Reactivity of singlet oxygen with tryptophan residues and with melittin inliposome systems

The reactivity of singlet oxygen, O-1(2), with amino acids, polypeptides an d proteins has been studied extensively in solution, in micelles and also i n vesicles. Here we attempt to examine its reactivity with N-acetyltryptoph an amide (NATA), with a tryptophan residue with a long aliphatic chain atta ched, Trp(CH2)(16), and with melittin-a small membrane protein-in a solutio n containing liposomes, In such a heterogeneous system the sensitizer and/o r the tryptophan residue can be located in the ambient D2O, in the liposome membrane or at the membrane-solution interface. The sensitizer meso-tetra( N-methyl-4 pyridyl)porphine tetratosylate (mTPTT) is located in the aqueous phase while hematoporphyrin (HP) is embedded in the membrane. The quenchin g of O-1(2), by the tryptophan residues and by melittin in solution, when u sing either of the sensitizers, was compared with the data in the liposome- containing system. It was found that the location of the sensitizer and of the quencher in the liposome membrane or in the surrounding solution greatl y affects the quenching rate constants of O-1(2).