ALPHA-HELIX AND ASSOCIATED LOOP SIGNATURES IN VIBRATIONAL RAMAN OPTICAL-ACTIVITY SPECTRA OF PROTEINS

A comparison of the aqueous solution vibrational Raman optical activity (ROA) spectra of poly-L-lysine in random coil and alpha-helix conformations with those of bovine serum albumin and insulin, which are both rich in alpha-helix, is reported. Possible ROA signatures of alpha-helix include a broad positive band in the range approximately 900-1000 cm-1 and a couplet, negative at low wavenumber and positive at high, centred at approximately 1103 cm-1, both features originating in backbone C(alpha)-C and C(alpha)N stretch modes; a negative-positive couplet centred at approximately 1275 cm-1 originating in backbone amide III C(alpha)H and NH deformations; and a positive band in the amide I region peaking at approximately 1665 cm-1. Bovine serum albumin, but not insulin, shows an intense positive ROA band at approximately 1339 cm-1 which might originate in some of the many rigid loops present in this protein: a similar ROA band occurs in alpha-helical, but not in random coil, poly-L-lysine at approximately 1335 cm-1, which suggests that the alpha-helical sections are connected by similar loop structures. Our results indicate that ROA is a more incisive probe of biopolymer conformation than conventional vibrational spectroscopy because only those few vibrational coordinates within a complicated normal mode which sample the skeletal chirality directly contribute to the corresponding ROA band intensity.