Effects of fluidity on the ensemble structure of a membrane embedded α-helical peptide

Melittin, the main hemolytic component of honeybee venom, is unfolded in an aqueous environment and folds into an α-helical conformation in a lipid environment. Membrane fluidity is known to affect the activity and structure of melittin. By combining two structurally sensitive optical methods, circular dichroism (CD) and deep-ultraviolet resonance Raman spectroscopy (dUVRR), we have identified distinct structural fluctuations in melittin correlated with increased and decreased 1,2-dimyristoyl-sn-glycero-3-phosphocholine bilayer fluidities. CD spectra have reduced intensity at temperatures above 22°C and high concentrations of the cholesterol analog 5α-cholestan-3β-ol indicating distortions in the α-helical structure under these conditions. No increase in the amide S is observed in the temperature-dependent dUVRR spectra, suggesting an increase in 310 -helical structure with increasing temperatures above 22°C. However, incorporation of 25 mol% 5α-cholestan-3β-ol resulted in a small increase in the amide S intensity indicating partial unfolding of melittin.