Showing papers on "Conformational isomerism published in 1976"
TL;DR: Detailed nuclear magnetic resonance studies of the dinucleoside monophosphates ApA, ApG, GpA, UpU, CpC, UpC, and CpU were undertaken, finding suggestive evidence that the orientation of the 3' phosphate group is coupled to the ribose conformational equilibrium.
Abstract: In order to obtain information about the conformational features in a polyribonucleotide at the nearest neighbor level, detailed nuclear magnetic resonance studies of the dinucleoside monophosphates ApA, ApG, GpA, UpU, CpC, UpC, and CpU were undertaken. Proton spectra were recorded at 100, 220, 270, or 300 MHz for D2O solutions, 0.01-0.03 M, pD 7.4 at 20+/-2 degrees C. Spectra of ApA, ApG, UpU, and UpC were also recorded in the temperature range of 70-90 degrees C. Unambiguous signal assignments of all proton resonances were made with the aid of selectively deuterated dimers. Complete, accurate sets of nuclear magnetic resonance (NMR) parameters were derived for each nucleotidyl unit by simulation-iteration methods. A complete set of chemical shift and coupling constant data was also obtained for all the constituent monomeric units at a concentration and ionization state comparable to that of the dimers. Conformational properties were evaluated quantitatively for most of the bonds in the dinucleoside monophosphates using procedures developed in earlier studies. All of the dimers have a flexible conformational framework in aqueous solution. While flexibility is allowed and alternate conformations are accessible, these molecules nevertheless attempt to achieve conformational identity by showing preferences--sometimes overwhelming preferences--for certain orientations. Thus the ribose rings exist as equilibrium mixtures of C2'-endo in equilibrium C3'-endo conformers with a bias for the C3'-endo pucker in most cases. The C4'-C5' bonds of both nucleotidyl units show significant preference (70-85%) for a gg conformation. Similarly, the dominant conformer (80-90%) about C5'-O5' is g'g'. Even though an unambiguous determination of the orientation about C3'-O3' cannot be made, there is suggestive evidence that the orientation of the 3' phosphate group is coupled to the ribose conformational equilibrium and it is likely that a 3Eg- in equilibrium 2Eg+ equilibrium exists with a bias for the 3Eg- coupled conformation in which the H3'-C3'-O3'-P dihedral angle is about 34-38 degrees. The individual nucleotidyl units in the dimers differ in several key ways from corresponding monomer conformations. Specifically, the ribose equilibrium C2'-endo in equilibrium C3'-endo shifts in favor of C3'-endo upon dimerization, the only exception being UpU. The C4'-C5' and C5'-O5' bonding network in the dimer forms a stable conformational unit and no correlation exists in the dimers between the conformational preference of this fragment and ribose conformer population. The temperature data for the dimers and dimerization data clearly indicate that the transition C2'-endo leads to C3'-endo is directly related to XCN changes brought about by dimerization and stacking...
159 citations
TL;DR: In this article, the authors considered derivatives of triptycene and fluorene whose barriers to rotation were of the order of 30 kcal/mol, and showed that the barrier to rotation is high enough to give rise to rotamers.
Abstract: Rotation about CC single bonds in suitably substituted compounds gives rise to rotational isomers (also called rotamers) which can be isolated if the barrier to rotation is high enough. The present article considers derivatives of triptycene and fluorene whose barriers to rotation are of the order of 30 kcal/mol.
114 citations
TL;DR: In this article, the minimum energy staggered rotamers for the αCH-βCH 2 side-chains of amino acids were estimated from data from model compounds, and the component vicinal coupling constants were derived from the stochastic equation.
81 citations
TL;DR: The i.r. spectra of piperidine (pentamethylene imine) and morpholine (diethyleneimide oxide) as vapours, liquids and as amorphous and crystalline solids at −180°C were recorded between 4000 and 200 cm−1.
71 citations
TL;DR: The 5, 6, 7, 8, 9, and 13-numbered lactams have been studied as simple models for the amide linkage and the free energy of activation for the interconversion of cis- and trans-azacyclononanone is found to be 17 kcal/mol.
