Characterization of the interaction of heparin with a synthetic peptide from GP120 of HIV by NMR spectroscopy and cis/trans isomerization of imide bonds in peptoids and peptides

The results from nuclear magnetic resonance studies of two different topics in the area of biological chemistry are reported.; In part I, the interaction of heparin with a synthetic peptide derived from the heparin-binding domain of envelop coat protein gp120 of the human immunodeficiency virus (HIV) has been studied using one and two-dimensional NMR spectroscopy. The peptide, R4K1, corresponds to amino acids 309--322 of the gp120 heparin binding domain. NOE data and temperature coefficients for the chemical shifts of the backbone amide NH protons suggest that a small, but significant population of heparin-bound peptide has secondary structure. Chemical shift titration data for the single lysine residue shows that electrostatic interactions between the four positively charged arginines and lysine and negatively charged heparin sites are responsible for the binding of the peptide to heparin. The D-amino acid form of the peptide shows the same binding behavior as the L-form, including essentially identical binding constants, which indicates that nonspecific electrostatic interactions drive the formation of the peptide-heparin complexes.; In part II, the results of studies of the kinetics of cis/trans isomerization of imide bonds in peptoids (N-substituted glycine oligomers) and proline-containing peptides are reported. It is found that peptoids, which are pharmaceutically interesting peptidomimetics, have an ensemble of conformations in solution. The number of conformations increases by a factor of two with each additional peptoid residue because of cis/trans isomerization across the peptoid bond. The cis conformations across the imide bonds of peptoids are more populated compared to those of proline-containing peptides. Cis/trans isomerization studies on several peptoids show that only a single bond rotation around the imide bond of the C-terminal peptoid residue takes place on the NMR time scale.; Proline is unique among 20 naturally occurring amino acids in that both cis and trans isomers can be significantly populated in proline-containing peptides. The effect of ring size and rigidity of the peptide backbone on the kinetics and equilibria of cis/trans isomerization across the Cys-Pro peptide bond of oxidized disulfide forms of several peptides with the sequence of Ac-Cys-Pro-Xaa-Cys-NH2 (Xaa=beta-Ala, gamma-Ala, delta-Ala, and 4-aminomethylbenzoic acid) were studied. The cis/trans isomerization kinetics and equilibria of the reduced dithiol peptides were also studied. Results are also reported for the rates of cis/trans isomerization of the proline peptide bond in the reduced dithiol and cyclic disulfide peptides with the following sequences from the active sites of thioredoxin, Ac-Cys-Pro-Xaa-Cys-NH 2 (Xaa=Ala, Val) and Ac-Cys-Xaa-Pro-Cys-NH2 (Xaa=Ala, Val). Results are also presented for the kinetics and equilibria across the Cys-Sar peptide bond of Ac-Cys-Sar-His-(Ala)n-Cys-NH2 (n=0-5), and for the cis/trans equilibria of the disulfide form of Ac-Cys-Pro-His-(Ala) n-Cys-NH2 (n=1-5).