Vibrational Study of the Chain Conformation of the Liquid n‐Paraffins and Molten Polyethylene

Femtosecond dynamics of electron localization at interfaces

The dynamics of two-dimensional small-polaron formation at ultrathin alkane layers on a silver(111) surface have been studied with femtosecond time- and angle-resolved two-photon photoemission spectroscopy. Optical excitation creates interfacial electrons in quasi-free states for motion parallel to the interface. These initially delocalized electrons self-trap as small polarons in a localized state within a few hundred femtoseconds. The localized electrons then decay back to the metal within picoseconds by tunneling through the adlayer potential barrier. The energy dependence of the self-trapping rate has been measured and modeled with a theory analogous to electron transfer theory. This analysis determines the inter- and intramolecular vibrational modes of the overlayer responsible for self-trapping as well as the relaxation energy of the overlayer molecular lattice. These results for a model interface contribute to the fundamental picture of electron behavior in weakly bonded solids and can lead to better understanding of carrier dynamics in many different systems, including organic light-emitting diodes.

FEMTOSECOND DYNAMICS OF ELECTRONS ON SURFACES AND AT INTERFACES

Two-photon photoemission is a promising new technique that has been developed for the study of electron dynamics at interfaces. A femtosecond laser is used to both create an excited electronic distribution at the surface and eject the distribution for subsequent energy analysis. Time- and momentum-resolved two-photon photoemission spectra as a function of layer thickness fully determine the conduction band dynamics at the interface. Earlier clean surface studies showed how excited electron lifetimes are affected by the crystal band structure and vacuum image potential. Recent studies of various insulator/metal interfaces show that the dynamics of excess electrons are largely determined by the electron affinity of the adsorbate. In general, electron dynamics at the interface are influenced by the substrate and adlayer band structures, dielectric screening, and polaron formation in the two-dimensional overlayer lattice.

Characterization of liquid chromatographic stationary phases by Raman spectroscopy. Effect of ligand type

This study represents the first Raman spectroscopic characterization of conventional chemically-bonded liquid chromatographic (LC) stationary phases under typical flow-rate and pressure conditions. Raman spectra were obtained for amino propyl (NH2), cyano propyl (CN), phenyl (Ph), octadecyl (C18), octyl (C8), and methyl (C1) chemically-bonded silica-based stationary phases in 100% aqueous mobile phases. The present experimental set-up has allowed Raman spectra of various stationary phase ligands, present in sub-monolayer coverages on the siliceous supports, to be obtained. This study: (1) demonstrates that conventional Raman spectroscopic techniques can be used to study LC stationary phases; (2) presents the experimental set-up, conditions, and approaches utilized to obtain Raman spectra of conventional stationary phases; (3) examines the spectroscopic differences observed for a variety of different types of bonded ligands that are typically used in reversed-phase (RPLC) and normal-phase (NPLC) liquid chromatographic separations; and (4) considers other future studies that are possible with this experimental approach, including mobile phase composition and temperature studies.

A Fourier Transform IR Study of the Phase Transitions and Molecular Order in the Hexadecyltrimethylammonium Sulfate/Water System

Variable temperature Fourier transform infrared (FTIR) spectroscopy was used to study the transition between the micellar and the coagel phases, and between the cubic and the coagel phases, for the hexadecyltrimethylammonium sulfate (CTAS)/water system. The phase transition takes place at 15°C for the coagel to micelle transition and 17°C for the coagel to cubic phase transition. CTAS in the solid state at two temperatures was also studied by FTIR to provide comparison with the aqueous samples on the molecular motion and packing in these phases. The present FTIR data also suggest the formation of rod-like micelles upon the addition of electrolyte, NaCl, to the CTAS/water system.

