Showing papers on "Conformational isomerism published in 1990"
TL;DR: In this paper, the 1D 1 H NOE difference spectra of 50 base and/or sugar-modified nucleosides were measured in (CD 3 ) 2 SO with irradiation of various protons.
Abstract: The 1D 1 H NOE difference spectra of 50 base- and/or sugar-modified nucleosides were measured in (CD 3 ) 2 SO with irradiation of various protons. The resulting NOE data were used for a conformational analysis with respect to their syn-anti conformer equilibrium. Measurement of the NOE spectra of sterically hindered nucleosides implied that both the chemical properties as well as the van der Waals radius of nucleobase substituent are of decisive importance for the conformation around the N-glycosylic bond
146 citations
TL;DR: In this paper, the thermodynamic stability of selected alkylated, dealkylated and rearranged 17α- and 17β-hopane isomers in the C27, C28, C29, C30 and C31 families were calculated using molecular mechanics (MM2) methods.
115 citations
TL;DR: The infrared spectra of aqueous dispersions of a homologous series of symmetric-chain, disaturated phosphatidylcholines, with fatty acyl chain lengths ranging from 12 to 19 carbons, have been measured at comparable reduced temperatures in their liquid-crystalline phases and yield a "per chain" intensity, which is directly related to the concentration of the different types of conformational defects.
Abstract: The infrared spectra of aqueous dispersions of a homologous series of symmetric-chain, disaturated phosphatidylcholines, with fatty acyl chain lengths ranging from 12 to 19 carbons, have been measured at comparable reduced temperatures in their liquid-crystalline phases. The infrared spectra of these compounds contain bands that are dependent on the conformation of the fatty acyl chains. In particular, in the 1400-1300-cm-1 spectral region, there are bands due to CH2 wagging which are specific for the different types of gauche conformers. Thus, gauche-trans-gauche sequences (or kinks) give a band at 1367 cm-1, end-gauche conformers a band at 1341 cm-1, and double-gauche conformers a band at 1355 cm-1. The intensities of these bands were determined and normalized to the intensity of the conformation-insensitive band due to symmetric methyl bending at 1378 cm-1. The intensities of the different "gauche" bands yield a "per chain" intensity, which is directly related to the concentration of the different types of conformational defects. We find that, within experimental error, the concentration of end-gauche and double-gauche conformers is relatively low and practically invariant with chain length when a series of homologous phosphatidylcholines are compared at the same reduced temperature. In contrast, the concentration of gauche-trans-gauche sequences (kink defects) is much higher and increases as the chain length increases. For dipalmitoylphosphatidylcholine we find that there are about 1.2 kink, 0.5-0.6 end-gauche, and 0.4 double-gauche conformers per hydrocarbon chain.(ABSTRACT TRUNCATED AT 250 WORDS)
109 citations
TL;DR: In this article, a conformational study of polythieno(3,4-b)thiophene with various fused fragments was performed to ascertain the relative stability of the aromatic vs quinoid forms as well as the torsional potentials.
Abstract: : Conformations and electronic structures of polymers based on polythiophene with various fused fragments were theoretically investigated. Thioethylenic, thiodimethylenic, and ethylenic fragments were explored, yielding polythieno(3,4-b)thiophene, polythieno(3,4-c)thiophene, and poly(3- thiabicyclo(3.2.0)-1,4,6-heptatriene), respectively. A conformational study, using the method of Partial Retention of Diatomic Differential Overlap, was performed to ascertain the relative stability of the aromatic vs. quinoid forms as well as the torsional potentials of the aromatic forms. The electronic structures of the polymers were obtained through modified extended Huckel band calculations. It was found that the stability of a conformer is mainly governed by the electronic effects associated with a given fragment. The more stable conformer of a polymer has a lower highest occupied crystal orbital level, a larger band gap, and a smaller highest valence band-width. The symmetries and the relative energy levels of the frontier orbitals of a fragment compared to those of a parent polymer play important roles in determining the electronic effects of a fragment and, in turn, the stability of a conformer. It is expected that polythieno(3,4-b)thiophene should be a very promising conducting polymer whose band gap is predicted to be comparable to that of polyacetylene.
