Showing papers on "Conformational isomerism published in 1991"

Conformational changes in proteins probed by hydrogen‐exchange electrospray‐ionization mass spectrometry

Abstract: Hydrogen-exchange electrospray-ionization mass spectrometry is demonstrated to be an effective new method for probing conformational changes of proteins in solutions. The method is based on the mass spectrometric measurement of the extent of hydrogen/deuterium exchange that occurs in different protein conformers over defined periods of time. Results are presented in which hydrogen-exchange electrospray-ionization mass spectrometry is used to probe conformational changes in bovine ubiquitin induced by the addition of methanol to aqueous acidic solutions of the protein.

The NMR structure of cyclosporin A bound to cyclophilin in aqueous solution.

Abstract: Cyclosporin A bound to the presumed receptor protein cyclophilin was studied in aqueous solution at pH 6.0 by nuclear magnetic resonance spectroscopy using uniform {sup 15}N- or {sup 13}C-labeling of cyclosporin A and heteronuclear spectral editing techniques. With an input of 108 intramolecular NOEs and four vicinal {sup 3}J{sub HN{alpha}} coupling constants, the three-dimensional structure of cyclosporin A bound to cyclophilin was calculated with the distance geometry program DISMAN, and the structures resulting from 181 converged calculations were energy refined with the program FANTOM. A group of 120 conformers was selected on the basis of the residual constraint violations and energy criteria to represent the solution structure. The average of the pairwise root-mean-square distances calculated for the backbone atoms of the 120 structures was 0.58 {angstrom}. The structure represents a novel conformation of cyclosporin A, for which the backbone conformation is significantly different from the previously reported structures in single crystals and in chloroform solution. The structure has all peptide bonds in the trans form, contains no elements of regular secondary structure and no intramolecular hydrogen bonds, and exposes nearly all polar groups to its environment. The root-mean-square distance between the backbone atoms of the crystal structure of cyclosporin Amore » and the mean of the 120 conformers representing the NMR structure of cyclosporin A bound to cyclophilin is 2.5 {angstrom}.« less

Natural Bond Orbital Analysis of Internal Rotation Barriers and Related Phenomena

Abstract: The origin of barriers to internal rotation in ethane-like molecules has been investigated with ab initio molecular orbital theory and natural bond orbital (NBO) analysis. The “hyperconjugative” (vicinal σ → σ*, n → σ*) nature of these interactions, as previously suggested on the basis of semiempirical INDO studies, is confirmed at higher 4–31G, 6–31G* levels of theory. Wave functions lacking such hyperconjugative interactions, but satisfying the Pauli principle, are shown to generally exhibit unphysical barrier behavior (i.e., wrong sign), contrary to the implication of the “Pauli-repulsions” model of Sovers et al. Each trans vicinal σ → σ* interaction is found to contribute about 0.7 kcal/mol to stabilization of the staggered conformer. Geminal interactions and Rydberg orbital contributions are found to be relatively unimportant. The consistency of NBO analysis is exhibited for the methyl series CH3-X (X = CH3, NH2, OH, F) and the family of fluoromethanes, to demonstrate the essential continuity that relates internal rotation barrier forces to static and kinetic anomeric effects, gauche effects, anionic hyperconjugation, bond strengthening in polyhalogenated methanes, (e, 2e) spectroscopic evidence of lone pair delocalization, and other stereoelectronic phenomena.

Solvent effect on the anomeric equilibrium in D-glucose: a free energy simulation analysis

Abstract: The calculated free energy difference is the result of near cancelation of two larger, statistically significant contributions, i.e., an intramolecular electrostatic term favoring the α anomer and an intermolecular solute-solvent interaction term favoring the β anomer. This result supports the conjecture that solvation stabilizes the β anomer in water. There is a large difference in the intramolecular contribution to the anomeric equilibrium calculated in solution from the free energy simulation and the gas-phase minimum; this suggests that conformational averaging, modulated by the solvent, is significant even for the internal terms. An examination of the rotamer distribution in the hydroxymethyl side chain shows that the trans, gauche conformer is strongly disfavored in aqueous solution, in accord with experiment

Conformational adaption of poly(ethylene oxide): A carbon-13 NMR study

Abstract: The 13C NMR chemical shift was used as a probe for the average partition of rotational conformers around the C-C bond in the O-CH2-CH2-O segments of polyethylene oxide). Dividing the conformers int ...

