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

Raman spectra of the cyclic hexapeptide cyclo-(L-prolylglycyl)3 and its Na+, K+, and Ca2+ complexes are reported for the solid state and for samples in solution. Model compounds and N-deuteration were used to aid mode identification. Spectra of the uncomplexed ionophore in solution are consistent with previously proposed solution conformations and permit the identification of spectral lines characteristic of proline-containing peptide bonds in the trans and the cis conformations. Upon cation complexation the prolyl carbonyl stretch bands sharpen and upshift 20-30 cm-1 (to 1690-1700 cm-1). The glycyl carbonyl stretch band is unaffected by Na+ complexation, upshifted approximately 15 cm-1 by K+ complexation, and downshifted approximately 20 cm-1 (to 1619 cm-1) by Ca2+ complexation. Arguments supporting the involvement of prolyl carbonyl groups in cation complexation are noted. Spectra of the Na+ complex of the tetramer cyclo(L-prolylglycyl)4 suggest an asymmetric structure.