78 citations
TL;DR: In this article, the internal coordinates of the disaccharide Man-α(1 → 3)-Man-α-O-Me were relaxed and minimized through an extensive molecular mechanics scheme.
Abstract: We report calculations of potential energy surfaces where all the internal coordinates of the disaccharide Man-α(1 → 3)-Man-α-O-Me were relaxed and minimized through an extensive molecular mechanics scheme. Flexibility within the mannopyranose rings plays a crucial role. Introduction of the relaxed principle into the conformational description of the disaccharide does not greatly alter the overall shape of the low-energy domains but it reveals new local minima. However, its principle effect is the lowering of energy barriers in the potential energy surface. New conformational transitions about the glycosidic bonds appear, permiting pathways among the low energy sections. This occurs with only little variation of the classical 4C1 conformation of the mannopyranose residues. All the conformations observed in the solid state, along with those already predicted through the joint use of NMR and modeling techniques, fall into the populations of stable conformers calculated in the present work. Moreover, a satisfactory agreement is reached between previously observed NOE values, and the theoretical one, calculated from the averaging of more than 500 microstates. The present results reconciliate most of the apparently conflicting data previously reported; they provide strong support for the application of the concept of conformational averaging to solution behavior. Some limitations of the proposed methodology are also discussed.
71 citations
TL;DR: In this paper, the effect of elevated hydrostatic pressure on the secondary structure of poly(L-lysine) was studied using Fourier transform ir spectroscopy, and it was shown that the polypeptidic conformations induced by elevated HOG are reversible, but that an apparent irreversibility can result from kinetic factors in the case of conformational changes involving extensive rearrangements.
Abstract: The effect of elevated hydrostatic pressure on the secondary structure of poly(L-lysine) was studied using Fourier transform ir spectroscopy. According to changes observed in the amide I band, both the β-sheet and the unordered polypeptide undergo a reversible, pressure-induced conformational change to α-helix. The conversion occurs at a much higher pressure from the unordered conformer (∼ 9 kbar) than from the β-sheets (∼ 2 kbar). The structural changes were found to be slower at pH > 11, especially at the highest concentration investigated (10 wt%), reflecting the fact that extensive hydrogen-bond networks have to reorganize. This study shows that alterations of polypeptidic conformations induced by elevated hydrostatic pressure are reversible, but that an apparent irreversibility can result from kinetic factors in the case of conformational changes involving extensive rearrangements. The present results also show that the strength of the hydrogen bonds between the backbone amide groups is not the only factor that determines the closest packing of the polypeptide molecules.
66 citations
TL;DR: In this paper, the authors used 2D NOESY experiments on DPDPE in aqueous versus dimethyl sulfoxide (DMSO) solution, and the observed interproton interactions were utilized for the quantitative calculation of the appropriate interpropton distances.
Abstract: Comparison of 'H and "C NMR parameters for the cyclic, conformationally restricted, 6 opioid receptor selective enkephalin analogue Tyr-D-Pen-Gly-Phe-WPen ((~-Pen~,~-Pen~)enkephalin, DPDPE) in aqueous versus dimethyl sulfoxide (DMSO) solution indicates that this peptide adopts similar conformations in these solvents. This suggestion that the conformation of DPDPE is relatively environment independent allows conclusions regarding the receptor-bound conformation of this peptide to be drawn from studies performed on experimentally convenient DMSO solutions of the peptide, alone. Accordingly, 2D NOESY experiments were conducted on DPDPE in DMSO, and the observed interproton interactions were utilized for the quantitative calculation of the appropriate interproton distances. A commonly encountered limitation, the general inability to stereospecifically assign diastereotopic and enantiotopic hydrogens within the amino acid residues, which results in increased error limits for calculated distances involving such hydrogens, was overcome by the synthesis of stereospecifically deuterated amino acids (2S,3R)-( 3-*H) tyrosine, (2S,3S)-(4,4,4-2H3)penicillamine, (R)-(2-2H)glycine, and (2S,3R)-( 3-*H)phenylalanine and their incorporation into DPDPE. As a result all resonances in the 'H NMR spectrum of DPDPE were assignable, and more stringent interproton distances were calculated from the observed NOE interactions. These interproton distances were employed as distance constraints for distance geometry calculations of conformations consistent with the experimental data. Energy minimization of conformers generated by distance geometry calculations was performed by using the AMBER force field, and the resulting low-energy conformers were reexamined for agreement with distance constraints and other confor- mation-dependent NMR parameters. From these studies a conformer was identified that displayed significantly lower energy than all others found while maintaining good agreement with experimental data. Details of this model conformer and comparisons with recently proposed conformations for DPDPE are discussed. , *
60 citations
TL;DR: In this article, the authors discuss the contribution entropique des motifs flexibles a la transition, i.e., le nombre de methylene dans le motif flexible varient.