Conformational disorder in unsaturated phospholipids by FTIR spectroscopy

Conformational disorder in liquid alkenes and in the L alpha and Hparallel phases of some unsaturated phospholipids has been monitored by FTIR spectroscopy. The CH2 wagging region (1330-1390 cm-1) in saturated chains contains vibrations of particular 2- and 3-bond conformational states as follows: 1341 cm-1, end-gauche (eg); 1352 cm-1, double gauche (gg); 1368 cm-1, the sum of kink and gtg states. In unsaturated chains, this spectral region revealed an additional band at 1362 cm-1 and (occasionally) a feature near 1348 cm-1. The 1362 cm-1 band is tentatively assigned to the wagging of CH2 groups adjacent to the C = C bond. Substantial populations of both gg and (kink+gtg) states are evident in the L alpha phases of unsaturated phosphatidylcholines (PC's). Unsaturated phosphatidylethanolamines (PE's) are more ordered than their PC counterparts, and possess fewer gg and eg states. Chain disorder in the Hparallel phase of PE's approaches that in L alpha phases of unsaturated PC's. Changes in conformer distributions during the L alpha-->Hparallel transition in 1,2-dioleoylphosphatidylethanolamine (DOPE), 1-palmitoyl,2-oleoylphosphatidylethanolamine (POPE), 1,2-dielaidoylphosphatidylethanolamine (DEPE), and N-methyl-DOPE(N-MeDOPE) were semi-quantitatively estimated. For DOPE and DEPE, slight cooperative increases in both gg and (kink+gtg) states occur, for POPE only the gg population increases and for N-MeDOPE only the kink+gtg populations increase. These disorder increases are consistent with the small calorimetric delta H for this transition. Difficulties in quantitative determination of conformational disorder in unsaturated chains are discussed.

Fluctuations in IR spectral parameters detected in mixed acyl chain membranes of Acholeplasma laidlawii B

Acholeplasma laidlawii B cells were grown at 37 degrees C on three binary C16:0-d(31)/C18:1 fatty acid mixtures at initial mol ratios of 3:2, 1:1, and 2:3. These mol ratios produced final C16:0-d(31)/C18:1 lipid acyl chain mol ratios of 1.66 +/- 0.23 (n=6), 1.3 +/- 0.20 (n=6) and 0.58 +/- 0.09 (n=10), respectively, in the membrane of the microorganism. Membrane conformational order for the deuterated and proteated acyl chains in intact cells was monitored by FT-IR spectroscopy through the thermotropic response of the acyl chain CD2 and CH2 stretching frequencies. Intact cells and isolated membranes revealed broad phase transitions centered well below the growth temperature. This result differs from previous studies (Moore, D.J. and Mendelsohn, R. (1994) Biochemistry 33, 4080-4085) of cells grown on a single saturated fatty acid source, where Tm was close to the growth temperature. Fluctuations in IR spectral parameters from the liquid crystalline phases were detected in ten separate samples of cells grown on a 2:3 mixture (final mol ratio 0.58:1) of C16:0-d(31)/C18:1, and in no other cell preparation. These were manifest by reduced precision in the measurement of CH2 and CD2 stretching frequencies and are attributed to fluctuations in the membrane conformational order. In addition to conformational order fluctuations in intact cells, similar behavior was noted for the simple binary phosphatidylcholine (PC) mixture, DOPC/1-C16:0-d(31),2-C18:1 PC (2:1 molar ratio). In this instance, the fluctuations were also detected through the temporal and thermotropic response of the relative intensity of the 1341 cm(-1) band assigned to end-gauche conformers about the penultimate C-C bond in the oleoyl chains. The relationship of these observations to the Raman spectroscopic detection of packing fluctuations in highly unsaturated PC's (Litman, B.J., Lewis, N., and Levin, I.W. (1991) Biochemistry 30, 313-319) is considered.

The effect of high external pressure on DPPC-cholesterol multilamellar vesicles: a pressure-tuning Fourier transform infrared spectroscopy study

