Induction of circular dichroism of symmetrical porphyrins bound to random coil polypeptides in aqueous solutions.

Absorption spectra and c.d. spectra have been measured in the region of Soret transition for aqueous solutions of porphine-meso-tetra(4-N-methylpyridinium) (TMpyP) tosylate in the presence of poly-L-glutamic acid, and of sodium porphine-meso-tetra(4-benzenesulphonate) (TPPS) or sodium porphine-meso-tetra(4-benzoate) (TPPC) in the presence of poly-L-lysine, all at high [P]/[D] ratios at neutral pH. The TmpyP-poly-L-glutamic acid system shows essentially no hypochromism and negligibly weak induced c.d., while the TPPS-poly-L-lysine or TPPC-poly-L-lysine system exhibits strong hypochromism and strong induced c.d. The former absorption band slightly shifts to red, but the latter shows a large blue shift. The primary interaction of the porphyrin with the polypeptide is their electrostatic binding. The difference in their interaction must arise from different degrees of hydrophobic interaction, and stronger interaction of TPPS or TPPC with poly-L-lysine would cause most of TPPS ions or all of TPPC ions bound on poly-L-lysine to dimerize, fix rigidly and couple together electronically on fully charged poly-L-lysine.