Carbon‐13 NMR spectra of cyclo‐Glycyl−L‐prolyl−glycyl−glycyl−L‐prolyl−glycyl: Assignment of the carbonyl resonances

The carbonyl carbon-13 nuclear magnetic resonances in peptides and depsipeptides are particularly sensitive to charge effects which arise during pH-titration [ 1,2] or upon complex formation with metal ions [3,4]. There is also some evidence that certain conformational changes in uncharged peptides might be manifested in the carbonyl resonances [ 51. On the other hand the assignment of the carbonyl resonances to specific amino acid residues is generally rather difficult even in small peptides, and hence the potential of the use of the carbonyl resonances for studies of the molecular conformations has as yet been little explored. This paper describes some proton-carbon-13 heteronuclear double resonance experiments which led to the identification of the carbonyl resonances in cycloglycyl-L-prolyl-glycyl-glycyl-L-prolyl-glycyl. On the basis of these data the effect of intramolecular hy drogen bonding on the carbonyl carbon resonance positions can be qualitatively assessed. c-(-Gly-L-Pro-Gly-)2 was particularly suitable for these studies because its molecular conformation had previously been extensively investigated. Already several years ago it was proposed on the basis of proton NMR data that one of two favorable o-type conformations with two transannular hydrogen bonds is present in solutions of this peptide [6,7]. More recently the investigation of an a-carbon-deuterated analogue, c-(-Gly-L-Pro-Gly(d2)-)2, showed that of the two likely /3-structures the one with the two intramolecular hydrogen bonds involving the amide protons and the carbonyl groups of the glycyl residues in positions 1 and 4 (structure type IIIb of fig. 2 in [7] and fig. 1 in [S]) is preferred [8,9] .