Table 3. RP-1 interaction with lipid systems by quantitative FTIR spectroscopy.

Lipid System	pH	% Conformation				Tilt Angle Θ	KA (M−1)	KD (M)
		α-helix	loop-turn	β-sheet	disordered
POPE∶POPG	7.5	51.5	31.6	6.5	10.4	51°	5.3×104	1.2×10−7
POPE∶POPG	5.5	50.2	32.9	8.7	8.2	46°	1.3×104	9.2×10−8
POPC∶CHO	7.5	42.9	33.8	13.2	10.1	34°	0.3×104	5.3×10−7
POPC∶CHO	5.5	40.2	34.2	14.9	10.7	31°	0.1×104	ND

The secondary structure and orientation of RP-1 was determined using representative bacterial (POPE/POPG; mole ratio 3∶1) and eukaryotic (POPC/cholesterol; mole ratio 1.2∶1) mimetic liposomal systems. For conformation assessment lipid-peptide films were dispersed in deuterated buffer (10 mM PIPES pH 5.5 or 10 mM HEPES pH 7.5) and FTIR spectra of the samples were averaged for 256 scans at a gain of 4 and a resolution of 2 cm−1. The relative amounts of α-helix, β-turn, β-sheet, and random (disordered) structures were estimated by Fourier self-deconvolution and the tabulated results represent means from four independent and highly reproducible determinations for each environment SE 5%, or better. K_assoc was measured by introducing RP-1 to solutions containing large unilamellar vesicles (10 mM PIPES pH 5.5 or 10 mM HEPES pH 7.5). The orientation of the RP-1 peptide in the lipid bilayer of eukaryotic and bacterial membrane mimetic systems was determined using polarization (0° to 90°) to determine the insertion, or tilt angle of the peptide helical axis in the lipid multilayers. The binding of the peptide to lipid was expressed as an association constant K_assoc [1], where [P] is the molar concentration of RP-1 peptide in solution, [L] is the molar concentration of lipid and [PL] is the molar concentration of peptide bound to lipid [39].