Investigation of intramolecular interactions in n-alkanes. Cooperative energy increments associated with GG and GTG' [G = gauche, T = trans] sequences

Abstract: Energies of rotational isomers of n-alkanes are largely determined by the number of individual gauche bonds (G). However, according to molecular mechanics calculations (MM2), within a group of rotamers with equal number of G bonds, there are characteristic energy variations due to cooperative effects involving sequences of several bonds. For example, the energy is increased by inserting a trans bond (T) between two consecutive G bonds of the same sign (e.g., TGGG

Thiocyclosporins: Preparation, Solution and Crystal Structure, and Immunosuppressive Activity

Abstract: The reaction of cyclosporin A (CsA) with Lawesson's reagent under different conditions yields various thiocyclosporins, in which carbonyl O-atoms and/or the hydroxy O-atom of the MeBmt residue are replaced by an S-atom. The position of the S-atom is determined by NMR spectroscopy, and the conformations of the products are studied by NMR spectroscopy and X-ray crystallography. Some of the thiocyclosporins show interesting conformational properties. Whereas one conformation strongly dominates for CsA in CDCL3, two conformers A and B, in a ratio 58:42 are found for [1ψ2, CS–NH]CsA. Extensive NMR studies including new 2D and 3D heteronuclear techniques and restrained MD calculations using ROE effects demonstrate that the major conformer A is identical to CsA, while the minor conformer B contains an additional cis peptide bond between the Sar3 and MeLeu4 residues. [4ψ5, CSNH; 7ψ8, CS–NH]CsA exhibits a conformation very similar to crystalline CsA. However, the D-Ala8NH, MeLeu6Co γ-turn H-bond is not present in this dithio analogue. Also different is the MeBmt1side-chain conformation, the dithio conformation showing a strong MeBmt1OH, Sar3CO H-bond. Immunosuppressive activities of thiocyclosporins are measured in IL-2 and IL-8 reporter gene assays. Their activities are discussed in relation to their conformations.

Liquid-crystalline polyethers based on conformational isomerism. 18. Polyethers based on a combined mesogenic unit containing rigid and flexible groups: 1-(4-hydroxy-4'-biphenyl)-2-(4-hydroxyphenyl)butane

Abstract: Polyethers (TPBX) based on 1-(4-hydroxy-4'-biphenyl)-2-(4-hydroxyphenyl)butane (TPB) and α,ω-dibromoalkanes containing from 4 to 20 methylenic units (X) are prepared by phase-transfer-catalysis polyetherification. TPBX are copolymers containing four different constitutional isomeric structural units derived from the two enantiomers of TPB. The phase behavior of TPBX was explained by analyzing the various trends of the dependences of the glass transition, nematic-isotropic, isotropic-nematic, melting and crystallization temperatures as a function of X.

Raman and infrared spectra, conformational stability, barriers to internal rotation, ab initio calculations and vibrational assignment of dichloroacetyl fluoride

Abstract: The Raman spectra (3200-10 cm−1) of dichloroacetyl fluoride (CHCL2CFO) of the gas, liquid (with qualitative depolarization ratios) and solid were recorded. The infrared spectra (3200-30 cm−1) of the gas and solid were also obtained. All of these data indicate a trans/gauche equilibrium in the gas and liquid whereas only the gauche conformation remains in the solid. A complete vibrational assignment is proposed for both conformers based on the Raman depolarization data, infrared band contours, group frequencies and normal coordinate calculations. From the studies of the Raman spectra at various temperatures, the enthalpy difference between the stable conformers was determined to be 315 ± 68 cm−1 (0.90 ± 0.19 kcal mol−1) (1 kcal = 4.184 kJ) and 180 ± 4 cm−1 (0.51 ± 0.01 kcal mol−1) for the gas and liquid, respectively, with the trans conformer being more stable in the gas but the gauche conformer more stable in the liquid. These experimental results are compared to those obtained from ab initio Hartree-Fock gradient calculations with the RHF/3-21G* and RHF/6-31G* basis sets, and to the corresponding quantities obtained for some similar molecules.

