Interaction of alkyltrimethylammonium bromides with DMPC-d_(54) and DMPG-d_(54) monolayers studied by infrared reflection absorption spectroscopy (IRRAS)

摘要

Infrared reflection absorption spectroscopy (IRRAS) was used to analyze the adsorption of the cationic surfactants n-dodecyl-, n-tetradecyl-, and n-cetyltrimethylammonium bromide (DTAB, TTAB and CTAB) at the air-water interface and to study their interaction with 1,2-dimyristoyl-d_(54)-sn-glycero-3-phosphocholine (DMPC-d_(54)) and 1,2-dimyristoyl-d_(54)-sn-glycero-3-phosphoglycerol (DMPG-d_(54)) Langmuir monolayers. From the Gibbs adsorption isotherms of the pure surfactants on 0.1 M NaCl as subphase the value of surface excess concentration Γ and the standard free energy of adsorption ΔG~0 was determined as a function of the interface coverage Θ. The conformational and orientational changes of the surfactant alkyl chains were followed by analyzing the frequencies of the antisymmetric and symmetric methylene stretching vibrational bands ν(CH_2) during adsorption at the air-water interface. The interaction of the surfactants with lipid monolayers was also followed by IRRAS. Deuterated lipids were used to distinguish between hydrocarbon chains of the surfactants and the phospholipids. Thus, information on the conformational changes of the lipid molecules during incorporation of surfactant molecules into the lipid monolayer could be obtained. The absorbance values of the antisymmetric and symmetric methylene stretching vibrational bands (ν(CD_2) and ν(CH_2)) were used for the direct calculation of the surface partition coefficients P of the surfactants. The influence of the surfactant headgroup charge and its alkyl chain length on the incorporation into lipid monolayers is discussed.