Conformation-Dependent Energy Transfer between Copolypeptide Carrying l-Ornithine and l-Tyrosine and Chlorophyll in Aqueous Sodium Dodecyl Sulfate Solution

Abstract

Potentiometric and spectroscopic studies of a chlorophyll a (Chl)−copolypeptide of l-ornithine and l-tyrosine (poly(l-Orn, l-Tyr)) system in aqueous solutions of sodium dodecyl sulfate (SDS) were carried out. The cooperative binding of SDS to poly(l-Orn, l-Tyr) studied by the potentiometric measurement of the binding isotherm showed that coil to α-helix transition of the poly(l-Orn, l-Tyr)−SDS complex takes place with an increase in SDS concentration. An increase in the helix content of poly(l-Orn, l-Tyr)−SDS complex can be well interpreted in terms of the hydrophobic interaction among bound SDS ions. On the basis of theoretical analysis of the cooperative binding isotherm, it was concluded that the formation of a micelle-like cluster consisting of at least seven SDS ions is required for the stabilization of a surfactant-induced helical structure. Singlet excitation energy transfer between l-Tyr-containing copolypeptide and Chl was investigated with steady-state fluorescence spectroscopy, and the efficiency of energy transfer from l-Tyr to Chl was evaluated as a function of SDS concentration or the degree of binding of SDS ion to poly(l-Orn, l-Tyr). We also determined energy-transfer parameters such as critical transfer distance and effective mean distance between l-Tyr and Chl, based on the Förster theory. It was shown that the effective mean distance varies from 49 to 34 Å with an increse in SDS concentration or the degree of binding, and the effective local concentration of Chl acceptor is about 350 times larger than the analytical concentration (1.0 × 10-5 M); thus it can be concluded that Chl molecules are highly concentrated in poly(l-Orn, l-Tyr)−SDS complexes and Chl and l-Tyr are located close to each other. Resonance Raman spectra of the Chl−poly(l-Orn, l-Tyr)−SDS system indicated that the keto-carbonyl at the 9-position of Chl interacts with phenol side chain of l-Tyr in poly(l-Orn, l-Tyr) via hydrogen bond. Therefore, it is reasonable to assume the formation of Chl−poly(l-Orn, l-Tyr) complexes in the presence of SDS. The present results on quantum yields of Chl fluorescence and energy transfer indicate that Chl molecules are incorporated into poly(l-Orn, l-Tyr)−surfactant complexes accompanying conformational change from the random coil to α-helix, and then the conformation-dependent energy transfer effectively occurs from l-tyrosine residue to Chl in the Chl−copolypeptide−surfactant system.