Studies on the molecular recognition of synthetic methyl β‐lactoside analogs by ricin, a cytotoxic plant lectin

Abstract: The binding of methyl β-lactoside and of all possible monodeoxy derivatives of methyl β-lactoside to the galactose-specific highly cytotoxin lectin ricin, has been investigated. The distribution of low-energy conformers of the disaccharide structures has been first determined using molecular-mechanics calculations and high-resolution NMR spectroscopy. The nuclear Overhauser enhancements and specific deshieldings observed are in agreement with a similar distribution of low-energy conformers for all studied compounds which may be described by a major conformer defined by φ (H1′-C1′-O1′-C4) and Ψ (C1′-O1′-C4-H4) torsion angles of 49° and 5°, respectively, with contribution of conformers with angles φ/Ψ 24°/– 59°, 22°/– 32° and 6°/– 44°. Assuming that the disaccharides bind to the lectin in these preferred conformations, the apparent dissociation constants for the ricin-disaccharide complexes have been interpreted in terms of specific polar and nonpolar interactions. In agreement with X-ray data, the hydroxyl groups at positions 3, 4 and 6 of the β-d-galactopyranose moiety appear as key polar groups in the interaction with ricin. These results are in contrast to previous results which have established that position 6 is not involved in lectin binding. An important nonpolar interaction involving position 3 of the β-d-glucopyranose moiety, seems to be operative. The distribution of low-energy conformers of these disaccharide structures permits this interaction to take place with the hydroxyl group at this position intramolecularly bonded, thus rendering this region of the molecule more lipophylic in character for acceptance into nonpolar regions of the combining site.

CD2 rocking modes as quantitative infrared probes of one-, two-, and three-bond conformational disorder in dipalmitoylphosphatidylcholine and dipalmitoylphosphatidylcholine/cholesterol mixtures.

Abstract: The use of CD2 rocking modes in the IR spectrum as quantitative probes of phospholipid conformational disorder has recently been described for aqueous dispersions of 1,2-dipalmitoylphosphatidylcholine (DPPC) and DPPC/cholesterol mixtures [Mendelsohn et al. (1989) Biochemistry 28, 8934-8939; Davies et al. (1990) Biochemistry 29, 4368-4373]. Initial studies focused at the 4, 6, and 10 acyl chain positions of DPPC. In the current work, the method is extended to the 2, 3, 12, and 13 positions. Conformational disorder in the L alpha phase is approximately the same (about 20% gauche) at positions 4, 10, and 13, but an unexpected higher value is observed (about 30%) at the 6 position. Cholesterol (33 mol%) restricts gauche rotamer formation by factors ranging from 6 to 9 at positions 4 and 6, respectively, to 1.5-2 at positions 10, 12, and 13. Quantitative analysis for the DPPC/cholesterol "liquid-ordered" phase indicates the occurrence of 1.2 gauche bonds/chain, a marked reduction from the 3.6-4.2 gauche bonds/chain for DPPC alone. Proximity to the ester moiety at acyl chain position 3 perturbs the vibrational coupling patterns of the CD2 rocking modes and eliminates their sensitivity to conformational change. In addition, the feasibility of a method based on the conformation-dependent coupling between CD2 rocking frequencies of two successive CD2 groups for the quantitative detection of specific, position-dependent king (gtg') and isolated gauche (gtt) conformers is demonstrated. Finally, comparisons between IR measurements and explicit theoretical predictions of acyl chain conformational order are presented.

Differential flexibilities in three branches of an N-linked triantennary glycopeptide.

Abstract: The solution conformation behavior of complex oligosaccharides was studied by resonance energy transfer, as measured by the time-resolved fluorescence method, to determine the conformational heterogeneity of a triantennary glycopeptide at various temperatures. Groups that acted as a fluorescence donor (naphthyl-2-acetyl, Nap) or acceptor (dansylethylenediamine, Dan) were selectively attached to the N terminus of the peptide and a Gal residue [either 6' (shown below), 6, or 8] of the oligosaccharide, respectively. [formula: see text] Time-resolved fluorescence energy-transfer measurements revealed two populations of conformers when Dan was attached to either Gal-6' or Gal-6. One conformer contained the antenna folded back toward the core region, and a second was in an extended conformation. The two conformations differed in donor-acceptor distance by about 10 A. Systematically increasing the temperature from 0 degrees C to 40 degrees C increased the ratio of extended to folded forms 2-fold for the Gal-6 isomer and 4-fold for the Gal-6' isomer, whereas the Gal-8 isomer showed only a single distance population throughout this temperature range. From these data, delta H and delta S for the reversible conformational change were calculated to be 3.1 kcal/mol and 10.8 cal/(mol.K) for the Gal-6 isomer and 7.1 kcal/mol and 25.8 cal/(mol.K) for the Gal-6' isomer. In addition to the structural microheterogeneity commonly associated with glycoproteins, the differential flexibilities of the different branches in the oligosaccharides contribute conformational heterogeneity and should be considered in conformational analysis. The data are discussed in terms of the most probable linkages that contribute to the observed flexibility of the individual triantennary branches, and the biological significance of flexible linkages in complex carbohydrates is considered.

