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

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Infrared reflection absorption (IRA) were measured for the Langmuir–Blodgett (LB) films of palmitoyl-lysine (PL) and palmitoyl-ornithine (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 D 2O 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.