Infrared Spectroscopic Determination of Conformational Disorder and Microphase Separation in Phospholipid Acyl Chains

Abstract

Infrared (IR) spectroscopy offers unique advantages for the study of phospholipid acyl chain structures and interactions. As is well known, the technique monitors molecular vibrations that produce dipole moment oscillations at infrared frequencies. The observed frequencies, intensities, and band shapes are dependent on molecular conformation, configuration, and chain packing. The method does not depend on probe molecules, is effective in sampling all relevant phases adopted by phospholipids (crystal, gel, liquid crystal, inverted hexagonal, cubic, etc.), and provides structural information not directly accessible to many other methods, e.g., the membranes of intact cells in normal and pathological states (Mantsch et al, 1988; Moore and Mendelsohn, 1994; Moore et al, 1995; Moore et al, 1993), and monolayer films (Dluhy, 1986; Flach et al, 1994; Ger-icke and Hühnerfuss, 1993; Mitchell and Dluhy, 1988). A major advantage for the study of phospholipids lies in the extensive IR spectra-structure correlations derived from alkanes. Assignments of the observed frequencies to specific normal modes, and the sensitivities of these modes to conformational change are in large part understood.