Liquid-crystalline polyethers based on conformational isomerism. 16. Hexagonal columnar phase (.PHI.h) in a nondiscotic copolyether based on 1,2-bis(4-hydroxyphenyl)ethane, 1,8-dibromooctane, and 1,12-dibromododecane, and the novel 2-dimensional-3-dimensional .PHI.h-sB transition

Abstract: Les copolyethers [OPh-CH 2 CH 2 -PhO-(CH 2 ) 8 ] x [OPh-CH 2 CH 2 -PhO-(CH 2 ) 12 ] y obtenus par polycondensation interfaciale presentent par refroidissement a partir de la phase isotrope une mesophase en colonne hexagonale suivie d'une phase smectique B toutes 2 enantiotropes. Cette transition represente un cas presque ideal de transition 2 dimensions-3 dimensions ou l'ordre longitudinal a longue distance est etabli sans affecter significativement l'ordre dans le plan

Benzoannelated centropolyquinanes. Part IX. Synthesis and conformational behavior of fenestrindans (tetrabenzo[5.5.5.5]fenestranes) with four bridgehead substituents

Abstract: All-cis-Tetrabenzo[5.5.5.5]fenestranes (fenestrindans) with four bridgehead substituents (4-10) and two doubly bridged, centrohexacyclic analogues, the bis-endo-peroxide 11 and the bis-disulfide 12, have been synthesized from the tetrabromofenestrindan 3. Pronounced steric interactions between the substituents at opposite bridgehead positions have been revealed by NMR and DNMR spectrometry, showing remarkable size-dependent hindrance of the degenerate interconversion of the two S4 symmetric conformers of the fenestrindans.

Barrier to asymmetric internal rotation, conformational stability, vibrational spectra and assignments, and Ab Initio calculations of n‐butane‐d0, d5 and d10

Abstract: The asymmetric torsional potential function of n-butane has been redefined based on a combination of results obtained from experimental and theoretical calculations. From the vibrational spectra of gaseous n-butane below 500 cm−1 the asymmetric torsional fundamentals of the more stable s-trans and high-energy gauche conformations of n-butane have been assigned at 121.28 and 116.60 cm−1, respectively, each with excited states falling to lower frequencies. Additionally, from variable-temperature studies of the Raman spectra for n-butane-d0 and d10, the conformational enthalpy difference has been determined to be 381 ± 41 cm−1 (1089 ± 117 cal mol−1) and 369 ± 72 cm−1 (1055 ± 206 cal mol−1), respectively. Optimized structural parameters, obtained from ab initio methods, for the s-trans and gauche conformers have been used to allow for structural relaxation during the asymmetric internal rotation and to place constraints on the determined potential function. The resulting potential coefficients are V1 = 584 ± 7, V2 = -96 ± 7, V3 = 1165 ± 3, V4 = 45 ± 3, V5 = -3 ± 2 and V6 = -40 + 1 and the s-trans to gauche, gauche to gauche and gauche to s-trans barriers are determined to be 1274, 1370 and 891 cm−1, respectively, with an enthalpy difference between the conformers of 383 ± 17 cm−1 (1095 ± 49 cal mol−1). This potential function is consistent with a dihedral angle for the gauche conformer of 65.4°. Additionally, this potential has a ‘syn’ barrier, which is the energy difference between the s-trans minimum and the gauche to gauche transition state, of 1753 cm−1 (5.01 kcal mol−1), which is in excellent agreement with the most recent value of 1836 cm−1 (5.25 kcal mol−1) obtained from a very high-level ab initio calculation. In order to provide a more complete description of the normal vibrational modes of n-butane, normal coordinate analyses have been completed for n-butane-d0, -d5 and -d10 based on force constants from ab initio calculations. The infrared (3500-40 cm−1) and Raman (3500-10 cm−1) spectra of n-butane-d5 and -d10 have been obtained and complete vibrational assignments are provided. The structural parameters, conformational stabilities, barriers to internal rotation and vibrational frequencies which have been determined experientally are compared with those obtained by ab initio methods.

