Induced CD and interaction of some porphyrin derivatives with α‐helical polypeptides in aqueous solutions

Abstract

AbstractAbsorption spectra and induced CD have been measured on aqueous solutions of water‐soluble porphyrins with α‐helical poly(L‐glutamic acid) or α‐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 α‐helix electrostatically, and the two components of the Soret transition of porphyrin are subject to dissymmetric perturbation. TMpyP ions bind to the α‐helix at isolated sites, while TPPS ions and TPPC ions are in pairs on the α‐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.