Vibrational Raman optical activity of α-helical and unordered poly(L-lysine)

Abstract

Measurements of the vibrational Raman optical activity (ROA) spectra of poly(L-lysine) in H₂O and D₂O solutions, under conditions said to generate model α-helical and unordered conformations, are reported. These provide new insights into the conformational elements actually present. In addition to several signatures characteristic of α-helix, the α-helical state (pH 11.0, 2 °C) of a sample with M_w= 26 000 shows a strong sharp positive ROA band at ca. 1340 cm^–1, previously assigned to rigid loop structure with local order possibly that of 3₁₀-helix, which suggests that the α-helical sections are connected by segments of the corresponding loop. On increasing M_w to 268 000 this loop signature grows to dominate the ROA spectrum with a concomitant decrease in the α-helix bands. The unordered state (pH 3.0, 20 °C) shows clear signatures of several distinct conformational elements, including α-helix, β-strand and, possibly, left-handed helix. Under conditions which tend to disrupt secondary structure (5 mol 1^–1 NaCl solution heated to 50 °C) the ROA spectrum of the unordered state shows a decrease in the intensity of the signature of the putative left-handed helix and an increase in bands assigned to α-helix.