Conformational study of peptides containing dehydrophenylalanine: helical structures without hydrogen bond.

The conformational behaviour of deltaZPhe has been investigated in the model dipeptide Ac-deltaZPhe-NHMe and in the model tripeptides Ac-X-deltaZPhe-NHMe with X=Gly,Ala,Val,Leu,Abu,Aib and Phe and is found to be quite different. In the model tripeptides with X=Ala,Val,Leu,Abu,Phe the most stable structure corresponds to phi1=-30 degrees, psi1=120 degrees and phi2=psi2=30 degrees. This structure is stabilized by the hydrogen bond formation between C=O of acetyl group and the NH of the amide group, resulting in the formation of a 10-membered ring but not a 3(10) helical structure. In the peptides Ac-Aib-deltaZPhe-NHMe and Ac-(Aib-deltaZPhe)3-NHMe, the helical conformers with phi = +/-30 degrees, psi = +/-60 degrees for Aib residue and phi=psi= +/-30 degrees for deltaZPhe are predicted to be most stable. The computational studies for the positional preferences of deltaZPhe residue in the peptide containing one deltaZPhe and nine Ala residues reveal the formation of a 3(10) helical structure in all the cases with terminal preferences for deltaZPhe. The conformational behaviour of Ac-(deltaZPhe)n-NHMe with n< or =4 is predicted to be very labile. With n > 4, degenerate conformational states with phi,psi values of 0 degrees +/- 90 degrees adopt helical structures which are stabilized by carbonyl-carbonyl interactions and the N-H-pi interactions between the amino group of every deltaZPhe residue with one C-C edge of its own phenyl ring. The results are in agreement with the experimental finding that screw sense of helix for peptides containing deltaZPhe residues is ambiguous in solution. The helical structures stabilized by hydrogen bond formation are found to be at least 3kCalmol(-1) less stable. Conformational studies have also been carried out for the peptide Ac-(deltaEPhe)6-NHMe and the peptide Ac-deltaAla-(deltaZPhe)6-NHMe containing deltaAla residue at the N-terminal. The N-H-pi interactions are absent in peptide Ac-(deltaEPhe)6-NHMe.

Designing of peptides with left handed helical structure by incorporating the unusual amino acids.

The conformational behaviour of delta Ala has been investigated by quantum mechanical method PCILO in the model dipeptide Ac-delta Ala-NHMe and in the model tripeptides Ac-X-delta Ala-NHMe with X = Gly, Ala, Val, Leu, Abu and Phe and is found to be quite different. The computational results suggest that in the model tripeptides the most stable conformation corresponds to phi 1 = -30 degrees, psi 1 = 120 degrees and phi 2 = psi 2 = 30 degrees in which the > C = 0 of the acetyl group is involved in hydrogen bond formation with N-H of the amide group. Similar results were obtained for the conformational behaviour of D-Ala in Ac-D-Ala-NHMe and Ac-Ala-D-Ala-NHMe. The conformational behaviour of the amino acids delta Ala, D-Ala, Val and Aib in model tripeptides have been utilized in the designing of left handed helical peptides. It is shown that the peptide HCO-(Ala-D-Ala)3-NHMe can adopt both left and right handed helix whereas in the peptide Ac-(Ala-delta Ala)3-NHMe the lowest energy conformer is beta-bend ribbon structure. Left handed helical structure with phi = 30 degrees, psi = 60 degrees for D-Ala residues and phi = psi = 30 degrees for delta Ala is found to be more stable by 4 kcal mole-1 than the corresponding right handed helical structure for the peptide Ac-(D-Ala-delta Ala)3-NHMe. In both the peptides Ac-(Val-delta Ala)3-NHMe and Ac-(D-Val-delta Ala)3-NHMe the most stable conformer is the left handed helix. Comparisons of results for Ac-(Ala-delta Ala)3-NHMe and Ac(Val-delta Ala)3-NHMe and Ac-(D-Ala-delta Ala)3-NHMe and Ac-(D-Val-delta Ala)3-NHMe also reveal that the Val residues facilitate the population of 3(10) left handed helix over the other conformers. It is also shown that the conformational behaviour of Aib residue depends on the chirality of neighbouring amino acids, i.e. Ac-(Aib-Ala)3-NHMe adopts right handed helical structure whereas Ac-(Aib-D-Ala)3-NHMe is found to be in left handed helical structure.

