Conformation of cyclo (-L-Pro-Gly-) ₃ and its Ca ²⁺ and Mg ²⁺ complexes

Abstract

The synthetic hexapeptide cyclo(-L-Pro-Gly-)3 is an ionophore that shows interesting conformational changes upon binding metal ions. X-ray crystallographic studies of this peptide show that when it is crystallized from an ethanol/ethyl acetate mixture the ring takes up an asymmetric conformation containing one cis peptide bond. In crystals of a Ca"s complex, the cation is sandwiched between two peptide molecules that differ markedly in conformation. However, both exhibit threefold symmetric forms, with all six peptide bonds in.the molecule occurring in the usual trans conformation. The Ca"' is octahedrally surrounded by six glycyl carbonyl oxygens from the two peptides at an average distance of 2.26 A and can easily be released by the disruption-of the peptide sandwich. In the magnesium complex, the peptide forms a 1:1 complex with the ion. The Mg2 is octahedrally coordinated to three glycyl carbonyls and three water oxygens. The average coordination distance between magnesium and the peptide oxygens is 2.03 A and that between magnesium and water oxygen-is 2.11 A. The two peptide molecules in the asymmetric unit have similar conformations and have approximate threefold symmetry. Naturally occurring cyclic peptides such as enniatin (1-3) and valinomycin (4, 5) are known to bind ions and mediate their transport across natural and prepared biological membranes; these peptides have been the subject of extensive investigations. The conformational changes that facilitate ion binding and transport can be studied in considerable detail by using x-ray crystallographic techniques, and such studies yield insight into the mechanism of ion binding and transport through membranes at the molecular level. These studies and. the interpretation of-the results are much simplified by the use of simple model peptides that mimic these properties and can easily be synthesized in the laboratory. One such synthetic peptide that mimics the ion binding properties of antamanide (6) and enniatin is. the hexapeptide cyclo(-L-Pro-Gly-)3 [hereinafter denoted (PC)3], which has been extensively studied by Blout and co-workers by spectroscopic and computed potential energy techniques (7,8). From their studies they concluded that (PG)3 conformation varies significantly writh the nature of the medium. In nonpolar medium as well as when forming complexes with cations, (PG)3 takes up a threefold symmetric conformation, and in polar solvents this changes to an asymmetric conformation with one ofthe peptide bonds in the cis configuration. Even though crystallographic results of a few cyclic hexapeptides have been reported, most of these contain two f-turns, with the peptide rings having at least an approximate twofold or inversion symmetry. The threefold nature of the chemical sequence of (PG)3 and the existence of alternating proline residues exert conformational restrictions that make the usual dou- ble 3-turn structure with intramolecular hydrogen bonds impossible for this hexapeptide. We report here the conformational features of this pep...

Speciation of cyclo(Pro-Gly)3 and its divalent metal-ion complexes by electrospray ionization mass spectrometry

“…ϩ is not observed, suggesting that protonation is necessary for this process to occur. 18 Olabeling experiments have shown that singly protonated peptides can lose water from the C-terminal carboxylicÅ acid,Å theÅ side-chainÅ hydroxylÅ groupsÅ ofÅ serineÅ or threonine residues or the amide carbonyl of a peptide bondÅ [27].Å InÅ theÅ caseÅ ofÅ CPG3,Å theÅ optionsÅ areÅ limited to loss of water from a proline or glycine carbonyl group. Furthermore, initial loss of water appears to be competitive with sequential loss of intact amino acid residues, because m/z 445 is the base peak in the product ion spectrum for [M ϩ H] ϩ and the b o ions are of relativelyÅ lowÅ abundanceÅ (TableÅ 2).Å TheÅ apparentÅ stability of the m/z 445 ion suggests that it is probably cyclic, while the absence of an equivalent product ion for [M ϩ Cu Ϫ H] ϩ further suggests that protonation of a prolyl carbonyl group is required for elimination of water from CPG3.…”

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

“…First, cycloCPro-Gly)^ has only 13 symmetric conformera (33)(34)(35)(36) and 18 asymmetric ones (33,(36)(37)(38). Second, cyclo(Pro-Gly)2 Is capable of binding and transporting cations (32) making It an excellent model for lonophores like antamanlde.…”

“…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).…”

“…X-ray crystallographlc studies have been done for several of the cation complexes: Na^ (126), Ca^* (35), and Mg^^ (34). Moreover, information on solution backbone conformations has been obtained by NMR, energy minimizations, and CD studies which have Included theoretical CD calculations (1,33,38).…”

“…The previous solution studies have placed the cation complexes in two however, exhibits 1:1 stoichlometry for the peptide and cation, but the unit cell contains two different peptide molecules (34). Bandwidths used for these forms are as follows: C^2, 4000 cm Cg3, 6000 cm S Cg5, 4000 cm ^; CgS, 4000 cm ^; Cg8, 6000 cm ^; and 0^9, 4000 cm The choice between the two bandwidths was made according to which one gave the better agreement with experiment.…”