We have investigated the effect of incorporation of cholesterol on the barotropic phase behavior of aqueous dispersions of 1,2-dipalmitoylphosphatidylcholine (DPPC) using Fourier-transform infrared spectroscopy (FTIR) in combination with the diamond anvil technique. Infrared spectral parameters, such as the frequencies, intensities, bandshapes and band splittings have been used to detect structural and dynamical changes upon incorporation of cholesterol into the DPPC bilayer. Analysis of these spectral parameters yields information on conformer population, reorientational fluctuations, interchain interaction, hydrogen bonding, interdigitation packing, and phase transformations of the DPPC/cholesterol mixtures. We present FTIR data of aqueous DPPC dispersions at 0, 10, 20, 30, 40, and 50 mol% cholesterol in the pressure range from 0.001 to 20 kbar at two temperatures, 25 degrees C and 55 degrees C. In addition, comprehensive temperature dependent measurements in the range from 20 degrees C to 80 degrees C were performed at ambient pressure. Analysis of the CH2 symmetric and antisymmetric stretching modes yields information of the effect of cholesterol concentration on the phase transition phenomena occurring in the lipid bilayer. Observation of the correlation field splittings of the CH2 bending and rocking modes monitors structural changes and dynamical properties of the lipid mixtures. Cholesterol induces more orientational disorder of the lipid molecules in terms of an increase of the reorientational fluctuations of the molecules and twisting/torsion motions of the acyl chains in the gel phase even at elevated pressures. It therefore appears that one important role of cholesterol is to make the membrane insensitive to changes in external environment, such as high hydrostatic pressure. Increase of pressure leads to a decrease in half width of the C = O band contour of pure DPPC and of DPPC/cholesterol mixtures, especially for cholesterol concentrations equal and higher than 30 mol%, which might be due to a marked increase in free carbonyl groups. At high pressure part of the bound water from the interfacial zone of the membrane is withdrawn. Increase of the cholesterol concentration and increase in pressure have opposite effects on the population of free and hydrated carbonyl ester groups of DPPC in the gel phases.

Raman spectroscopic study of conjugates of butyrylcholinesterase with organophosphates

Raman spectra of human butyrylcholinesterase (BuChE; E.C. 3.1.1.8) were analyzed in the native state and after conjugation with organophosphates (soman, DFP and paraoxon). The secondary structure of the native BuChE in Tris-HCl buffer (pH 7.5), determined from analysis of the amide I polypeptide vibration band, indicates 47% alpha-helices, 26% beta-sheets, 16% turns and 12% undefined structure. We obtained the same values for paraoxon-phosphorylated BuChE, but 39% helical structure, 31% beta-sheets, 17% turns and 13% undefined structure for 'aged' DFP-BuChE conjugates and 36% helical structure, 34% beta-sheets, 20% turns and 10% undefined structure for 'aged' soman-BuChE conjugates. The approximately 10% decrease of alpha-helical structure observed upon phosphorylation by DFP and phosphonylation by soman, probably corresponds to the 'aging' process, which does not take place in the case of paraoxon. Considerable differences have been observed between native, paraoxon inhibited and 'aged' BuChE in aromatic ring vibrations, suggesting that the dealkylation of organophosphate conjugates modifies the environment or the interactions of aromatic amino-acid residues. In the aliphatic side chains an increase of the number of gauche configurations has been observed in 'aged' DFP-BuChE and soman-BuChE.

FTIR study of lamellar and reversed micellar phases in the mono-oleoylglycerol/water system

The lamellar liquid crystalline (L alpha) and the reversed micellar (L2) phases of mono-oleoylglycerol (MO) in water were investigated by Fourier transform infrared (FTIR) spectroscopy. From polarized infrared spectra of aligned bilayers of MO/water, order parameters (S) were determined for vibrational modes in the hydrocarbon chain region (symmetric CH2 stretch and olefinic CH=CH stretch) and the aggregate surface region (C=O stretch and asymmetric CO-O-C stretch). The phase transition between L alpha and L2 (10 wt% water) occurs in the temperature range 43-46 degrees C. Already at about 32 degrees C the LD value for the C-OH stretching vibration has decreased to 50% of its value at 25 degrees C. A similar relative decrease in LD is obtained for the C=O stretch and the asymmetric CO-O-C stretch at about 38 degrees C and for the CH=CH stretch at about 43 degrees C. It is concluded that the molecular ordering in the region of the aggregate surface starts to decrease at a lower temperature than for the acyl chain region. Infrared spectra show a significant decrease in the conformational order of the hydrocarbon chains at the L alpha/L2 phase transition.