Abstract: The 5-, 6-, 7-, 8-, 9-, and 13-numbered lactams have been studied as simple models for the amide linkage. ^(15)N and ^(13)C spectra have been determined in various solvents and assignments made for the majority of the resonance lines. The ^(15)N and ^(13)C carbonyl chemical shifts show no apparent relationship to the amide ring size or to each other. The 9-membered lactam, azacyclononanone, exists as an equilibrium mixture of cis and trans conformers, the relative amounts of which depend on solvent and temperature. Carbon-13 NMR spectra of pure trans-zacyclononanone have been obtained from solutions of the solid at -45 oC in CDCl_3. At -35 oC two trans conformers are seen; at room temperature approximately equal quantities of the cis and trans isomers are present in ethanol, dimethyl sulfoxide, and tetrachloroethylene, whereas the cis conformer predominates in chloroform. Further increases in temperature cause rapid (on the NMR time scale) equilibrium between the two isomers such that a single averaged spectrum is seen at 120 oC. From the coalescence temperature of the carbonyl peaks the free energy of activation for the interconversion of cis- and trans-azacyclononanone is found to be 17 kcal/mol. This value is discussed in terms of the twofold periodic potential function for the distortion of amides from a coplanar conformation.
69 citations
TL;DR: In this paper, a detailed ab-initio investigation of the rotational surface of methyl vinyl ether shows that two stable isomers exist, the more "crowded" cisoid-staggered conformer being the structure of lowest energy.
Abstract: A detailed ab-initio investigation of the rotational surface of methyl vinyl ether shows that two stable isomers exist, the more "crowded" cisoid-staggered conformer being the structure of lowest energy. This conformational preference is in agreement with experimental results and can be understood in terms of a nonbonded attractive interactions between the methyl group and the double bond. Furthermore, the methyl rotational barriers in the cisoid and transoid confotmations have been computed and their relative magnitudes can be also interpreted in terms of nonbonded interactions. In our continuing studies of nonbonded interactions, we have investigated the conformational properties of the molecule methyl vinyl ether, H3COCH=CH2. This particular molecule was selected because it can serve as a prototype for the study of nonbonded attractive interactions and their influence upon molecular conformation. In this paper, the results of ab-initio calculations are reported and compared with experimental data. The computational results, in agreement with experimental evidence,2 show that the most stable conformation of methyl vinyl ether is the one which a chemist might have dismissed on grounds of being "sterically" unfavorable. Subsequently, it is shown that this conformational preference can be understood in terms of a qualitative MO analysis based upon the ideas developed by one of the author^^-^ and that these ideas are in full accord with the computational data. Ab-Initio Calculations In order to study the conformational properties of methyl vinyl ether two rotational motions have to be investigated (see Figure l ) , Le., rotation around the (21-0 bond (described here in terms of the torsional angle 0) and rotation around the 0 C 2 bond (described in terms of the torsional angle a). The zero values of the two torsional angles correspond to the cisoid-eclipsed ( C E ) conformation. The computations were performed with the GAUSSIAN 70 series of programs6 a t two different levels of sophistication. W e first made a detailed study of these two rotational motions with a STO-3G7 basis set. These computations were carried out for various values of the angle @ (@ = 0, 45, 90, 120, 135, 155, and 1 80') and a t each @ for various values of the angle 0 (6' = 0, 30, 60, and 100'). At this computational level the following bond angles and bond lengths were kept constant (see Figure 1): LHClH = 109.5'; LHC3H = 120'; r(C3-H) = r(C2-H) = 1.07 A; and r(C1-H) = 1.09 A. The remaining parameters were first optimized in the T E conformation, where important nonbonded interactions a re absent, in the following order: (a) L O C ~ C ~ ; (b) L C I O C ~ ; (c) r(C2C3); (d) r ( C 2 0 ) ; (e) LCzC3H; and (f) r ( O C l ) . Furthermore, for the various conformations investigated, the following parameters were reoptimized: (i) L O C ~ C ~ and LCzOCl for all the conformations with 0 = 0 and 60'; (ii) L C ~ O C I for all the conformations with 0 = 30 and 100' because LOC2C3 was found to have, in all cases, almost identical values a t 0 = 0 and 60' (in the computations a t 0 = 30' L C ~ C ~ O was kept a t the value obtained a t 0 = O', while for 0 = 100' a t the value obtained a t 6 = 60'); (iii) r ( C 2 0 ) for the conformations with 0 = 0' (for all the conformations with the same @, r ( C 2 0 ) was kept fixed a t the value obtained a t 6 = 0'). In order to obtain more accurate