The interactions of horse heart apocytochrome c with phospholipid vesicles and surfactant micelles: time-resolved fluorescence study of the single tryptophan residue (Trp-59).

The interactions of horse heart apocytochrome c with membrane interfaces were studied on membrane models including micelles of the anionic surfactant sodium dodecyl sulfate (SDS), the micelle forming lipid analogs dodecylphosphoglycol (C12PG), tetradecylphosphoglycol (C14PG), and dodecylphosphocholine (C12PN), and the negatively charged phospholipid 1-palmitoyl-2-oleoylsn-glycero phosphocholine (POPS) forming small unilamellar vesicles (SUV). The time-resolved fluorescence of the single tryptophan residue (Trp-59) emission was monitored to characterize the modifications of the conformational equilibrium and of the internal dynamics of the protein, which can be brought about by its binding to these model membranes. In most of the cases, as for the protein in solution, the excited state lifetime distribution of the Trp emission was described by four discrete classes, whose relative proportions and barycenters vary significantly in the different complexes formed. In the complex with POPS, however, the decay analysis showed only 3 lifetime classes: the long lifetime class displayed a barycenter value smaller than that observed for the protein in aqueous solution but with a much higher proportion, indicating a stabilization of this conformer in the membrane-bound form of the protein. A similar sensitivity of the Trp-59 excited state to deactivation by thermal collisions in water and in the protein/POPS complex was observed, indicating a probable location of Trp-59 at the membrane/water interface. The effects of protein binding to C12PN, C12PG and C14PG micelles on the long lifetime class proportion were similar to that of POPS but, in addition, there was a large contribution of a short lifetime component which was absent in POPS vesicles.(ABSTRACT TRUNCATED AT 250 WORDS)