Abstract: Les polyethers prepares presentent une mesophase nematique virtuelle; les temperatures de transition virtuelle isotrope-nematique et les parametres thermodynamiques associes des polyethers contenant de 4 a 20 groupes methylene dans le motif flexible varient selon que le nombre de methylene est pair ou impair. Discussion de la contribution entropique des motifs flexibles a la transition
59 citations
TL;DR: In this article, the authors performed high temperature Annealed molecular dynamics simulations (HTAMDS) on the bicyclic 222 cryptand and on model M+/222 cryptates with different representations of M+.
Abstract: We have performed “High Temperature Annealed Molecular Dynamics Simulations” (HTAMDS) on the bicyclic 222 cryptand, and on model M+/222 cryptates with different representations of M+. The analysis of four sets of 500 structures allows assessment of the ability of HTAMDS to: (1) interconvert experimentally known conformers starting from one of them, (2) locate the energy minima, (3) generate new conformers of low energy, and (4) account for the average structure observed on the NMR time scale. In view of the ionophoric behavior of 222, structures are also analyzed in terms of the “in/out” orientation of the binding sites. It is found that simulations on the free molecule, although widely sampling the conformational space, do not give structures adequate for cation inclusion. They generate however the lowest energy structure known experimentally and other new closely related ones. Inclusion of the substrate in the simulation (either as a purely electrostatic “driver,” or as a charged sphere) is required to generate conformations found in several complexes. These results suggest that in the field of drug design, conformation of drugs suitable for binding to a given receptor may not be found when the simulations are performed on the isolated drug or substrate.
59 citations
TL;DR: The ground-state conformation of a rotationally constrained tryptophan derivative, 3-carboxy-1,2,3,4-tetrahydro-2-carboline, W(1), is determined from single-crystal X-ray diffraction, MM2 calculations, and 1 H NMR coupling constants as discussed by the authors.
Abstract: The ground-state conformation of a rotationally constrained tryptophan derivative, 3-carboxy-1,2,3,4-tetrahydro-2-carboline, W(1), is determined from single-crystal X-ray diffraction, MM2 calculations, and 1 H NMR coupling constants. The solid-state structure represents the predominant solution conformation. W(1) populates only two minimum-energy conformations in solution, which correspond to the half-chair forms of cyclohexene. The conformers are distinguished mainly by distance of the carboxylate from the indole ring
58 citations
TL;DR: The infrared spectra for carbon monoxide complexed to hemoglobins were examined in the C-O stretch region and provide qualitative and quantitative information on the structural dynamics, stability, and ligand binding properties of hemoglOBins.
Abstract: The infrared spectra for carbon monoxide complexed to hemoglobins were examined in the C-O stretch region. Deconvolution of the spectra requires four bands and supports the presence of four distinct conformers at the ligand binding site. Most typical hemoglobins exhibit only one predominant conformer for each subunit represented by a band at 1951 cm-1 in contrast to myoglobins, which typically exist in two major conformations. Several hemoglobins with an enlarged heme pocket are shown to shift the C-O frequency into the higher frequency conformer regions. Many factors, including pH, temperature, solvents, and divalent metals, are also shown to be capable of expanding the heme pocket. Only very specific structural changes that can reduce the size of the heme pocket will result in the lower frequency conformers. The weighted averages of the multiple CO vibrational frequencies are linearly related to the single 13CO NMR chemical shift values and to the exponential of fast CO on-rates. Conformer interconversion occurs at a rate greater than 10(4) s-1. The infrared C-O stretch spectra provide qualitative and quantitative information on the structural dynamics, stability, and ligand binding properties of hemoglobins.
