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

Measurements of the vibrational Raman optical activity (ROA) spectra of poly(L-lysine) in H2O and D2O solutions, under conditions said to generate model a-helical and unordered conformations, are reported. These provide new insights into the conformational elements actually present. In addition to several signatures characteristic of alpha-helix, the alpha-helical state (pH 11.0, 2 degrees C) of a sample with (M) over bar(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 3(10)-helix, which suggests that the alpha-helical sections are connected by segments of the corresponding loop. On increasing (M) over bar(w) to 268 000 this loop signature grows to dominate the ROA spectrum with a concomitant decrease in the alpha-helix bands. The unordered state (pH 3.0, 20 degrees C) shows clear signatures of several distinct conformational elements, including alpha-helix, beta-strand and, possibly, left-handed helix. Under conditions which tend to disrupt secondary structure (5 mol l(-1) NaCl solution heated to 50 degrees 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 alpha-helix.