High Helicities of Lys-Containing, Ala-Rich Peptides Are Primarily Attributable to a Large, Context-Dependent Lys Stabilization

Peptides 1K, YKGGGAAAAAAAAKAAAAAAAAAGGGK-NH₂; 2K, YKGGGAAAAAKAAAAAKAAAAAAGGGK-NH₂; and 3K, YKGGGAAAAKAAAAKAAAAKAAAGGGK-NH₂ have been prepared by solid-phase synthesis, purified, and characterized by amino acid analysis, MALDI mass spectrometry, and ultracentrifugation. Their circular dichroism (CD) spectra of unaggregated solutions are reported for measurements in 0.01 M NaCl at 2, 25, and 60 °C and at 2 °C in aqueous guanidinium hydrochloride (0−3 M) and aqueous trifluoroethanol (TFE, 0−15 mol %). The CD spectra exhibit a helical signature in 0.01 M NaCl or in water−TFE at 2 °C, and the intensities of the mean residue ellipticities at the minimums of 222 nm in 0.01 M NaCl are (1K) −9100, (2K) −18 100, and (3K) −19 900 deg cm^-1 dmol^-1. These ellipticities are accurately modeled using a Lifson−Roig algorithm by the helical propensities previously reported by Renold et al. (Renold, P.; Tsang, K.-Y.; Shimizu, L. S.; Kemp, D. S. J. Am. Chem. Soc. 1996, 118, 12234−12235.) but not by those of Doig and Baldwin (Doig, A. J.; Baldwin, R. L. Protein Sci. 1995, 4, 1325−1336.). The helicities of peptides such as 1K, 2K, and 3K are best attributed to a lysine stabilization and not to an intrinsic helix propensity of alanine.