PROPENSITY FOR HELIX FORMATION IN THE HYDROPHOBIC PEPTIDES K-2(LA)(X)(X = 6, 8, 10, 12) IN MONOLAYER, BULK, AND LIPID-CONTAINING PHASES - INFRARED AND CIRCULAR-DICHROISM STUDIES

A series of hydrophobic peptides K-2(LA)(x) (x = 6, 8, 10, 12) has bee n synthesized. IR and CD studies in MeOH solution are reported, along with IR studies of these species in vesicles with 1,2 dipalmitoylphosp hatidylcholine, and IR Reflection-Absorption Spectroscopy (IRRAS) stud ies of peptide and lipid/peptide monolayer Films in situ at the air/wa ter interface, In bulk phases, tile propensity toward helix formation increases with increasing chain length, there being essentially no hel ix in the shortest peptide, varying and concentration-dependent helica l content in K-2(LA)(8), and >90% helix formation in both K-2(LA)(10) and K-2(LA)(12). In monolayers at the air/water interface, peptide sec ondary structure was inferred from both the Amide I and Amide A bands. The shortest peptide adopted an antiparallel beta-sheet structure, wh ile the remainder of the series (when spread at low surface pressure) appeared to adopt varying proportions of parallel beta-sheet forms, Th e secondary structure adopted by K-2(LA)(10) and K-2(LA)(12) depended remarkably on the initial spreading pressure; when spread at high pres sures, the molecules were alpha-helical. The current experiments demon strate the unique advantages of IRRAS for evaluation of peptide confor mations in situ at the air/water interface and reveal large difference s in the propensity for helix formation in monolayers compared with bu lk phases.