In-situ investigation of partially deuterated fatty acid and phospholipid monolayers at the air-water interface by IR reflection-absorption spectroscopy

Abstract

Partially deuterated fatty acid and phospholipid Langmuir films were examined by infrared reflection-absorption spectroscopy, thereby permitting a conformational analysis of different regions of the film-forming molecules from the frequencies of the CH 2 and CD 2 stretching vibrations. For hexadecanoic acid-d 17, half-deuterated at the tail end, the observed CH 2-stretching frequencies of ≈2 915 and 2 849 cm −1 were considerably lowered from those of completely protiated hexadecanoic acid, indicating that the conformational order adjacent to the headgroup is much higher than at the tail end. With the half-deuterated fatty acid as the acyl chain component in DPPC-d 34, it was possible to probe different parts of the hydrocarbon moiety of phospholipid monolayers at the air-water interface. Results similar to those for the fatty acids were noted, with greater conformational disorder occurring toward the bilayer center.