Consistent helicities from CD and template t/c data for N-templated polyalanines: progress toward resolution of the alanine helicity problem.

The 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 s 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.