This study uses low-angle (LAXS) and wide-angle (WAXS) X-ray synchrotron scattering, volume measurements and thin layer chromatography to determine the structure and interactions of SOPC, SOPC/cholesterol mixtures, SOPC/peptide and SOPC/cholesterol/peptide mixtures. N-acetyl-LWYIK-amide (LWYIK) represents the naturally-occurring CRAC motif segment in the pretransmembrane region of the gp41 protein of HIV-1, and N-acetyl-IWYIK-amide (IWYIK), an unnatural isomer, is used as a control. Both peptides thin the SOPC bilayer by approximately 3 A, and cause the area/unit cell (peptide+SOPC) to increase by approximately 9 A2 from the area/lipid of SOPC at 30 degrees C (67.0+/-0.9 A2). Model fitting suggests that LWYIK's average position is slightly closer to the bilayer center than IWYIK's, and both peptides are just inside of the phosphate headgroup. Both peptides increase the wide-angle spacing d of SOPC without cholesterol, whereas with 50% cholesterol LWYIK increases d but IWYIK decreases d. TLC shows that LWYIK is more hydrophobic than IWYIK; this difference persists in peptide/SOPC 1:9 mole ratio mixtures. Both peptides counteract the chain ordering effect of cholesterol to roughly the same degree, and both decrease KC, the bending modulus, thus increasing the SOPC membrane fluidity. Both peptides nucleate crystals of cholesterol, but the LWYIK-induced crystals are weaker and dissolve more easily.