estimates of the energy differences of interest, we carried out calculations for the conformations listed in Table I using a 4-31G basis s e t 8 In the first stage we used the geometries optimized a t the STO-3G level and these results are shown in Table I. In the second stage we reoptimized at the 4-31G level certain geometrical parameters which, on the basis of the STO-3G computations, appear to affect the energy the most, i.e., r (O-C2) , L O C ~ C ~ , LClOC2, and LH,C10. The corresponding values, together with the total energies, are listed in Table 11. The potential energy curves for rotation about the 0 C 2 bond, determined a t the STO-3G and 4-31G levels, are shown in Figure 2. Here, we report only one curve at the 4-3 1G level, since the relative energies with and without geometry reoptimization are almost identical. The two curves a t the STO-3G and 4-31G levels are very similar and predict that only two stable isomers exist: the C S (@ = 0') and TS (@ = 180') conformations. In both cases the planar C S conformer is the most stable isomer. The experimental studies2 have suggested the existence of two rotamers of methyl vinyl ether with the more stable isomer having a planar cis conformation in accord with the a b initio result. However, there is some disagreement between the theoretical and experimental results about the structure of the less stable rotamer. Electron diffraction dataZC suggest that it has a nonplanar heavy-atom skeleton with @ in the range 80-1 loo , while the present computational results indicate a planar transoid form (@ = 180'). It must be emphasized that this latter result has been obtained after a sophisticated geometry optimization. On the other hand, various assumptions were made in the least-squares refinement of the structural Bernardi, Epiotis et al. / Nonbonded Attraction in Methyl Vinyl Ether
66 citations
TL;DR: It is proposed that destacking occurs primarily by rotation about P-O5' and/or O3'-P, with change of ribose conformation occurring in consequence of an alteration of chiCN, the latter in turn being governed by the need for maximum eta overlap of stacked adenine rings.
Abstract: A detailed 220-MHz NMR study has been made of the conformational properties for the homodinucleotide adenylyl-3' leads to 5'-adenosine, ApA, in D2O. Unambiguous signal assignments of all proton signals were made with the aid of selectively deuterated nucleotidyl units, ApA, ApA, and D-8ApA, and complete, accurate sets of NMR parameters were derived by simulation-iteration methods. Sets of limiting chemical shifts and coupling values were also obtained for ApA and constituent monomers 3'-AMP and 5'-AMP at infinite dilution and at identical ionization states for assessment of dimerization effects. Conformational properties were evaluated quantitatively for most of the conformational bonds of ApA and these are consistent with two compact folded dynamically averaged structures, a base-stacked right helical structure, I, characterized as anti, C3'-endo, g-, w,w' (320,330 degrees), g'g', gg, C3'-endo, anti, and a more loosely base-stacked loop structure, II, with anti, C3'-endo, g-, w,w' (80 degrees, 50 degrees), g'g', gg, C3'-endo, anti orientations. Dimerization produces a number of nucleotidyl conformational changes including a shift in ribose equilibrium C2'-endo (S) in equilibrium C3'-endo (N) in favor of C3'-endo in both Ap- and -pA (60:40 vs. 35:65 in monomers), a change in glycosidic torsion angle chiCN toward 0 degrees, and a greater locking-in of rotamers along bonds involved in the phosphodiester backbone. Moreover, there is clear evidence that the transitions from S leads to N forms and chiCN leads to 0 degrees are directly related to base stacking in ApA. Finally, ApA exists in solution as an equilibrium between I, II and an unstacked form(s) with as yet undetermined conformational features. Since C4'-C5', C5'-O5', and C3'-O3' bonds possess exceptional conformational stabilities, it is proposed that destacking occurs primarily by rotation about P-O5' and/or O3'-P. Predominant factors influencing the overall ApA conformation are thus base-base interaction and flexibility about P-O5' and O3'-P, with change of ribose conformation occurring in consequence of an alteration of chiCN, the latter in turn being governed by the need for maximum eta overlap of stacked adenine rings.
65 citations
TL;DR: In this paper, the chemical shift increments of the aromatic protons on complex formation of C6H5CH-t-Bu2, as a reference for the eclipsed conformation with the substituent staggered with respect to the carbonyl ligands, have been used to estimate the contribution of this conformer to the conformational equilibrium of the other monoalkylsubstituted complexes.
62 citations
TL;DR: In this article, the 13C NMR spectra have been determined of aliphatic compounds having at one end a functionalized sulphur atom (SH, S−, SMe, S(O)Me, SO2Me and S+Me2) and saturated sulphur heterocycles variously substituted at the S-atom.
