Biophysical studies of model stratum corneum lipid monolayers by infrared reflection-absorption spectroscopy and Brewster angle microscopy

One, two, and three component lipid models of the stratum corneum (SC) cons isting of non hydroxy fatty acid (NFA) ceramide, cholesterol, and perdeuter ated palmitic acid have been investigated as monolayers at the air/water in terface, Infrared reflection absorption spectroscopy (IRRAS), Brewster angl e microscopy (BAM), and pi-A isotherms have provided a molecular level unde rstanding and a macromolecular picture of SC lipid organization and phase b ehavior. BAM studies of pure ceramide monolayers reveal discrete domains of highly ordered molecules, which upon addition of fatty acid and cholestero l become more loosely packed fluid-like phases. IRRAS measurements of the m ulticomponent films reveal that the ceramide molecules remain conformationa lly ordered in these apparently more fluid films. However, both the chain p acking and the headgroup hydrogen bonding are very different in monolayers than in the bulk phase, multilamellar system. Chain packing is hexagonal in the monolayer, whereas it is orthorhombic for multilayers. Hydrogen bondin g patterns in the headgroup region of NFA ceramide molecules in monolayers is significantly different from multilayers. The splitting of headgroup ami de modes, previously observed in multilamellar samples, is not observed in monolayers. This lack of amide mode splitting in monolayers is consistent w ith the hypothesis (Moore et al. J. Phys. Chem. 1997, 101, 8933-8940) that amide mode splitting in NFA ceramide multilamellar samples is a transverse interaction between opposite headgroups in adjacent bilayers going down thr ough the bilayers along the z-axis. The consequences of the observed lipid phase behavior, and headgroup bonding interactions, for skin barrier lipid cohesion and organization are discussed. The implications of this structure on skin barrier function are then considered.