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

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journal contribution
posted on 18.01.2002, 00:00 by Robert J. Kennedy, Kwok-Yin Tsang, Daniel S. Kemp
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-An-tLInp2K4W-NH2 of length 4 ≤ n ≤ 14 that bears the helix-initiating and monitoring N-cap Ac-Hel and the spaced solubilizer tLInp2K4W-NH2 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.