Conformational Interconversions of the Cyclic Hexapeptide Cyclo(Pro-Gly) ₃

Abstract

A cyclic hexapeptide, cyclo(Pro-Gly-ProGly-Pro-Gly), hasbeen synthesized; its solution conformations were examined by 220-MHz nuclear magnetic resonance spectroscopy. The solution structures have been deduced, and shown to vary as a function of solvent polarity. In addition, it has been found that this cyclic peptide binds alkali metal cations. While the predominant conformation of this cyclic peptide is 3-fold symmetric in the apolar solvent methylene chloride, an asymmetric structure is preferred in some polar solvents (water, dimethylsulfoxide). However, addition of alkali metal salts, such as sodium thiocyanate, to dimethylsulfoxide solutions of the peptide shifts the conformational equilibrium in favor of a second type of C3-symmetric structure, presumably the result of the formation of a stable peptide-metal ion complex. Nuclear magnetic resonance data suggest that the peptide in methylene chloride solution takes up a conformation containing three cis' Pro CaC=O bonds and three cis Gly-Pro peptide bonds; that water and dimethylsulfoxide stabilize a conformer in which one (or two) sets of such bonds of a given Pro-Gly unit have undergone interconversion to trans'/trans forms; and that alkali metal cations complex the cyclic peptide in a Cs-symmetric all-trans'/trans structure.The biological activity and structural simplicity of many cyclic peptides have generated considerable interest in the determination of their conformations, with the ultimate goal of ascertaining structure-function relationships. Highresolution nuclear magnetic resonance (NMR) spectroscopy, useful for the study of solution conformations of cyclic peptides, can provide information for structural determinations (1,2).Many naturally-occurring cyclic peptides contain proline, and it is believed that investigations of synthetic prolinecontaining cyclic peptides composed of repeating oligopeptide units (3-5) may clarify the structural role of the prolyl residue in biologically active cyclic peptides. An additional advantage of the investigation of proline peptides is that the number of possible conformers of the peptide backbone is limited by the restrictions to rotation about the N-CCa bond of the prolyl residues. Another consideration is the recent demonstration of the presence of cis-and trane-X-Pro peptide bonds (where X is any amino-acid residue preceeding a proline residue), in both linear (6-8) and cyclic (4, 5) proline-containing peptides. Since the barrier to rotation about the peptide

Cyclo(L-prolylglycyl)3 and its sodium, potassium, and calcium ion complexes: a Raman spectroscopic study

“…Our data also identify the carbonyl groups responsible for cation binding in solution and in the solid state. In general, our solution data are consistent with conformations previously proposed for c(PG)3 on the basis of proton and 13C NMR, circular dichroism, and minimum energy calculation studies (Madison et al, 1974;Deber et al, 1972;Bartman et al, 1977;Madison, 1973). There are no previous studies of c(PG)3 or its cation complexes in the solid state with which to compare our data.…”

“…c(PG)3 is known to form C3-symmetric 1:1 complexes with K+, Li+, Rb+, and Cs+. It also forms complexes with Mg2+, Ca2+, Ba2+, and Mn2+, respectively (Deber et al, 1972;Madison et al, 1974;Deber et al, 1976; Bartman et al, 1977). We find that c(PG)3 in 2:1 (v/v) CH30H-CHC13 readily complexes with NaSCN.…”

Propensity for cis‐Proline Formation in Unfolded Proteins

“…However, in peptide bonds between any amino acid (X) and proline (X−Pro), the trans and cis conformers have a substantially lower energy difference owing to the cyclic nature of the proline side chain. Thus, cis ‐peptidyl‐prolyl ( cis ‐Pro) conformations in unfolded polypeptide chains are populated to significantly higher levels, with values that range from 5 to 80 % in model peptides, depending on the precise amino‐acid composition, with virtually no detectable dependence on temperature . In folded proteins, local interactions around X−Pro bonds typically induce 100 % population of either the cis or trans conformation …”

“…For proteins with multiple Pro residues, knowledge of the fraction of cis ‐Pro at each X−Pro bond is critical for the resultant analysis of refolding, which typically contains multiple phases. However, the quantification of cis ‐Pro propensity in intact, denatured proteins has remained limited, with most studies instead opting to quantify cis ‐Pro populations in small, model peptides …”

Proline side chain effects on theoretical [pi-pi] * absorption and circular dichroic spectra of proline-containing peptides

“…It also makes a good model for the binding of substrate to enzymes becuse it binds larger moieties such as amino acid salts (192). The cations which cyclo(Pro-Gly)2 binds Include Mg^* (33,34,190), Ca^* (33-35, 126, 190), Na"^ (126,191), k"^, L1+, Rb*, Cs"^ (191), and RNH^ and RNHg (e.g., Val-OMe and Pro-OBz, respectively) (192).…”