Effect of micellar charge on the conformation and dynamics of melittin.

Electrostatic interactions play a crucial role in modulating and stabilizing molecular interactions in membranes and membrane-mimetic systems such as micelles. We have monitored the change in the conformation and dynamics of the cationic hemolytic peptide melittin bound to micelles of various charge types, utilizing fluorescence and circular dichroism (CD) spectroscopy. The sole tryptophan of melittin displays a red-edge excitation shift (REES) of 3-6 nm when bound to anionic, nonionic, and zwitterionic micelles. This suggests that melittin is localized in a restricted environment, probably in the interfacial region of the micelles, and this region offers considerable restriction to the reorientational motion of the solvent dipoles around the excited state tryptophan in melittin. Further, the rotational mobility of melittin is considerably reduced in these micelles and is found to be dependent on the surface charge of micelles. Interestingly, our results show that melittin does not partition into cetyltrimethylammonium bromide (CTAB) micelles owing to electrostatic repulsion between melittin and CTAB micelles, both of which carry a positive charge. In addition, the fluorescence lifetime of melittin is modulated in micelles of different charge types. The lowest mean fluorescence lifetime is observed in the case of melittin bound to anionic sodium dodecyl sulfate (SDS) micelles. CD spectroscopy shows that micelles induce significant helicity to melittin, with maximum helicity being induced in the case of melittin bound to SDS micelles. Fluorescence quenching measurements using the neutral aqueous quencher acrylamide show differential accessibility of melittin in various types of micelles. Taken together, our results show that micellar surface charge can modulate the conformation and dynamics of melittin. These results could be relevant to understanding the role of the surface charge of membranes in the interaction of membrane-active, amphiphilic peptides with membranes.

Radical-induced chain oxidation of proteins and its inhibition by chain-breaking antioxidants.

Exposure of proteins to oxygen-centred radicals results in dramatic changes in their structure, stability and function; properties that have been studied in many laboratories from a qualitative viewpoint. To allow a quantitative evaluation, we subjected aerated solutions of BSA to hydroxyl and superoxide anion radicals generated radiolytically under conditions where all radicals formed reacted with the protein (as judged by maximum damage to BSA). We observed that for each radical generated approx. 15 amino acids were consumed initially. Similar results were found with lysozyme and melittin. Such a massive consumption of BSA's amino acids was not observed when irradiations were carried out under anaerobic conditions. When bilirubin or Trolox (a water-soluble analogue of vitamin E) was added at a 2-fold molar excess over BSA, initial consumption of all measured amino acids, except tryptophan, decreased 4-fold. The total mass of amino acids initially protected (from consumption) exceeded the mass of antioxidants consumed by more than a factor of 20. Such protection of amino acids was not observed when the antioxidant-inactive acetyl Trolox was used. These results suggest that radical-mediated oxidation of proteins can proceed via a previously unrecognized chain reaction that may be inhibited by chain-breaking antioxidants.

The conformational status of a protein influences the aerobic photolysis of its tryptophan residues: melittin, beta-lactoglobulin and the crystallins.

We have studied the aerobic photolysis of the tryptophan residues of the proteins melittin and beta-lactoglobulin when the proteins are in ordered conformations and when they are in randomly coiled states. The results suggest that the conformational status of the protein is a factor that influences the photolysis of the constituent tryptophan residues. This point appears to be of relevance to the photo-oxidation of the tryptophan residues of the eye lens proteins crystallins.