INFLUENCE OF LIPID PEPTIDE HYDROPHOBIC MISMATCH ON THE THICKNESS OF DIACYLPHOSPHATIDYLCHOLINE BILAYERS - A H-2 NMR AND ESR STUDY USING DESIGNED TRANSMEMBRANE ALPHA-HELICAL PEPTIDES AND GRAMICIDIN-A/

We have investigated the effect of a series of hydrophobic polypeptide s (WALP peptides) on the mean hydrophobic thickness of (chain-perdeute rated) phosphatidylcholines (PCs) with different acyl chain length, us ing H-2 NMR and ESR techniques. The WALP peptides are uncharged and co nsist of a sequence with variable length of alternating leucine and al anine, flanked on both sides by two tryptophans, and with the N- and C -termini blocked, e.g., FmAW(2)(LA)(n)W(2)AEtn, H-2 NMR measurements s howed that the shortest peptide with a total length of 16 amino acids (WALP16) causes an increase of 0.6 Angstrom in bilayer thickness in di -C-12-PC, a smaller increase in di-C-14-PC, no effect in di-Cls-PC, an d a decrease of 0.4 Angstrom in di-C-18-PC, which was the largest decr ease observed in any of the peptide/lipid systems. The longest peptide , WALP19, in di-C-12-PC caused the largest increase in thickness of th e series (+1.4 Angstrom), which decreased again for longer lipids towa rd di-Cis-PC, in which no effect was noticed. WALP17 displayed an infl uence intermediate between that of WALP16 and WALP19. Altogether, inco rporation of the WALP peptides was found to result in small but very s ystematic changes in bilayer thickness and area per lipid molecule, de pending on the difference in hydrophobic length between the peptide an d the lipid bilayer in the liquid-crystalline phase. ESR measurements with spin-labeled lipid probes confirmed this result. Because thicknes s is expected to be influenced most at the lipids directly adjacent to the peptides, also the maximal adaptation of these first-shell lipids was estimated. The calculation was based on the assumption that there is little or no aggregation of the WALP peptides, as was supported by ESR, and that lipid exchange is rapid on the H-2 NMR time scale. It w as found that even the maximal possible changes in first-shell lipid l ength were: relatively small and represented only a partial response t o mismatch. The synthetic WALP peptides are structurally related to th e gramicidin channel, which was therefore used for comparison. In most lipid systems, gramicidin proved to be a stronger perturber of bilaye r thickness than WALP19, although its length should approximate that o f the shorter WALP16. The effects of gramicidin and WALP peptides on b ilayer thickness were evaluated with respect to previous P-31 NMR stud ies on the effects of these peptides on macroscopic lipid phase behavi or. Both approaches indicate that, in addition to the effective hydrop hobic length, also the physical nature of the peptide surface is a mod ulator of lipid order.