Abstract: The 13C NMR spectra have been determined of: (i) aliphatic compounds having at one end a functionalized sulphur atom (SH, S−, SMe, S(O)Me, SO2Me and S+Me2) and (ii) saturated sulphur heterocycles variously substituted at the S-atom . The results are discussed in terms of the familiar deshielding effects for α- and β-carbons and shielding effects for γ-carbons, exerted by the sulphur atom itself and/or by the atoms or groups of which the sulphur function is made up. The γ-effect of the S-atom appears to be nearly independent of the nature of the S-function and of comparable magnitude to that of an aliphatic carbon (−2·5 + −3·0 ppm). Surprisingly, however, a SCH3 group shields the carbon in γ position with respect to CH3 by an amount (−5·4 ppm) which is more than twice that (−2·5 ppm) exerted by the aliphatic γ-carbon on the S-CH3 carbon itself. As to the cyclic compounds, the shieldings of the α- and β-carbons can be rationalized in terms of the conformational orientation of the substituent at sulphur, and the equilibrium distribution of the conformers. The results confirm the great value of 13C NMR for configurational and conformational assignment of S-heterocycles.
54 citations
52 citations
TL;DR: In this paper, the 1H NMR spectra of the six acetylated glycals as well as that of tetra-O-acetyl-2-hydroxy-Dglucal have been carried out.
Abstract: The complete analysis of the 1H NMR spectra of the six acetylated glycals as well as that of tetra-O-acetyl-2-hydroxy-D-glucal has been carried out. The conformation of these compounds, as determined from the proton couplings (±0.05Hz) obtained, has been interpreted in terms of a rapid interconversion equilibrium between the two possible dihydropyran half-chair conformations. A computer treatment of all observed couplings has been carried out to obtain optimized values for the populations and couplings characteristic of the two alternative half-chair conformations. The rotamer populations of the acetoxy–methyl group are discussed on the basis of the measured 5J16 and 5J36 couplings.
TL;DR: The 690nm absorption spectrum of CF3NO has been studied in the gas phase at various temperatures and in the condensed phase at 77 K, and assigned to an (nπ*) transition as mentioned in this paper.
Abstract: The 690 nm absorption spectrum of CF3NO has been studied in the gas phase at various temperatures and in the condensed phase at 77 K, and assigned to an (nπ*) transition. Analysis of the vibrational structure shows that, while only the eclipsed conformer is stable in the ground state, there are two stable isomers of comparable energy in the excited electronic state. One has an eclipsed conformation, but with the CF3 group tilted away from the oxygen atom. The other has a staggered conformation. In both of the excited state isomers the barrier to internal rotation is higher than in the ground state.
TL;DR: It is proposed that PV may have a higher affinity for cations than valinomycin because of its higher potential energy in the uncomplexed state.
Abstract: The solution conformation of cyclo-[D-Val-L-Pro-L-Val-D-Pro]3 (PV) and its alkali-metal ion complexes was investigated by proton nuclear magnetic resonance spectroscopy. It is concluded that the cation complexes of PV have S6 symmetry and are essentially isostructural with the K complex of valinomycin. In contrast to valinomycin, the Li- and Na-PV complexes are stable in methanol and have dissociation rate constants that are several orders of magnitude slower than the corresponding valinomycin complexes. Also in contrast to valinomycin, free PV exists in two different conformational states which interconvert at very slow rates (less than 1 s-1). One of these conformers has S6 symmetry and is structurally similar to that of the cation complexes. The other species, which has lower symmetry than S6, is the more stable conformer. Depending upon concentration and solvent polarity, the latter represents between 50 and 75% of the total mixture. It is proposed that PV may have a higher affinity for cations than valinomycin because of its higher potential energy in the uncomplexed state.
TL;DR: The effect of pressure on the equilibrium between conformers of 1,1,2-trichlorocyclohexane has been studied using infrared spectra obtained with the diamond anvil cell.
Abstract: The effect of pressure on the equilibrium between conformers of 1,1,2‐trichlorocyclohexane has been studied using infrared spectra obtained with the diamond anvil cell. Changes in relative intensities of infrared bands show that the population of the more polar conformer (Cs symmetry) increases relative to that of the less polar conformer (C1 symmetry) with increasing pressure. Volume differences for converting the less polar to the more polar conformer are −3.8 cm3/mol in pure liquid and −1.8 cm3/mol in CS2 solution. These results are in agreement with values calculated by K. R. Crook and E. Wyn‐Jones [J. Chem. Phys. 50, 3445 (1969)].
