THE HELICAL S-CONSTANT FOR ALANINE IN WATER DERIVED FROM TEMPLATE-NUCLEATED HELICES

FORMATION of alpha-helices from disordered polypeptides depends on the degree to which amino acids favour the helical state. The folding of helical oligopeptides can be modelled by two parameters: sigma which reflects helix initiation and s which reflects propagation of a pre-existing helix and measures helical bias 1,2. Scheraga has reported s values for oligopeptides of about 1.1, implying a weak helical bias for amino-acid residues 3. By contrast, certain helical peptides studied by Baldwin seem to require much larger s values for alanine 4. Resolution of this inconsistency requires experiments that disentangle the ease of propagation from that of initiation. In this study varying lengths of polyalanine are linked to a 'template' that initiates helical structure and permits study solely of propagation. We report here that the s value for alanine in water is close to 1, supporting the earlier results of Scheraga but not the more recent results of Baldwin.