Quantitative functional group orientation in Langmuir films by infrared reflection - Absorption spectroscopy: C=O groups in behenic acid methyl ester and sn2-¹³C-DSPC

IR reflection - absorption spectroscopy (IRRAS) is used to determine the orientation of carbonyl groups in behenic acid methyl ester (BME) and in sn2-¹³C 1,2 distearoylphosphatidylcholine (sn2-¹³C-DSPC) in aqueous monolayer films. The C=O stretching contour of BME consists of three overlapped peaks arising from unhydrated, monohydrated, and doubly hydrated forms at 1737, 1721, and 1702 cm^-1, respectively. Dichroic ratios constructed from the p- and s-polarized IRRAS intensities of the resolved components of the contour were simulated with the optical theory of V. L. Kuzmin. On the basis of valence bond and molecular modeling considerations, the carbonyl group is essentially perpendicular to the chain axes. The transition moment is determined by IRRAS to indeed lie along the C=O bond for the unhydrated groups (thus providing good support for the theory) but changes direction for hydrated groups. A satisfactory fit between simulation and experiment could not be obtained by assuming a uniaxial distribution of the zigzag all-trans methylene chain to which the carbonyl group is rigidly attached. A slight preferential orientation of the chain planes in the direction perpendicular to that of the through barrier movement is found for both BME and sn2-¹³C DSPC. In addition, spectral quality in the 1000-1300 cm^-1 region for BME was sufficient to determine that the orientation of the single-bond C-O stretch of the ester moiety was within 20° of the vertical. For sn2-¹³C-DSPC, each C=O bond stretching contour was split into a doublet arising from hydrated and unhydrated forms. The best fit to the IRRAS data for the unhydrated C=O mode in each chain led to a suggested chain tilt of 30°C. This value, in good accord with X-ray diffraction measurements, was confirmed through analysis of the CH₂ stretching vibrations. The transition moments of the hydrated C=O bonds in sn2-¹³C-DSPC were again found to not lie along the bond direction. The current analysis provides a new method for determining functional group orientation in monolayers. The approach, when coupled with X-ray and neutron reflectivity measurements, will provide unique information about the structures and interactions of film constituents.