The crystal structure of Afc-containing peptides.

ABSTRACT

A systematic structural analysis of Afc (9-amino-fluorene-9-carboxylic acid) containing peptides is here reported. The crystal structures of four fully protected tripeptides containing the Afc residue in position 2 Z-X(1)-Afc(2)-Y(3)-OMe (peptide a X = Y = Gly; peptide b X = Aib, C(alpha, alpha)-dimethylglycine, Y = Gly; peptide c X = Gly, Y = Aib; peptide d X = Y = Aib) have been solved by x-ray crystallography. All the results suggest that the Afc residue has a high propensity to assume an extended conformation. In fact, the Afc residue adopts an extended conformation in three peptides examined in this paper (peptides a-c). In contrast, Afc was found in a folded conformation, in the 3(10)-helical region, only in the peptide d, in which it is both preceded and followed by the strong helix promoting Aib.