INFRARED REFLECTION-ABSORPTION OF MELITTIN INTERACTION WITH PHOSPHOLIPID MONOLAYERS AT THE AIR WATER INTERFACE/

The interaction of melittin with monolayers of 1,2-dipalmitoylphosphat idylcholine and 1,2-dipalmitoylphosphatidylserine has been investigate d with infrared external reflection-absorption spectroscopy. Improved instrumentation permits determination of acyl chain conformation and p eptide secondary structure in situ at the air/water interface. The IR frequency of the 1,2-dipalmitoylphosphatidylcholine antisymmetric acyl chain CH2 stretching vibration decreases by 1,3 cm(-1) upon melittin insertion, consistent with acyl chain ordering, whereas the same vibra tional mode increases by 0.5 cm(-1) upon peptide interaction with the 1,2-dipalmitoylphosphatidylserine monolayer, indicative of chain disor dering. Thus the peptide interacts quite differently with zwitterionic compared with negatively charged monolayer surfaces. Melittin in the monolayer adopted a secondary structure with an amide I(I') frequency (1635 cm(-1)) dramatically different from the alpha-helical motif (ami de I frequency 1656 cm(-1) in a dry or H2O hydrated environment, amide I' frequency 1645 cm(-1) in an H --> D exchanged alpha-helix) assumed in bilayer or multibilayer environments. This work represents the fir st direct in situ spectroscopic indication that peptide secondary stru cture in lipid monolayers may differ from that in bilayers.