Temperature dependence of the rates of conformational changes reported by fluorescein 5'-isothiocyanate modification of H+,K(+)- and Na+,K(+)-ATPases.

Abstract: Stopped-flow fluorometry has been used to measure the forward and reverse rates of the conformational change from E1 to E2 in the fluorescein-modified proton and sodium pumps (1) as a function of Na+ and K+ concentrations to verify the proposed mechanism of ion interaction with the enzymes and (2) as a function of temperature to gain insight into the nature of the conformational transition. (1) The fluorescence changes caused by Na+ and K+ are consistent with rapid competitive binding of the two ions to the E1 conformations of the enzymes followed by rate-limiting transitions between E1K and E2K. (2) Reaction coordinate diagrams for the E1K to E2K transitions in the H,K-ATPase and Na,K-ATPase are qualitatively similar. Enthalpy barriers to reaction are partially compensated by increased entropy in the transition states. However, there are striking quantitative differences between the two enzymes. The E2K to E1K reaction of the H,K-ATPase is more than 2 orders of magnitude faster (tau 1/2 = 6 ms at 22 degrees C) than the reverse rate of the Na,K-ATPase transition (tau 1/2 = 1.6 s), explaining repeated failure to detect a K(+)-"occluded" form of the H,K-enzyme. The E2K conformer of the Na,K-ATPase is 3 orders of magnitude more stable than E1K, while the E1K and E2K conformations of the H,K-ATPase are nearly equivalent energetically.

Optical isomers of rocastine and close analogues: synthesis and H1 antihistaminic activity of its enantiomers and their structural relationship to the classical antihistamines.

Abstract: The enantiomers of 2-[2-(dimethylamino)ethyl]-3,4-dihydro-4-methylpyrido[3,2-f]-1,4- oxazapine-5(2H)-thione (rocastine) and two of its more potent analogues were prepared with an enantiomeric purity of greater than 99.9%. The antihistaminic activity of these compounds was assessed by their ability to block histamine-induced lethality in guinea pigs and to inhibit [3H]mepyramine binding to guinea pig cortex. In this series, compounds having the R configuration at the 2-position are at least 300 times more potent than the S isomers. Conformational analysis and molecular modeling suggest that rocastine can adopt a conformation in which the pyridine ring, ether oxygen, and protonated amine functions are positioned similarly to the corresponding elements of the probable binding conformers of some of the more classical antihistamines. This conformation, boatlike in the oxazepine ring with the side chain quasi-equatorial and folded back toward the ring, is the likely binding conformer at the histamine H1 receptor, and the available structure-activity relationship data is consistent with this interpretation.

The conformational composition of 4-fluoro-1-butene as studied by microwave spectroscopy and ab initio computations

Abstract: The microwave spectrum of 4-fluoro-1-butene has been reinvestigated in the 10-26.5 GHz spectral range. The ground and several vibrationally excited states have been assigned for two «new» rotamers denoted Skew-Gauche I and Skew-Gauche II. The dipole moments of the «new» conformers have been determined

High pressure study on molecular conformational equilibria of n‐pentane

Abstract: The n‐pentane molecule has the basic conformations, TT, TG, and GG. It is important for understanding the conformational behavior of chain molecules. We have studied the effect of pressure on the conformations of n‐pentane in the condensed phase by using high pressure infrared spectroscopy with a diamond anvil cell up to 28.9 kbar. Compression of liquid n‐pentane increases the concentration of the more globular conformers, while the TT conformer only exists as a high pressure solid. The volume changes for TT to TG, TG to GG, and TT to GG forms are −1.0±0.2, −1.1±0.3, and −2.1±0.3 cm3/mol, at 20 °C, respectively. The conformational volume changes in liquid n‐alkanes are discussed in terms of the solvent‐excluded volume.

