Ultrafast Electron Crystallography. 2. Surface Adsorbates of Crystalline Fatty Acids and Phospholipids

In this account, the second in this series, we report our detailed studies of the structures and dynamics of adsorbates of crystalline fatty acids and phospholipids, using ultrafast electron crystallography (UEC). These macromolecules serve as model systems for biomembranes and allow for systematic studies under controlled conditions. The systems investigated are arachidic (eicosanoic) acid and dimyristoyl phosphatidic acid (DMPA), deposited on a substrate by the Langmuir−Blodgett technique. We studied these systems in monolayer, bilayer, and multilayer structures, and under different conditions (pH, pressure, and temperature) and on different substrates (hydrophobic and hydrophilic). The subunit cell −CH 2 −CH 2 −CH 2 − distances were determined for all structures. For fatty acid samples, the structure is orthorhombic, and a 0 and b 0 have the values, depending on conditions, of 4.7−4.9 Å and 8.0−8.9 Å, respectively. The c 0 value is 2.54 Å and for a given sample the accuracy is milliangstrom. Structural dynamics, after an ultrafast temperature jump in the underlying substrate, were studied by observing changes of the diffraction patterns: Bragg spot position, intensity, width, and the rocking curves. All structures exhibit a coherent anisotropic nonequilibrium expansion along the aliphatic chains, accompanied by transient structural ordering on the ultrafast time-scale. This wave-type, not diffusive, motion is followed by contraction and restructuring at longer times due to energy redistribution and diffusion. The transient behavior is entirely different from that reported here at equilibrium temperatures, in the range of 100−380 K. From these results, we are able to draw a general picture for the structural dynamics of amphiphilic chain molecules and elucidate the important role of nonequilibrium behavior at short times.