Ultraviolet resonance Raman study of side chain electrostatic control of poly-L-lysine conformation

We used 204 nm excitation UV resonance Raman (UVRR) spectroscopy to examine the role of side chain electrostatic interactions in determining the conformation of poly-L-lysine (PLL). We examined the pH and ionic strength dependence of the UVRR. The pH dependence of PLL UVRR spectra between pH 7.1 and 11.7 cannot be described by a two-state model but requires at least one additional state. The AmIII(3) region fitting with pH 7.1 and 11.7 basis spectra reveals a small pH-induced decrease in the relative fraction of the 2.5(1)-helix conformation compared to the PPII conformation. We performed a 2D general correlation analysis on the PLL pH dependence UVRR spectra. The asynchronous spectrum shows enhanced spectral resolution. The 2D asynchronous spectrum reveals multiple components in the C(α)-H b band and the AmII band whose origins are unclear. The cross peaks in the 2D asynchronous spectrum between the AmIII band and the other bands reveals that increasing pH induces three new structures: π-helix, α-helix, and some turn structure. We find that 2.5 M NaCl does not change the equilibrium between the PPII and 2.5(1)-helix conformations by screening side chain electrostatic repulsion. The result indicates that NaCl does not penetrate the region between the side chain and the peptide backbone. We also compared PLL conformations induced by high pH to that induced by 0.8 M ClO(4)(-). Both conditions induce α-helix-like conformations. ClO(4)(-) (0.8 M) induces 6% more α-helix-like conformations than at pH 12.4. Higher pH gives rise to longer α-helices and less turn structures.