Induced Circular Dichroism of a Cationic Porphyrin Bound to α-Helical Poly( l -glutamic acid) and β-Form Poly(S-carboxymethyl- l -cysteine) in Aqueous Solutions

Absorption spectra and circular dichroism (CD) of aqueous solutions of a water-soluble porphyrin derivative, porphine-meso-tetra(4-N-methylpyridinium) tosylate (TMpyP tosylate), have been measured in the presence of poly(l-glutamic acid) or poly(S-carboxymethyl-l-cysteine) at different pH. At neutral and alkaline pH where poly(l-glutamic acid) is randomly coiled, essentially no CD is induced in the Soret region over a wide range of [P]/[D] ratios. At acid pH where poly(l-glutamic acid) is helical, a positive CD band at 415 nm and a stronger negative CD band at 442 nm are induced if [P]/[D] is high, but the induced CD is considerably weak as [P]/[D] is lowered, owing to partial disruption of the helical conformation by densely bound TMpyP ions, and it substantially vanishes at [P]/[D] as low as 1. While random coil poly(S-carboxymethyl-l-cysteine) does not induce CD in the Soret region of TMpyP, β-form poly(S-carboxymethyl-l-cysteine) induces a weak, positive CD band at 510 nm, accompanied by an extensive weakening of the absorption bands.