conformers is very slow relative to the n.m.r. time scale.
TL;DR: In this paper, the NMR spectra of succinic and methyl succinic acids have been obtained and analysed, and from the measured vicinal couplings a value for the energy difference between gauche and trans conformers obtained.
TL;DR: In this article, a variable-temperature 1H n.m.n.r. spectra have been recorded for (RSC2H4SR) MX2(R = Me, Et, Prn, Pri, or Bun; M = Pt or Pd; X = Cl, Br, or I).
Abstract: Variable-temperature 1H n.m.r. spectra have been recorded for (RSC2H4SR) MX2(R = Me, Et, Prn, Pri, or Bun; M = Pt or Pd; X = Cl, Br, or I). The pyramidal conformation at sulphur allows the detection of meso and (±)-isomeric forms at low temperatures. At higher temperatures, rapid interconversion of these isomers leads to the observation of ‘averaged’ spectra corresponding to a weighted mean of the parameters of the individual conformers. The interconversion process is inversion at single sulphur atoms. The rate of inversion increases as the trans-influence of X increases, and this appears to be an inductive effect. Inversion in the palladium compounds is more rapid than in the platinum derivatives.
TL;DR: The room-temperature e.s.r. spectrum of the photolytically generated radical anion of thiophen-2,5-dicarbaldehyde shows the existence of two of the three possible rotational isomers as mentioned in this paper.
Abstract: The room-temperature e.s.r. spectrum of the photolytically generated radical anion of thiophen-2,5-dicarbaldehyde shows the existence of two of the three possible rotational isomers. The less stable could be straightforwardly identified as the O-cis–trans because of the non-equivalence of the thiophen proton splittings. Examination of a number of derivatives having different steric requirements allowed the assignment of the O-cis–cis conformation to the other rotamer.
TL;DR: In this paper, extended Huckel type calculations suggest that the bond populations in SS-dimethyl-N-methyl-sulphonylsulphimide (Me2S:N·SO2Me) are independent of the dihedral angle of the N-SVI bond rotation; the barriers of the rotation can be correlated to steric and lone pair effects.
Abstract: Extended Huckel type calculations suggest that the bond populations in SS-dimethyl-N-methyl-sulphonylsulphimide (Me2S:N·SO2Me) are independent of the dihedral angle of the N–SVI bond rotation; the barriers of the rotation can be correlated to steric and lone pair effects. A theoretical interpretation of the experimentally observed interconversion of chiral conformers is given.