# Consistent Helicities from CD and Template t/c Data for N-Templated Polyalanines: Progress toward Resolution of the Alanine Helicity Problem

journal contribution

posted on 18.01.2002, 00:00 by Robert J. Kennedy, Kwok-Yin Tsang, Daniel S. KempThe helicity reporting parameters t/c and [θ]

_{222}have been measured at 2, 25, and 60 °C in water for the solubilized polyalanine series Ac-Hel-A*-*_{n}^{t}^{}LInp_{2}K_{4}W-NH_{2}of length 4 ≤*n*≤ 14 that bears the helix-initiating and monitoring N-cap Ac-Hel and the spaced solubilizer^{t}^{}LInp_{2}K_{4}W-NH_{2}as a C-cap. Correlation of t/c with length shows that the helical propensity for*n*≤ 6 is ca. 1.0, consistent with our early reports, but that a dramatic increase in temperature dependence and helical propensity occurs for*n*≥ 8. A model based on hydrogen-bonding cooperativity is proposed to explain this finding, and both t/c and [θ]_{222}are modeled successfully by length-dependent alanine propensities at 2 °C of 1.03 for*n*= 6, 1.15, for 7 ≤*n*≤ 9 and 1.26 for*n*≥ 10. The implications of these results for the energetics of helix formation by alanine-rich peptide sequences are discussed.