Abstract
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.
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Notes
We have used the term maximum of fluorescence emission in a somewhat wider sense here. In every case, we have monitored the wavelength corresponding to the maximum fluorescence intensity, as well as the center of mass of the fluorescence emission. In most cases, both these methods yielded the same wavelength. In cases where minor discrepancies were found, the center of mass of the emission has been reported as the fluorescence maximum
Abbreviations
- CD:
-
circular dichroism
- CHAPS:
-
3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate
- CMC:
-
critical micelle concentration
- CTAB:
-
cetyltrimethylammonium bromide
- DOPC:
-
dioleoyl-sn-glycero-3-phosphocholine
- DPH:
-
1,6-diphenyl-1,3,5-hexatriene
- REES:
-
red edge excitation shift
- SDS:
-
sodium dodecyl sulfate
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Acknowledgements
We thank Y.S.S.V. Prasad and G.G. Kingi for technical help. This work was supported by the Council of Scientific and Industrial Research, Government of India. We thank members of our laboratory for critically reading the manuscript. H.R. thanks the University Grants Commission for the award of a Senior Research Fellowship.
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Raghuraman, H., Chattopadhyay, A. Effect of micellar charge on the conformation and dynamics of melittin. Eur Biophys J 33, 611–622 (2004). https://doi.org/10.1007/s00249-004-0402-7
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DOI: https://doi.org/10.1007/s00249-004-0402-7