Modeling, design, chiral aspects and role of para-substituents in aryloxypropranolamine based beta-blockers.

The conformation of the beta-blockers viz. metoprolol, atenolol, bisoprolol, betaxolol and celiprolol has been investigated using Perturbative Configuration Interaction of Localized Orbitals (PCILO) method. The conformational energy maps have been constructed for both the enantiomers (R and S) by rotating the molecule from the para-substituent end. The aryloxypropranolamine moiety adopts the same conformation for all antagonists. The graphical view of R- and S- form of these antagonists in the lowest energy conformation reveals that it is only in the S- form of beta-blockers, all the three functionalities--aromatic moiety, amino and beta-hydroxyl groups are available for interaction with beta-adrenoceptors. The para-substituents of the beta-blockers adopt a conformation which is perpendicular to the aryloxy moiety resulting in an L-shaped structure. The beta-antagonists possibly partition into the lipid bilayer through the para-substituents and the aryloxypropranolamine moiety containing the functionalities, thus, lies parallel to the plane of lipid bilayer for interaction with beta-adrenoceptors. Superimposition of S-bisoprolol in lowest energy conformation with the 3rd putative transmembranous segment of the beta-adrenoceptors reveals that the aromatic moiety, amino and beta-hydroxyl groups of antagonists are involved in interaction with the side chains of Trp-109, Asp-113 and Thr-110 respectively. This has been further substantiated by the interaction studies on the model systems.

Safranine-O as membrane potential probe: a mechanistic study using fluorescence spectroscopy.

Use of safranine-o has been examined as membrane potential probe in 1-palmitoyl-2-oleoyl-3-phosphatidylcholine (POPC) vesicles both in presence and absence of cholesterol. The fluorescence signal increases in presence of vesicles and the increase in fluorescence intensity on hyperpolarization with valinomycin is diffusion potential dependent. The fluorescence spectra recorded after time driven experiments reveals the blue shift in gamma max of fluorescence with increasing diffusion potential. The fluorescence spectra of vesicles-associated dye is at variance with those of the safranine-o in organic solvents. In organic solvents with increasing hydrophobic character of the solvent the gamma max is slightly red shifted. The electronic spectra of the dye molecule and the charges on different atomic centers have been calculated by quantum chemical method GRINDOL. The predicted first excited state originating from the phenazine moiety is in very good agreement with the excitation wavelength. On the basis of charges on various atoms the binding of safranine with vesicles has been discussed. The nonlinear behaviour of fluorescence signal with delta phi, anisotropy measurements and the computational results, reveal the penetration of bound dye molecules (along with orientation) as a function of diffusion potential. Addition of microaliquots of 1.5 M K2SO4 to already hyperpolarized vesicles decreases the fluorescence signal and the fluorescence spectra recorded on stabilization of signal after each addition showed a shift in gamma max of fluorescence in opposite direction i.e. red shifted.

Conformational structure of some beta 1-blockers, their partitioning in lipid and the role of parasubstituents.

The conformational structure of beta1-blockers metoprolol, atenolol and practolol has been investigated by PCILO method. The aminoalkanol moiety adopts the same conformation in all these compounds. These beta-antagonists differ only in the conformation adopted by the substituent para to the aminoalkanol moiety. The graphical representation of the B1-antagonists for the final conformation reveals that only in the S-form, three interacting sites, namely, aromatic moiety, the beta-hydroxyl group and the -NH2(+) groups of aminoalkanol moiety are available for interactions with the receptor. The interaction of the aryloxy oxygen of the beta-antagonists with water molecule has also been taken into account. A linear relationship was obtained between log K (the partitioning of the beta-blocker in DMPC and also in octanol/water) and the potencies of these beta1-antagonists. Possibly, the role of para substituent is to act as an anchor by partitioning in the lipid bilayer so that the beta1-antagonist adopts the proper orientation for binding to the receptor.