TL;DR: A conformational model for the carbohydrate chain, deduced from the NMR data and consistent with hard-sphere exo-anomeric calculations shows that the rotamer population about the C-5--C-6 bond of beta-Man is restricted to the P omega = 180 rotamer, mainly.
Abstract: 1H- and 13C-NMR assignments for the carbohydrate part of the glycopeptide alpha-D-Man-(1----6)-[beta-D-Xyl-(1----2)]-beta-D-Man-(1----4)-beta-D- GlcNAc-(1----4)-[alpha-L-Fuc-(1----3)]-beta-D-GlcNAc-(1----N)-Asn approximately, derived from the proteolytic enzyme bromelain (EC 3.4.22.4), have been obtained using homo- and heteronuclear correlation spectroscopy, two-dimensional homonuclear Hartmann-Hahn and nuclear Overhauser enhancement experiments. A conformational model for the carbohydrate chain, deduced from the NMR data and consistent with hard-sphere exo-anomeric calculations shows that the rotamer population about the C-5--C-6 bond of beta-Man is restricted to the P omega = 180 rotamer, mainly.
TL;DR: TRNOE measurements on me4FGal2 and me4f2dGal2 in the presence of the specific antibody indicate that the pyranose ring pucker of each galactose ring remains unchanged, but rotations about the glycosidic linkage occur upon binding to X24.
Abstract: Transferred nuclear Overhauser enhancement spectroscopy (TRNOE) was used to observe changes in a ligand's conformation upon binding to its specific antibody. The ligands studied were methyl O-beta-D-galactopyranosyl(1----6)-4-deoxy-4-fluoro-beta-D-galactopyra nos ide (me4FGal2) and its selectively deuteriated analogue, methyl O-beta-D-galactopyranosyl(1----6)-4-deoxy-2-deuterio-4-fluoro-beta -D- galactopyranoside (me4F2dGal2). The monoclonal antibody was mouse IgA X24. The solution conformation of the free ligand me4F2dGal2 was inferred from measurements of vicinal 1H-1H coupling constants, long-range 1H-13C coupling constants, and NOE cross-peak intensities. For free ligand, both galactosyl residues adopt a regular chair conformation, but the NMR spectra are incompatible with a single unique conformation of the glycosidic linkage. Analysis of 1H-1H and 1H-13C constants indicates that the major conformer has an extended conformation: phi = -120 degrees; psi = 180 degrees; and omega = 75 degrees. TRNOE measurements on me4FGal2 and me4F2dGal2 in the presence of the specific antibody indicate that the pyranose ring pucker of each galactose ring remains unchanged, but rotations about the glycosidic linkage occur upon binding to X24. Computer calculations indicate that there are two sets of torsion angles that satisfy the observed NMR constraints, namely, phi = -152 +/- 9 degrees; psi = -128 +/- 7 degrees; and omega = -158 +/- 6 degrees; and a conformer with phi = -53 +/- 6 degrees; psi = 154 +/- 10 degrees; and omega = -173 +/- 6 degrees. Neither conformation is similar to any of the observed conformations of the free disaccharide.(ABSTRACT TRUNCATED AT 250 WORDS)
TL;DR: Comprehensive conformational analysis using molecular mechanics calculations has been carried out for the potent and selective dopamine D-1 receptor agonist 7,8-dihydroxy-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine and several analogues, and it is concluded that the most probable receptor-bound conformation in both cases is a chair conformation with an equatorial phenyl ring.
