Effect of micelle diameter on tryptophan dynamics in an amphipathic helical peptide in phosphatidylcholine.

The effect of dimyristoylphosphatidylcholine (DMPC) on the conformation and environment of the single tryptophan residue of a model amphipathic helical polypeptide has been investigated by fluorescence quenching with a water-soluble, neutral quencher (acrylamide) and multiple-frequency phase fluorometry. The peptide H-Ser-Ser-Ala-Asp-Trp-Leu-Lys-Ala-Phe-Tyr-Asp-Lys-Val-Ala-Glu-Lys-Leu-Ly s-Glu- Ala-Phe-Ser-Ser-Ser-OH [18As; Kanellis, P., Romans, A.Y., Johnson, B.J., Kercret, H., Chiovetti, R., Jr., Allen, T.M., & Segrest, S.P. (1980) J. Biol. Chem. 255, 11464] was synthesized by solid-phase techniques. Peptide was incubated at 26 degrees C with DMPC at various peptide:lipid weight ratios. The diameter of the resulting disk-shaped micelles increases with increasing lipid concentration from 12.0 +/- 0.4 nm at a 1:1 weight ratio of peptide to lipid to a maximum of 48.7 +/- 1.0 nm at a 1:13 ratio. At a weight ratio of 1:5, the average diameter is 22.7 +/- 0.6 nm. Decreasing the peptide:lipid ratio of the micelle resulted in a blue-shift in the fluorescence emission maximum (from 337 nm at 1:1 to 334 nm at 1:5), an increase in the fluorescence lifetime of the tryptophan measured by the phase shift method at 18 MHz (from 3.12 ns at 1:1 to 3.61 ns at 1:5), a decrease in the rate of fluorescence quenching by acrylamide (from 0.87 x 10(9) M-1 s-1 at 1:1 to 0.42 x 10(9) M-1 s-1 at 1:5), and an increase in the activation energy for quenching (from 6.7 kcal/mol at 1:1 to 12.7 kcal/mol at 1:5).(ABSTRACT TRUNCATED AT 250 WORDS)