Conformational and Structural Analysis of the Equilibrium Between Simple and Double Strand beta-Helix of a D,L-Alternating Oligonorleucine

Alternating sequences of D and L residues in peptides are directly related to the formation of several kinds of regular helical conformations usually called -helices. The major feature of these structures is that they can be associated with the transmembrane ion-conducting channel activity in some natural antibacterial peptides. The study of alternating D,L synthetic peptides is critical to understand how factors such as surrounding media, main chain length, type of side chain and terminal groups, among others, can determine the adoption of a specific kind of -helix. The influence of terminal groups on the final stable conformation of Nformylated peptides has been studied in this work. The initial basic NMR data analysis of a synthetic alternating D,L-oligopeptide with ten norleucines, N-methylated on the residue 7 and having HCO-- and --OMe as terminal groups clearly indicates the coexistence of two different conformations in equilibrium. NMR data and molecular dynamics calculations point to a dimeric antiparallel -helix structure for the main conformation. On the other hand, NMR data suggest a single -helix structure for the second conformation. Finally, a thermodynamic analysis of the equilibrium between both conformations has been carried out by one-dimensional NMR measurements at ten different temperatures. The temperature at which 50% of dimer conformation is dissociated is 319.