Abstract: Comprehensive conformational analysis using molecular mechanics calculations (MM2(85)) has been carried out for the potent and selective dopamine D-1 receptor agonist 7,8-dihydroxy-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine (1; SKF SCH 23390), and several analogues, including conformationally constrained ones Calculated conformational energies have been related to pharmacological and biochemical data in an attempt to identify the biologically active conformations of 1 and 8 It is concluded that the most probable receptor-bound conformation in both cases is a chair conformation with an equatorial phenyl ring and for 8 an equatorial N-methyl group It is suggested that the orientation of the phenyl ring in the receptor-bound molecule does not deviate in terms of dihedral angles by more than about 30 degrees from the preferred phenyl group rotamer in which the planes of two aromatic rings are essentially orthogonal
TL;DR: In this paper, rotational coherence spectroscopy was used to obtain excited state rotational constants for different conformers of jet-cooled tryptamine, 3-indole acetic acid, and 3indole propionic acid.
TL;DR: Spectroscopic studies of methyl salicylate (MS), salicylamide (SAM), and o-hydroxyacetophenone (OHAP) isolated in 12 K matrices of, variously, SF{sub 6}, Ar, Kr, or Xe are presented in this paper.
Abstract: Spectroscopic studies of methyl salicylate (MS), salicylamide (SAM), and o-hydroxyacetophenone (OHAP) isolated in 12 K matrices of, variously, SF{sub 6}, Ar, Kr, or Xe are presented. Irradiation in the S{sub 1} electronic absorption bands of the normal intramolecularly hydrogen bonded conformers generates matrix-stabilized rotamers. Ground-state photorotamer conformations deduced from infrared spectra are correlated with steady-state electronic absorption, excitation, and emission spectra, as well as with emission lifetime data. Matrix-isolated SAM and OHAP photolyze to yield phosphorescent, nonintramolecularly hydrogen bonded rotamers via photochemically reversible pathways. In contrast, irradiation of MS in SF{sub 6} proceeds via a photochemically irreversible pathway to generate a rotamer with a weak intramolecular hydrogen bond between the phenol hydrogen and the methoxy oxygen of the ester moiety. The MS photorotamer exhibits both UV fluorescence and visible phosphorescence.
TL;DR: In the structure of bovine trypsin, the importance of including a solvation energy term in the search is demonstrated for solvent accessible residues, while molecular mechanics alone is enough to correctly predict the conformation of internal residues.
Abstract: A "knowledge-based" method of predicting the unknown structure of a protein from a homologous known structure using energetics to determine a sidechain conformation is proposed. The method consists of exchanging the residues in the known structure for the sequence of the unknown protein. Then a conformational search with molecular mechanics energy minimization is done on the exchanged residues. The lowest energy conformer is the one picked to be the predicted structure. In the structure of bovine trypsin, the importance of including a solvation energy term in the search is demonstrated for solvent accessible residues, while molecular mechanics alone is enough to correctly predict the conformation of internal residues. The correctness of the model is assessed by a volume error overlap of the predicted structure compared to the crystal structure. Finally, the structure of rat trypsin is predicted from the crystal structure of bovine trypsin. The sequences of these two proteins are 74% identical and all of the significant changes between them are on external residues. Thus, the inclusion of solvation energy in the conformational search is necessary to accurately predict the structure of the exchanged residues.
TL;DR: Measurements of carbonyl absorption frequencies in a number of solvents of the methyl esters of the acyl groups used to make acyl-enzymes have permitted determination of the apparent dielectric constants experienced bycarbonyl groups in the enzyme active centre as well as a discussion of the effects of polarity.
