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    Supported Phospholipid Monolayers. The Molecular Structure Investigated by Vibrational Sum Frequency Spectroscopy

    Year: 2011

    Journal: J. Phys. Chem. C, 2011, 115 (21), 10617–10629, 20131009

    Authors: Jonathan F. D. Liljeblad, Vincent Bulone, Mark W. Rutland, C. Magnus Johnson

    Organizations: School of Chemistry, Division of Surface and Corrosion Science, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden; School of Biotechnology, Division of Glycoscience, KTH Royal Institute of Technology, SE-106 91 Stockholm, Sweden; YKI, Institute for Surface Chemistry, Box 5607, SE-114 86 Stockholm, Sweden; Swedish Center for Biomimetic Fiber Engineering (Biomime), KTH Royal Institute of Technology, SE-106 91 Stockholm, Sweden

    The molecular structure, packing properties, and hydrating water of Langmuir–Blodgett monolayers of the phospholipids 1,2-distearoyl-sn-glyerco-phosphatidylcholine (DSPC, 18:0 PC), its deuterated analogue (18:0 PC-d83), and 1,2-distearoyl-sn-glyerco-phosphatidylserine (DSPC, 18:0 PS) deposited on planar calcium fluoride (CaF2) substrates have been investigated using the surface-specific nonlinear optical technique vibrational sum frequency spectroscopy (VSFS). Compression isotherms were recorded before the deposition of the monolayers at a surface pressure of 35 mN/m, mimicking the conditions of biological cell membranes. The CH and CD stretch regions, the water region, and the lower wavenumber region, containing phosphate, ester, carboxylate, and amine signals, thus partly covering the fingerprint region, were probed to obtain a complete map of the molecules. The data indicate that all deposited monolayers formed a well-ordered and stable film, and probing the water region revealed significant differences in hydration for the different headgroups. In addition, the tilt angle of the aliphatic chains relative to the surface normal was estimated to be approximately 4° to 10° based on orientational analysis using the antisymmetric methyl stretching vibration. Orientational analysis of the ester C═O groups was also performed, and the result was consistent with the estimated tilt angle of the aliphatic chains.