Conformation and calcium-binding properties of a bicyclic nonapeptide

The conformation and calcium binding properties of the bicyclic nonapeptide BCP2, cyclo-(Glu(1)-Ala(2)-Pro(3)-Gly(4)-Lys(5)-Ala(6)-Pro(7)-Gly(8))-cycl o-(1 gamma --> 5 epsilon) Gly(9), have been investigated by means of NMR sp ectroscopy. Interproton distances, evaluated by, nuclear Overhauser effect (NOE) contacts, and phi angles, from (3)J(NH-alpha CH), have been used to o btain a feasible model for the BCP2-Ca2+ (BCP: bicyclic peptide) complex by means of restrained molecular dynamics (RMD). The NMR analysis of the free peptide, carried out in CD3CN, shows the presence in solution of at least four conformers in intermediate exchange rate. The addition of calcium ions caused the appearance of a new set of resonances, differing from those obs erved for the free BCP2. A comparison with published data about the conform ational behavior of the closely analogous peptide BCP3, differing from BCP2 for two Leu residues instead of two Ala residues in positions 2 and 6, sho ws that this simple substitution dramatically increases the peptide flexibi lity. On the contrary, upon calcium ion addition, both BCP2 and BCP3 reach a strictly, close conformation, as strongly testified by the almost identic al H-1-NMR spectra exhibited by both peptides. The RMD molecular model of t he BCP2-Ca2+ complex, here reported, is a quite symmetric structure, presen ting a three-dimensional cavity ideal for the binding of spherical cations. Four carbonyls from the main ring (Ala(2), Gly(4), Ala(6) and Gly(8)) poin t toward it, offering, together with the two carbonyls of the peptide bridg e (Gly(9) and gamma Glu(1)), putative coordinations to the cation. (C) 2001 John Wiley & Sons, Inc.