Abstract: I.r. difference spectra are presented for 3-(indol-3-yl)acryloyl-, cinnamoyl-, 3-(5-methylthien-2-yl)acryloyl-, dehydrocinnamoyl- and dihydrocinnamoyl-chymotrypsins at low pH, where the acyl-enzymes are catalytically inactive. At least two absorption bands are seen in each case in the ester carbonyl stretching region of the spectrum. Cinnamoyl-chymotrypsin substituted at the carbonyl carbon atom with 13C was prepared. A difference spectrum in which 13C-substituted acyl-enzyme was subtracted from [12C]acyl-enzyme shows two bands in the ester carbonyl region and thus confirms the assignment of the features to the single ester carbonyl group. The frequencies of the ester carbonyl bands are interpreted in terms of differential hydrogen-bonding. In each case a lower-frequency relatively narrow band is assigned to a productive potentially reactive binding mode in which the carbonyl oxygen atom is inserted in the oxyanion hole of the enzyme active centre. The higher-frequency band, which is broader, is assigned to a non-productive binding mode in each case, where a water molecule bridges from the carbonyl oxygen atom to His-57; this mode is equivalent to the crystallographically determined structure of 3-(indol-3-yl)acryloyl-chymotrypsin, i.e. the Henderson structure. A difference spectrum of dihydrocinnamoyl-chymotrypsin taken at higher pH shows resolution of a feature centred upon 1731 cm-1, which is assigned to a non-bonded conformer in which the carbonyl oxygen atom is not hydrogen-bonded. Perturbation of the protein spectrum in the presence of acyl groups is interpreted in terms of enhanced structural rigidity. It is reported that the ester carbonyl region of the difference spectrum of cinnamoyl-subtilisin is complicated by overlap of features that arise from protein perturbation. Measurements of carbonyl absorption frequencies in a number of solvents of the methyl esters of the acyl groups used to make acyl-enzymes have permitted determination of the apparent dielectric constants experienced by carbonyl groups in the enzyme active centre as well as a discussion of the effects of polarity. The ester carbonyl bond strengths of the various conformations were estimated by using simple harmonic oscillator theory and an empirical relation between the force constants and bond strengths. The fractional bond breaking induced by hydrogen-bonding was used to calculate rate enhancement factors by using absolute reaction rate theory.(ABSTRACT TRUNCATED AT 400 WORDS)
TL;DR: In this article, the 1 : 1 cyclic complexes of water with the three possible conformers of glycine have been determined and the most stable complex involves a doubly hydrogen-bonded arrangement of water and the carboxyl group of the monomer I, which is also the lowest energy conformer of the glycine monomer.
TL;DR: In this paper, the excitation spectra of seven tryptophan derivatives entrained in a supersonic expansion have been recorded using both resonantly enhanced two-photon ionization and laser induced fluorescence.
Abstract: Electronic excitation spectra of seven tryptophan derivatives entrained in a supersonic expansion have been recorded using both resonantly enhanced two‐photon ionization and laser induced fluorescence. Two derivatives, tryptophan amide and tryptophan methyl amide, were found to have substantial low frequency vibrational progressions in their excitation spectra, yet in both compounds this behavior was apparent in only one conformer. Other derivatives did not display as much vibronic activity. Conformers which had vibrational progressions were found to emit in a broad band far to the red of excitation. All other conformers were found to fluoresce most strongly in resonance with excitation. The presence of low frequency vibrational activity and red shifted fluorescence correlates well with the ability of the derivative to form an intramolecular hydrogen bond between the amine and the carboxylic acid. Backbone conformers that contain an intramolecular hydrogen bond are expected to have large dipole moments, which may strongly perturb the electronic structure of the indole chromophore.
TL;DR: In this paper, the conformational behavior of the three macrocyclic octaols (4, 5, 6) obtained by acid-catalysed condensation of resorcinol with heptanal is elucidated for the first time.
Abstract: The conformational behaviour of the three macrocyclic octaols (4)–(6), obtained by acid-catalysed condensation of resorcinol with heptanal, is elucidated for the first time. Two of them, namely the diamond (5a) and the chair (6a) stereoisomers, undergo a ring-inversion conformational process in acetone to give the corresponding crown conformers (5b) and (6b). In DMSO or on addition of acetic acid to an acetone solution of diamond octaol (5), conformer (5a) is favoured. The presence of such equilibria and solvent effects are interpreted as an interplay between the tendency of the phenolic OH groups to form intramolecular hydrogen bonds and the alky chains to assume the endo position, avoiding steric repulsions and allowing self-aggregation.
TL;DR: In this article, the proton and 13C NMR of azulene and dialkylaminohydroxyphenyl squaraines were investigated and the four-membered ring carbon resonances were attributed to the...
