Infrared reflection absorption and sum frequency generation spectroscopic study on the structures of the LB films of palmitoyl-L- and DL-ornithine and palmitoyl-L- and DL-lysine

Infrared reflection absorption (IRA) were measured for the Langmuir-Blodgett (LB) films of palmitoyl-lysine (PL) and palmitoyl-omithine (PO) with various mixing ratios of the L- and D-enantiomers. The introduction of the D-enantiomers to the LB films consisting of the L-enantiomers in PO-LB does not cause any frequency shifts of the amide I and II bands of the peptide groups in PO-LB, while the introduction causes appreciable frequency changes in PL-LB. The IRA spectra of the LB films soaked in D2O indicated a much higher H/D exchange rate of the peptide groups for PL-LB than that for PO-LB. These results proved the existence of a more flexible molecular assembly in PL-LB compared to that in PO-LB. The SFG spectrum of PL-LB (D/L = 0/10) gives rise to peaks at 3220 and 3160 cm(-1), which are absent in the SFG spectra of PO-LB (D/L = 0/10). The peaks are ascribable to water molecules in an ordered arrangement, which play as a channel for the H/D exchange process. The bound water molecules render PL-LB more hydrophilic than PO-LB, which explains the fact that the PL-LB forms a Z-type structure and PO-LB a Y-type structure. (C) 2002 Elsevier Science B.V. All rights reserved.