Potentiometric and spectroscopic studies of a chlorophyll a (Chl)-copolypeptide of L-ornithine and L-tyrosine (poly(L-Om, 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 alpha-helix transition of the poly(L-Om, L-Tyr)-SDS complex takes place with ail 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 Chi was investigated with steady-state fluorescence spectroscopy, and the efficiency of energy transfer from L-Tyr to Chi 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 Chi, based on the Forster theory. It was shown that the effective mean distance varies from 49 to 34 Angstrom with an increse in SDS concentration or the degree of binding, and the effective local concentration of Chi acceptor is about 350 times larger than the analytical concentration (1.0 x 10(-5) M); thus it can be concluded that Chi 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 Chi interacts with phenol side chain of L-Tyr in poly(L-Orn L-Tyr) via hydrogen band. Therefore, it is reasonable to assume the formation of Chl-poly(L-Om, L-Tyr) complexes in the presence of SDS. The present results on quantum yields of Chi fluorescence and energy transfer indicate that Chi molecules are incorporated into poly(L-Orn, L-Tyr)-surfactant complexes accompanying conformational change from the random coil to alpha-helix, and then the conformation-dependent energy transfer effectively occurs from L-tyrosine residue to Chl in the Chl-copolypeptide-surfactant system.