Lowest energy excited singlet states of isomers of alkyl substituted hexatrienes

Abstract: Vibrationally resolved S0→S1 excitation spectra for the alkyl substituted linear polyenes heptatriene, octatriene, and decatriene seeded into a supersonic He expansion have been measured by resonance enhanced multiphoton ionization spectroscopy. As is the case for the parent compound hexatriene, the lowest energy excited singlet state in all of these molecules is the 2 1Ag state. The measurement of S0→S1 excitation spectra of three of the four double bond isomers of heptatriene gives a detailed picture of the dependence of the electronic structure of the 2 1Ag state on molecular conformation. The three isomers for which spectra are presented have the cis configuration at either the central or the alkyl substituted double bond, or both. For the case of the mono cis species with the cis configuration at the alkyl substituted double bond the spectra show the presence of two single bond conformers. Because of the increased number of distinguishable isomers and conformers for octatriene and decatriene we were ...

High-field nuclear magnetic resonance and molecular dynamics investigations of alditol conformations in aqueous and non-aqueous solvents

Abstract: 3 J coupling constants for five 2H-exchanged alditols in D2O and [2H5]pyridine have been determined at 25 °C after refinement of the fully resolved 1H NMR spectra obtained at 620 MHz. Concurrently, molecular dynamics simulations have been performed of two of the molecules (mannitol and glucitol) in SPC/E water and in an argon-like solvent, by means of which the torsional angles and their flexibilities have been determined and compared to those derived from an analysis of the 3J values with the aid of the Karplus equation. The results are interpreted in terms of different populations of rotamers which are sensitive to stereochemical detail and to the solvent medium. Although there is a reasonable level of agreement between the NMR-based and simulation results, some discrepancies remain and are discussed.

Cyclic retro-inverso dipeptides with two aromatic side chains. II. Conformational analysis.

Abstract: The conformations of cis and trans cyclic retro-inverso dipeptides--2-[(4-hydroxy)benzyl]-5-benzyl-4,6(1H,2H,3H,5H)-pyrimidinedi one (c[mTyr-gPhe]), and 2-benzyl-5-amino-5-[(4-hydroxy)benzyl]-4,6(1H,2H,3H,5H)-pyrimidinedione (c[mTyr-gPhe]), and 2-benzyl-5-amino-5-[(4-hydroxy)benzyl]-4,6(1H,2H,3H,5H)-pyrimidinedione (c[(alpha-amino)mTyr-gPhe])--and the parent cyclic dipeptides--c[tyrosyl-phenylalanine] (cis-c[L-Tyr-L-Phe]) and c[tyrosyl-D-phenylalanine] (trans-c[L-Tyr-D-Phe])--were studied by using 1H-nmr spectroscopy and semiempirical energy calculations. In the cis compounds of all the cyclic retro-inverso and parent dipeptides, the most stable conformer has both aromatic side chains sharing the space over the backbone ring in a "face-to-face" fashion. All the trans compounds predominantly assume a "sandwich" conformation in which the two aromatic rings are folded back over the backbone ring on opposite sides. However, different conformational preferences were observed for the backbones between the retro-inverso and parent cyclic dipeptides. The parent cyclic dipeptide trans-c[L-Tyr-D-Phe] adopts two types of boat structures with different side-chain orientations in almost equal amounts: one with the Tyr side chain in a pseudoaxial position and the Phe side chain in a pseudoequatorial position, the other with the Tyr side chain in a pseudoequatorial position and the Phe side chain in a pseudoaxial position. On the other hand, the cyclic retro-inverso dipeptides trans-c[mPhe-gTyr] and trans c[mTyr-gPhe] assume only one type of boat structure in which the malonyl side chain is in a pseudoequatorial and the gem-diamino side chain is in a pseudoaxial position. In addition to the preferred conformations, the conformational energies of the C alpha--C beta bonds in the malonyl and gem-diamino residues were estimated from the temperature variation of vicinal 1H--1H coupling constants for the H--C alpha--C beta--H groupings observed for the trans isomers of cyclic retro-inverso dipeptides. The energies were evaluated to be 1.1 and 1.8 kcal mol-1 for the malonyl and gem-diamino residues, respectively. Applying these energies to the parent cyclic dipeptide trans-c[L-Tyr-D-Phe], the observed fractions of three side-chain conformations are reasonably reproduced. The conformational energies as well as conformational properties of the molecules estimated in this investigation may be useful to refine force constants for both parent and retro-inverso peptides with aromatic side chains.