INDUCED CD AND INTERACTION OF SOME PORPHYRIN DERIVATIVES WITH ALPHA-HELICAL POLYPEPTIDES IN AQUEOUS-SOLUTIONS

Absorption spectra and induced CD have been measured on aqueous solutions of water-soluble porphyrins with alpha-helical poly(L-glutamic acid) or alpha-helical poly(L-lysine) at different mixing ratios. For the former, porphyrin is porphine-meso-tetra(4-N-methylpyridinium) (TMpyP), and for the latter, it is porphine-meso-tetra(4-benzenesulfonate) (TPPS) or porphine-meso-tetra(4-benzoate) (TPPC). All the solutions of porphyrin-polypeptide systems show hypochromism in the Soret band and induced CD in the Soret region. The CD is characterized by a positive band at a shorter wavelength and a stronger negative band at a longer wavelength. The hypochromicity and the magnitude of molar ellipticities are much larger for the TPPS- and TPPC-poly(L-lysine) systems than for the TMpyP-poly(L-glutamic acid) system. Porphyrin ions bind to the alpha-helix electrostatically, and the two components of the Soret transition of porphyrin are subject to dissymmetric perturbation. TMpyP ions bind to the alpha-helix at isolated sites, while TPPS ions and TPPC ions are in pairs on the alpha-helix, that is, two ions bind consecutively and dissymmetrically. In the TMpyP-poly(L-glutamic acid) system a single CD band is associated with each of the two components of the Soret transition, and these are of opposite sign. In the TPPS- and TPPC-poly(L-lysine) systems, a pair of positive and negative CD bands is associated with each of the two components, thus giving apparently a single pair of CD bands with a shoulder, owing to partial overlapping.