TL;DR: Carbon-phosphorus couplings across three bonds revealed the preferred populations for certain backbone rotamers, demonstrating that significant changes in conformation about the "c(3')-O and C(5') -O bonds do not occur in the temperature or salt-induced unstacking of adenylyl-(3',5')-adenosine.
Abstract: The solution conformation of adenylyl-(3',5')-adenosine and adenylyl-(2',5')-adenosine in both the stacked and unstacked states was studied by carbon-13 magnetic resonance spectroscopy. Large chemical shift differences between the base carbons in the dimers and those in the corresponding monomers are attributed in part to the influence of base-base interaction. Carbon-phosphorus couplings across three bonds revealed the preferred populations for certain backbone rotamers, demonstrating that significant changes in conformation about the "c(3')-O and C(5')-O bonds do not occur in the temperature or salt-induced unstacking of adenylyl-(3',5')-adenosine. However, rotations about the C(2')-O and C(5')-O bonds occur in the temperature-mediated unstacking of adenylyl-(2',5')-adenosine.
TL;DR: In this paper, it was shown that the four planar conformations of enamino-ketones can be predicted with some accuracy from model dihydropyridones by employing the analysis of B atts and S pinner.
TL;DR: The conformational analysis of a number of N-hexopyranosylimidazoles and their tetra-O-acetyl derivatives has been carried out using 1H-n.m.r. data obtained after computer simulation of spectra as mentioned in this paper.
TL;DR: In this paper, the structures and complete force fields of cis and trans nitrous acid have been calculated with a (7, 3) basis set, and the differences between the two stable isomers are reproduced well.
TL;DR: In this paper, the solvent shift method of BELLAMY is re-examined, particularly as a tool for the identification of vibrations that include and are coupled to carbonyl, all of which show solvent sensitivity.
TL;DR: In this article, the stereospecific spin-spin coupling constants over five bonds between the α-proton in the side chain and the protons in the heterocycle in 2-vinylfuran, in its β-nitro and β-aldehydic derivatives, and in 2vinylthiophene are used to demonstrate the preponderance of the s-trans conformers in polar and nonpolar solutions.
Abstract: The stereospecific spin–spin coupling constants over five bonds between the α-proton in the side chain and the protons in the heterocycle in 2-vinylfuran, in its β-nitro and β-aldehydic derivatives, and in 2-vinylthiophene are used to demonstrate the preponderance of the s-trans conformers in polar and nonpolar solutions. These conclusions are compared with predictions made by molecular orbital theory at the STO-3G, INDO, CNDO/2, and MINDO/3 levels. Long-range coupling constants between the protons in the side chain and protons in the heterocycle are calculated by CNDO/2 and INDO–MO–FPT and are compared with experiment. It is concluded that the five-bond couplings involving the α-proton are most sensitive to conformation and that they are transmitted mainly via a σ electron mechanism. The other long-range coupling constants are discussed in terms of σ and π electron mechanisms. The STO-3G calculations yield barriers to internal rotation of greater than 4.8 kcal/mol.
TL;DR: Analysis of the coupling constants for the alpha anomers indicates that the phase angles, conformer populations, and rotational isomers are generally insensitive to dephosphorylation, whereas both the phase angle and conforming populations are strongly dependent on the redox state of the base, alphaNMN being predominantly 2'-endo and alphaNMNH exclusively 2'-exo.
