Vibrational solvatochromism in Vaska's complex adducts.

The vibrational solvatochromism of bis(triphenylphosphine) iridium(I) carbonyl chloride (Vaska's complex, VC) was investigated by FTIR spectroscopy. The carbonyl stretching frequency (ν(CO)) was measured in 16 different organic solvents with a wide range of Lewis acidities for VC and its dioxygen (VC-O(2)), hydride (VC-H(2)), iodide (VC-I(2)), bromide (VC-Br(2)), and sulfide (VC-S(X)) adducts. The ν(CO) of the VC-O(2) complex was sensitive to the solvent electrophilicity, whereas minimal correlation was found for VC and the other adducts.

Polarization-multiplexed vibrational sum frequency generation for comprehensive simultaneous characterization of interfaces.

We describe a novel three-pulse experimental arrangement for the simultaneous generation and subsequent resolution of all four electric-dipole allowed vibrational sum frequency generation polarization combinations. For noncentrosymmetric and achiral systems, this represents full characterization of all symmetry-allowed elements of the second-order susceptibility, providing a comprehensive intensity level assessment of the system under study. By measuring all relevant signals simultaneously, this approach enables assessment of molecular orientation and structure in dynamic, temporally evolving systems that were previously inaccessible by means of sequentially scanned acquisition of the individual tensor elements.

Surface chemistry and annealing-driven interfacial changes in organic semiconducting thin films on silica surfaces.

Vibrational sum frequency generation (VSFG) spectroscopy was used in conjunction with steady-state IR spectroscopy, atomic force microscopy (AFM), and spectroscopic ellipsometry to characterize organic semiconductor thin films that were vapor deposited on silica- and trimethoxy(octadecyl)silane (ODTMS)-functionalized silica surfaces. The growth of perylene derivative N,N'-dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C(8)) was found to proceed differently on simple glass slides relative to that of native oxide on silicon and fused quartz slides. VSFG was applied to these samples to isolate structural changes that occurred specifically at the buried interface between the organic semiconductor and the silica dielectric upon thermal annealing.

Free energy and entropy of activation for phospholipid flip-flop in planar supported lipid bilayers.

Basic transition state theory is used to describe the activation thermodynamics for phospholipid flip-flop in planar-supported lipid bilayers (PSLBs) prepared by the Langmuir-Blodgett/Langmuir-Schaeffer method. The kinetics of 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) flip-flop were determined as a function of temperature and lateral surface pressure using sum-frequency vibrational spectroscopy (SFVS). From the temperature and lateral pressure dependent DSPC flip-flop kinetics, a complete description of the activation thermodynamics for flip-flop in the gel state, including free energy of activation (DeltaG(++)), area of activation (Deltaa(++)), and entropy of activation (DeltaS(++)), was obtained.

Kinetics and thermodynamics of flip-flop in binary phospholipid membranes measured by sum-frequency vibrational spectroscopy.

In order to better characterize the dependence of lipid flip-flop rate and thermodynamics on the nature of the lipid headgroup, we have studied the kinetics of flip-flop for single-lipid and mixed-lipid bilayers consisting of 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) and 1,2-distearoyl-sn-glycero-3-phosphoethanolamine (DSPE) as a function of both pressure and temperature. The kinetics of flipping were studied by sum-frequency vibrational spectroscopy (SFVS), which does not require exogenous chemical labeling of the lipid species of interest. Additionally, SFVS may be employed to track only a single species (DSPE or DSPC) within a binary mixture by selective deuteration of the matrix lipid to make it spectroscopically inactive.

Phospholipid flip-flop modulated by transmembrane peptides WALP and melittin.

Select transmembrane proteins found in biogenic membranes are known to facilitate rapid bidirectional flip-flop of lipids between the membrane leaflets, while others have no little or no effect. The particular characteristics which determine the extent to which a protein will facilitate flip-flop are still unknown. To determine if the relative polarity of the transmembrane protein segment influences its capacity for facilitation of flip-flop, we have studied lipid flip-flop dynamics for bilayers containing the peptides WALP(23) and melittin.

Lateral pressure dependence of the phospholipid transmembrane diffusion rate in planar-supported lipid bilayers.

The dependence of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) flip-flop kinetics on the lateral membrane pressure in a phospholipid bilayer was investigated by sum-frequency vibrational spectroscopy. Planar-supported lipid bilayers were prepared on fused silica supports using the Langmuir-Blodgett/Langmuir-Schaeffer technique, which allows precise control over the lateral surface pressure and packing density of the membrane. The lipid bilayer deposition pressure was varied from 28 to 42 mN/m.

Facile lipid flip-flop in a phospholipid bilayer induced by gramicidin A measured by sum-frequency vibrational spectroscopy.

The first direct experimental evidence that gramicidin A (gA), a transmembrane peptide, facilitates the translocation of unlabeled lipids in a phospholipid bilayer was obtained with sum-frequency vibrational spectroscopy (SFVS). SFVS was used to investigate the effect of gA on lipid flip-flop in a planar 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) lipid bilayer. The kinetics of lipid translocation were determined by an analysis of the SFVS intensity versus time at different temperatures in the presence of 2 mol % gA.