Abstract: The proton and 13C NMR of azulene and dialkylaminohydroxyphenyl squaraines have been investigated. The four-membered ring carbon resonances occur in the 180–190 ppm range and are attributed to the ...
TL;DR: In this paper, the chromatographic performance of ramipril, an angiotensin-converting enzyme inhibitor, and its active metabolite, was studied on an octadecyl-bonded silica stationary phase and with aqueous eluents.
TL;DR: According to a VT-NMR study, Paracyclophane (1-D4) exists as a mixture of chair and boat conformers in the ratio of 10: 13 (ΔGo = 1 kcal/mol) at -70°C with an energy barrier for the chair boat inversion of 120 kcal/mol (270 MHz, ca 1% CD2Cl2 solution, Tc = - 15°C) as discussed by the authors.
Abstract: According to a VT-NMR study, [2,2,11,11-D4][33]paracyclophane (1-D4) exists as a mixture of chair and boat conformers in the ratio of 10: 13 (ΔGo = 01 kcal/mol) at -70°C with an energy barrier for the chair-boat inversion of 120 kcal/mol (270 MHz, ca 1% CD2Cl2 solution, Tc = - 15°C) The conformer ratio (chair/boat) is dependent on the concentration of 1-D4; it is 1:1 in ca 1% CD2Cl2 solution but 1:2 in ca 5% CD2Cl2 solution, due to preferential crystallization of the chair conformer
TL;DR: In this paper, the n→3s Rydberg transition in several methyl-substituted cyclohexanes and tetrahydropyrans (THP's) cooled in a free-jet expansion reveals the presence of two distinct molecular species.
Abstract: Examination of the n→3s Rydberg transition in several methyl-substituted cyclohexanes and tetrahydropyrans (THP's) cooled in a free-jet expansion reveals the presence of two distinct molecular species. These have been assigned as pairs of conformational isomers that freeze out in the molecular beam at their room temperature populations as a result of the substantial inversion barrier of the saturated six-membered ring system
TL;DR: A theoretical conformational analysis of nine cyclic tetrapeptides, structurally related to the highly μ‐receptor‐selective dermorphin analogue, was performed and it was concluded that a tilted stacking arrangement of the two aromatic rings may represent a structural requirement for high μ-receptor affinity of the examined cyclic der morphin analogues.
Abstract: A theoretical conformational analysis (molecular mechanics study) of nine cyclic tetrapeptides, structurally related to the highly mu-receptor-selective dermorphin analogue H-Tyr-D-Orn-Phe-Asp-NH2, was performed. These compounds display considerable diversity in their mu-receptor affinity and selectivity. A systematic search and subsequent energy minimization in absence of the exocyclic Tyr1 residue and Phe3 side chain revealed the constrained nature of the 11-13-membered ring structures contained in these analogues. No more than four low-energy conformers (within 2 kcal/mol of the lowest energy conformation) were found in each case. After attachment of the Tyr1 moiety and Phe3 side chain to the "bare" low-energy ring structures, a systematic search and energy minimization of these exocyclic moieties resulted in a limited number of low-energy conformational families for all compounds. Five analogues with high mu-receptor affinity--H-Tyr-D-Orn-Phe-Asp-NH2, H-Tyr-D-Orn-Phe-D-Asp-NH2, H-Tyr-D-Asp-Phe-Orn-NH2, H-Tyr-D-Asp-Phe-A2bu-NH2 (A2 bu: alpha, gamma-diaminobutyric acid) and H-Tyr-D-Cys-Phe-Cys-NH2--all showed a tilted stacking interaction between the Tyr1 and Phe3 aromatic rings in the lowest or second lowest energy conformation found. The same kind of stacking was not possible in low-energy conformers of the four analogues with poor affinity for the mu-receptor [H-Tyr-L-Orn-Phe-Asp-NH2, H-Tyr-D-Orn-D-Phe-Asp-NH2, H-Tyr-D-Orn-Phe(NMe)-Asp-NH2 [Phe(NMe): N alpha-methylphenylalanine], and H-Tyr-D-Orn-Phg-Asp-NH2 (Phg: phenylglycine)].(ABSTRACT TRUNCATED AT 250 WORDS)
TL;DR: In this article, the size of the ionophoric cavities of conformationally isomeric calix[4]arenes was estimated from two-phase solvent extraction of alkali picrates, and they were shown to be in the order, partial-cone-3 > tetrakis(ethoxycarbonylmethoxy)-p-t-butylcalix[ 4]arene (cone) > cone-3.