Abstract: The chemical shifts and coupling constants are reported for the proton nuclear magnetic resonance (NMR) spectra of the alpha and beta anomers of the oxidized and reduced pyridine mononucleotides and nucleosides. The pseudorotational conformational analyses of the ribose coupling constants indicate that the ribose conformation for beta-nicotinamide mononucleotide, beta NMN, can best be described by a 3:1 mixture of interconverting 3'-exo (S) and 2'-exo (N) conformers. Reduction of betaNMN to betaNMNH results in phase angles consistent with interconverting 2'-endo (S) and 3'-endo (N) conformers without changes in the conformer populations. Cleavage of the 5'-phosphate from betaNMN has a significant effect on the phase angles (becoming more like those for betaNMNH), conformer population (the N and S conformers become nearly equal), and the distribution of the rotational isomers around the ribose 4'-5' bond to the exocyclic methylene (the gauche-gauche population decreases by about 25%). In contrast, for betaNMNH these parameters are all insensitive to dephosphorylation. The pseudorotational analysis has been extended to define the conformational parameters of alpha nucleotides. Analysis of the coupling constants for the alpha anomers indicates that the phase angles, conformer populations, and rotational isomers are generally insensitive to dephosphorylation, whereas both the phase angle and conformer populations are strongly dependent on the redox state of the base, alphaNMN being predominantly 2'-endo and alphaNMNH exclusively 2'-exo. The rotational isomers around the 4'-5' and 5'- O bonds are found to be insensitive to.the large changes in ribose conformation in the absence of any interaction with the base. The results are discussed in terms of relative contributions from base-ribose, ribose-side chain, and base-side chain interactions to the general conformational restraints imposed by the cis-2'-3'-hydroxyl interaction in beta nucleotides and the additional cis-2'-hydroxyl-base interaction in alpha nucleotides. The significance of these interactions with respect to the enzymatic and nonenzymatic properties of the pyridine nucleotides is also considered.
TL;DR: The photoelectron spectra of certain hydrazines, disulfides, peroxides, and aminophosphines have been assigned to a unique conformer, being present under normal PE spectroscopic conditions.
Abstract: The photoelectron spectra of certain hydrazines, disulfides, peroxides, and aminophosphines have been assigned to a unique conformer, being present under normal PE spectroscopic conditions. In contrast, different rotamers could be detected in the PE spectra of hexahydropyridazines and tetrasubstituted diphosphines and diarsines and were assumed for polysilanes. The composition of the rotameric mixture (transgauche) obtained for tetramethyldiphosphine from the relative PE peak areas had to be revised.
TL;DR: In this article, the authors applied Piekara factor (ΔIµ/E2) measurements together with electric permittivity to the conformational equilibria in the liquid phase of 1,2-dichloroethane and dimethyl carbonate at several temperatures.
Abstract: The method of studying molecular association by the use of Piekara factor (ΔIµ/E2) measurements together with electric permittivity (Iµ) is applied to the conformational equilibria in the liquid phase of 1,2-dichloroethane and dimethyl carbonate at several temperatures. Dipole moments of the gauche conformer in the first molecule as well as the temperature and solvent dependence of the equilibrium constant are in good agreement with other data. Results on dimethyl carbonate indicate the presence of 3 conformers in the liquid; the equilibria are studied by assuming values for the dipole moments.
TL;DR: In this paper, the conformational analysis of the [Co(dien)2]3+ and [Co (linpen)]3+ (dien=H2N(CH2CH2NH)2H, linpen= H2 N(CH 2CH 2NH)5H) complexes has been carried out.
Abstract: The conformational analysis of the [Co(dien)2]3+ and [Co(linpen)]3+ (dien=H2N(CH2CH2NH)2H, linpen= H2N(CH2CH2NH)5H) complexes has been carried out. The former, which can exist in three geometric isomers, was selected as a standard for the determination of several parameters in the strain-energy calculation; they were determined so that the calculated geometries of these isomers were almost the same as those obtained from the crystal structure analysis studies. These results were applied to the strain-energy calculation for the latter, [Co(linpen)]3+, which can exist in four geometric isomers. Further, in three of these isomers, some conformational isomers are possible. From the results of the energy-minimized conformational-analysis calculations of these isomers, it has turned out that the assignment of the structures of the configurational and conformational isomers is possible.
TL;DR: In this paper, a temperature study of the partially deuterated monomeric methanol species CH2DOH(D) and CHD2O(D), isolated in Ar, Kr, Xe, and N2 matrices is reported.
TL;DR: Two peaks separated by ca 2 PPM are observed in the 31 P NMR spectrum of diisopropyl phosphoryl α-chymotrypsin, and the two signals are assigned to two slowly interconverting conformational isomers of the enzyme complex.
TL;DR: In this paper, two structures of opposite helicity are proposed for the two principal types of procyanidin polymer found in Nature, based on the observation that the structure of the interflavan bond is opposite.
Abstract: Conformational isomerism has been demonstrated for the four major naturally occurring procyanidins, and the phenomena have been interpreted in terms of two different forms of restricted rotation about the interflavan bond; based on these observations structures of opposite helicity are proposed for the two principal types of procyanidin polymer found in Nature.