INCREASING SEQUENCE LENGTH FAVORS ALPHA-HELIX OVER 3(10)-HELIX IN ALANINE-BASED PEPTIDES - EVIDENCE FOR A LENGTH-DEPENDENT STRUCTURAL TRANSITION

Ala-based peptides form marginally stable helices at low temperature and are conventionally considered as mixtures of alpha-helix and random coil. However, recent work with doubly spin-labeled peptides suggests that short 16-residue sequences contain a significant fraction of 3(10)-helix near the N-terminus (positions 4-8). Using the same double-label strategy, we report on the helix geometry of the peptides Ac-(AAAAK)(n)A-NH2 with n = 3 and n = 4. The 16-mer (n = 3) is now examined at a region near the C-terminus, and there is evidence for 3(10)-helix here as well. The 21-mer (n = 4) is examined in three regions of the sequence. In dramatic contrast to the 16-mer, the 21-mer exhibits the signature of alpha-helix at the N-terminus and on through the middle of the peptide. The 21-mer C-terminus, however, adopts the 3(10)-helix geometry as is often found for C-termini in protein alpha-helices. These data indicate that the proportion of alpha-helix and 3(10)-helix in Ala-based peptides depends upon the sequence length.