Induction of Circular Dichroism on the Soret Bands of a Symmetric Water-Soluble Porphyrin by Poly(l-lysine) in Three Conformations

Abstract Absorption spectra and circular dichroism (CD) of aqueous solutions of porphine-meso-tetra(4-benzenesulfonic acid) (TPPS) in the presence of poly(l-lysine) have been measured at different pH and at different mixing ratios, and also after heated at alkaline pH higher than 10.5. Even at pH 3.0—8.0, where poly(l-lysine) is in the random coil conformation, strong CD is induced in the Soret region, giving many CD bands, and their magnitudes are not very dependent on the mixing ratio, P/D, higher than 5. TPPS bound to random coil poly(l-lysine) is mostly in the dimeric form having a blue-shifted absorption band at 399 nm. The pH-induced helix-coil transition of poly(l-lysine) part occurs, accompanied by the conversion of bound TPPS from the dimeric form to the monomeric form. At pH 11 where poly(l-lysine) is in the perfect α-helix, strong CD is induced in the Soret region, consisting of a pair of positive and negative bands, and their magnitudes increase with decreasing P/D ratio and have a maximum at an intermediate P/D ratio. TPPS bound to helical poly(l-lysine) is monomeric, having an absorption band at 418nm. The induction of strong CD on the Soret region of TPPS when bound to either coiled or helical poly(l-lysine) can be attributed to dissymmetric electronic coupling of two or more TPPS ions bound to poly(l-lysine). Only weak CD is induced on the Soret transition of TPPS bound to β-form poly(l-lysine) that has been formed by heating aqueous solution of helical poly(l-lysine) at alkaline pH.