Abstract: “Cone” and “partial cone” isomers of 25,27-bis(2-pyridylmethoxy)-26,28-bis(ethoxycarbonylmethoxy)-p-t-butylcalix[4]arene (3) were isolated by a TLC method and the structures were identified by 1H NMR. From two-phase solvent extraction of alkali picrates, the size of the ionophoric cavities was estimated to be in the order, partial-cone-3 > tetrakis(ethoxycarbonylmethoxy)-p-t-butylcalix[4]arene (cone) > cone-3. This is the first quantitative estimation of the ionophoricity of conformationally-isomeric calix[4]arenes.
TL;DR: In this article, the effect of side-chain conformation of the carboxamide side chain on the energy and stability of the corresponding conformational states of the enzyme dihydrofolate reductase was investigated.
Abstract: As part of a study of factors controlling biological redox reactions of nicotinamide cofactors [nicotinamide adenine dinucleotide (phosphate) NAD(P)H], we have investigated the effect on a model reaction of the conformational state (cis or trans) of the carboxamide side chain, using quantum chemical methods. The reaction is that for the enzyme dihydrofolate reductase between the NADPH analogue, 1-methyl-dihydronicotinamide, and the protonated forms of the folate and dihydrofolate substrate analogues, pyrazine and dihydropyrazine. Some calculations on pterin and dihydropterin substrate analogues were also carried out in order to gauge the effects of inter-ring coupling. The influence of carboxamide side-chain conformation of nicotinamide on the energetics of the hydride-ion transfer, and on the structures of the transition states and stable intermolecular-interaction complexes, are examined as a function of the orientation of approach of the reactants. These approach geometries include those corresponding to the observed binding of cofactor and either substrate or inhibitor in the enzyme active site. Reactant, product, reactants-complex, and transition-state geometries were optimized at the semiempirical AM1 level, while ab initio SCF/STO-3G and SCF/3-21G single-point calculations were carried out at the AM1 optimized geometries for all species, as well as full geometry optimizations for isolated reactants and products. The results show that reactants-complex and transition-state energies are lower for the trans conformer of dihydronicotinamide than for the cis conformer, due to more favorable H-bonding or electrostatic interactions with the protonated substrate. Also, consideration of the structural parameters, including reaction coordinate bond lengths, ring geometries, and charge distributions, indicate that the trans transition states are more product-like than those for the cis. For the (trans) approaches corresponding to the enzymic orientation for substrate, the intermolecular interaction for the folate reaction lacks the stabilizing influence of the formal H-bond which is present for the dihydrofolate reaction, and consequently the reactants-complex and transition state are less stable.
TL;DR: In this article, the microwave spectrum of CH 2 C(CH 3 )C 35 ClO has been recorded from 12.4 to 39.0 GHz and an R-branch assignment has been made for the s-trans conformer.
TL;DR: In this paper, the mesomorphic phase behavior of ternary copolyethers was characterized by DSC and optical polarized microscopy, and three-dimensional representations of experimental data demonstrated that isomorphic-mesomorphic phase transition temperatures and enthalpy changes of all copolymers are situated on planar surfaces.
Abstract: Ternary copolyethers [MBPE-5/7/9(A/B/C)] were synthesized, and their mesomorphic phase behavior was characterized by DSC and optical polarized microscopy. Three-dimensional representations of experimental data demonstrated that isomorphic-mesomorphic phase transition temperatures and enthalpy changes of all copolymers are situated on planar surfaces that represent the weight-averaged values of the corresponding thermal transitions and enthalpy changes of the MBPE-5, MBPE-7, and MBPE-9 homopolymers