NMR and molecular modeling of the heavy-metal complexes of phytochelatins and the cis/trans isomerization kinetics of proline-containing peptides

The results from nuclear magnetic resonance studies of two different topics in the area of biological chemistry are reported.;In part I, the results of studies of the interaction of phytochelatins with metals and metalloids are reported. Phytochelatins are polydisperse peptides that plants, algae, and certain fungi synthesize to complex toxic metals and metalloids that are present in their environment. These peptides, being class III metallothioneins, are cysteine rich and have the formula (γ-Glu-Cys) n-Gly (n = 2–11). The peptides studied in this work were end-capped analogues with the structure Ac-(γ-Glu-Cys)n-Gly-NH 2 in order to focus on the chemistry of sulfur. In this research, one- and two-dimensional high-resolution NMR has been used to characterize the interaction of Ac-(γ-Glu-Cys)3-Gly-NH2 with mercury, cadmium, selenium, arsenic, copper, zinc, bismuth, and silver. Also, the interaction of Ac-(γ-Glu-Cys)3-NH2 and Ac-(γ-Glu-Cys) 4-NH2 interactions with cadmium and mercury are reported.;It was found that Ac-(γ-Glu-Cys)2-Gly-NH2 binds mercury and selenium at a one-to-one ratio through the thiolate groups of cysteine to produce 1D 1H NMR spectra with a minimal amount of exchange broadening. The other metal and metalloid complexes of Ac-(γ-Glu-Cys) 2-Gly-NH2 had spectra dominated by exchange broadening, with metal binding involving the thiolate and carboxylate groups of the peptide. Ac-(γ-Glu-Cys)3-NH2 and Ac-(γ-Glu-Cys) 4-NH2 both produced distinct complexes in solution with mercury of the stoichiometries Hg3[Ac-(γ-Glu-Cys)3-NH 2]2 and Hg2[Ac-(γ-Glu-Cys)4-NH 2].;In part II, the results of studies of the kinetics of cis/trans isomerization of proline peptide bonds in reduced dithiol and cyclic disulfide peptides are reported. Proline is unique among the 20 naturally occurring amino acids in that a significant population of the peptide bond preceding the prolyl residue exists in the cis form. Previous work reported that, upon formation of cyclic peptides, the rate of cis/trans isomerization decreased due to steric constraints. It is reported in this research that the Cys-Pro and Xaa-Pro peptide bonds in peptides having the sequences Ac-Cys-Pro-Xaa-Cys-NH 2, Ac-Cys-Xaa-Pro-Cys-NH2, Ac-Cys-Pro-Phe-Xaa-Cys-NH 2, and Ac-Cys-Pro-Phe-Xaa-Xaa-Cys-NH2 show increased rates of cis/trans isomerization in the cyclic disulfide form due to the formation of an intramolecular hydrogen bond between the pyramidalized proline nitrogen of the transition state and the NH proton of the following residue in the peptide.