Biophysical Studies of Model Stratum Corneum Lipid Monolayers by Infrared Reflection−Absorption Spectroscopy and Brewster Angle Microscopy

Abstract

One, two, and three component lipid models of the stratum corneum (SC) consisting of non hydroxy fatty acid (NFA) ceramide, cholesterol, and perdeuterated palmitic acid have been investigated as monolayers at the air/water interface. Infrared reflection absorption spectroscopy (IRRAS), Brewster angle microscopy (BAM), and π−A isotherms have provided a molecular level understanding and a macromolecular picture of SC lipid organization and phase behavior. BAM studies of pure ceramide monolayers reveal discrete domains of highly ordered molecules, which upon addition of fatty acid and cholesterol become more loosely packed fluid-like phases. IRRAS measurements of the multicomponent films reveal that the ceramide molecules remain conformationally ordered in these apparently more fluid films. However, both the chain packing 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 bonding patterns in the headgroup region of NFA ceramide molecules in monolayers is significantly different from multilayers. The splitting of headgroup amide modes, previously observed in multilamellar samples, is not observed in monolayers. This lack of amide mode splitting in monolayers is consistent with 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 through 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.