Effect of solvent on the cis-trans conformational equilibrium of a proline imide bond of short model peptides in solution.

Proton NMR spectra of proline-containing short peptides with N-terminal sequences of N-acetyl-prolyl- (Ac-Pro-) N-tert-butoxycarbonyl-phenylalanyl-prolyl- (Boc-Phe-Pro-) and N-tert-butoxycarbonyl-leucyl-prolyl- (Boc-Leu-Pro-) were measured in mixed solvents of hexadeuterodimethylsulfoxide and deuterochloroform (CDCl3). Population ratios of cis and trans conformers with respect to the proline imide bond and chemical shifts of NH protons were obtained as a function of a CDCl3 fraction of solvent. With increasing fraction of CDCl3, the trans percentages of the Ac-Pro-imide bonds increased. On the other hand, those of Boc-Phe-Pro- decreased, and those of Boc-Leu-Pro- exhibited middle tendency. From the solvent-dependent variation of the chemical shifts of the NH protons, intramolecular hydrogen bonds that stabilize the trans form of Ac-Pro- and the cis form of Boc-Phe-Pro- were discussed. For the Ac-Pro- peptides, only the trans forms are found to the compatible with 7-, 10-, and 13-membered hydrogen-bonded rings that would be similar to the ordinary secondary structures, gamma- and beta-turns and alpha-helix, respectively. For the cis form of Boc-Phe-Pro-R (R = O-methyl or glycyl-O-ethyl), the hydrogen-bonded structure is found to be similar to the type-VIa beta-turn. On the other hand, for Boc-Phe-Pro-Pro-Leu-Gly-NH2, it has been suggested that two different hydrogen bonds, which are different from that of the type-VIa beta-turn, support each other and cooperatively stabilize the cis form.

Coupling of intramolecular hydrogen bonding to the cis-to-trans isomerization of a proline imide bond of small model peptides.

A relationship between intramolecular hydrogen bonding and the cis-trans isomerization of a proline imide bond for proline-containing short peptides were studied by proton NMR and infrared spectroscopy using DMSO-d6/CDCl3 mixed solvents. The percentage of the trans form increases with increasing fraction of CDCl3 in the mixed solvents except for compounds without possibility of intramolecular hydrogen bonding. Chemical shift variations of amide protons with solvent mixing ratios were found to be useful for judging whether the amide protons take part in the intramolecular hydrogen bonding to a considerable degree or not. These results and infrared spectra were used to specify intramolecularly hydrogen bonded structures of the peptides. Formation of the 10-membered or 13-membered hydrogen bonded ring which includes the carbonyl group precedent to the prolyl residue facilitates the cis-to-trans isomerization and these hydrogen bonded rings are strong enough to restrict the proline imide bond to the trans form in CDCl3 solution. On the other hand, a 7-membered hydrogen bonded ring is not so effective in restricting the proline imide bond.

[The effect of stressor experiences and optimism upon stress responses].

The present studies investigated whether or not optimism/pessimism is a cognitive mediator of future depression for people who have experienced many negative life events. Subjects were administered optimism scales, stress response scales at Time 1. They then completed the stressor scale and stress response scales at Time 2, about six weeks later. The results showed the interaction of stressor experiences and optimistic diathesis: Subjects who have higher stressor experiences and higher stable and global explanatory style for negative events showed higher depressive responses. Other indices of optimistic diathesis--Life Orientation, Cognitive Style, and Internality dimension of Attributional Style--did not produce this interaction effect. Moreover, this interaction did not appear in the psychological stress response other than depression. These results were consistent with diathesis-stress model of depression.

Conformations of isolated fragments of pancreatic polypeptide.

In spite of its short polypeptide chain, the pancreatic polypeptide molecule consists of a polyproline II type helix and alpha-helix. To understand the stability and formation of the alpha-helical region, we prepared some peptide fragments including the helical segment of chicken pancreatic polypeptide and studied their conformations by circular dichroism (CD). PP7-36 (a peptide fragment corresponding to residues 7-36 of chicken pancreatic polypeptide) showed a CD spectrum characteristic of the helix at pH 4.6 and at peptide concentrations as low as 1 microM. PP11-36 was able to form a helical conformation only at high peptide concentrations and not at concentrations lower than 10 microM. However, acetyl PP11-36 (in which the alpha-amino group is acetylated so that no positive charge exists at the N terminus) was able to form the helical conformation at pH 4.6 and at the peptide concentrations where PP11-36 could not. Succinyl PP12-36 (in which the alpha-amino group is succinylated to introduce a negative charge) was also able to form the helical conformation. The CD spectra of PP12-36 and PP13-36 were not characteristic of the helical conformation at all the pH values and peptide concentrations studied. Acetyl PP13-36, which has no charge at the N terminus, did not form the helix. On the other hand, succinyl PP13-36, which has a negative charge at the N-terminal end, did form the helix at pH 4.6. These findings indicate that the presence of the negative charge of carboxylate at the N-terminal region of a peptide fragment is important for helix formation.(ABSTRACT TRUNCATED AT 250 WORDS)