Characterization and control of lipid layer fluidity in hybrid bilayer membranes.

The main gel-to-liquid-crystal (LC) phase transition temperature, T(m), of the distal lipid layer in hybrid bilayer membranes (HBMs) under water was investigated using vibrational sum frequency spectroscopy (VSFS). VSFS has unique sensitivity to order/disorder transitions in the lipid acyl chains and can determine T(m) for the lipid monolayers in HBMs. We recently reported the observation that T(m) is raised and the transition width is broadened for the overlying phospholipid monolayer in HBM systems formed on densely packed crystalline self-assembled monolayers (SAMs) as compared to that of vesicles in solution. In this report, we establish that T(m) for the lipid layer of HBMs can be controlled by proper choice of the SAM underlayer. The SAM underlayer of the HBM was systematically altered by using an alkane thiol, a saturated thiolipid, a mixed SAM of a saturated lipid-pyridine disulfide, and finally a mixed SAM of an unsaturated lipid-pyridine disulfide. T(m) was measured for two different chain length saturated phosphatidylcholine lipid overlayers on these SAMs. The values obtained show that Tm of the lipid layer of HBMs is sensitive to the composition and/or packing density of molecules in the underlying SAM.

Determination of lipid phase transition temperatures in hybrid bilayer membranes.

The main gel-to-liquid-crystal (LC) phase transition temperature, T(m), of the lipid monolayer in hybrid bilayer membranes (HBMs) was investigated using vibrational sum frequency spectroscopy (VSFS). In the gel phase, the acyl chains of the lipid molecules assume an ordered, all-trans configuration, whereas in the LC phase, the acyl chains exhibit a significant number of disordered gauche conformers. VSFS has unique sensitivity to the order/disorder transitions in the acyl chains and was used to determine T(m) for a series of saturated phosphatidylcholine lipids on octadecanethiolate self-assembled monolayers (SAMs). The values obtained for T(m) for all lipids studied are significantly higher than for the corresponding lipids in vesicles in solution. Additionally, the transition widths are broader for the lipids in HBMs. The underlying SAM clearly influences the phase behavior of the overlying lipid monolayer.

Electron-transfer dynamics from Ru polypyridyl complexes to In2O3 nanocrystalline thin films.

Photoinduced electron injection dynamics from Ru(dcbpy)(2)(X)(2) (dcbpy = 4,4'-dicarboxy-2,2'-bipyridine; X(2) = 2SCN(-), 2CN(-), and dcbpy; referenced as RuN3, Ru505, and Ru470) to In(2)O(3) nanocrystalline thin films were studied using ultrafast transient IR absorption spectroscopy. After 532 nm excitation of the adsorbates, the dynamics of electron injection from their excited states to In(2)O(3) were studied by monitoring the IR absorption of the injected electrons in the semiconductor. The injection kinetics were non-single-exponential. For samples exposed to air, the half rise times, defined as the time of 50% injection yield, were 5 +/- 0.8, 85 +/- 20, and >200 ps for RuN3, Ru505, and Ru470, respectively. For samples in pH 2 buffer, the corresponding half time for injection from these complexes became 6 +/- 1, 105 +/- 20, and 18 +/- 5 ps. The injection kinetics from RuN3 to In(2)O(3) was found to be similar to that to SnO(2). These kinetics traces showed a negligible <100 fs injection component and were very different from those to TiO(2). The dependences of the injection kinetics on adsorbate energetics and the nature of the semiconductors are discussed.

Ultrafast electron transfer at the molecule-semiconductor nanoparticle interface.

Electron transfer across the molecule-semiconductor interface is a fundamental process that is relevant to many applications of nanoparticles, such as dye-sensitized solar cells and molecular electronics. This review summarizes recent progress in understanding electron transfer dynamics from molecular adsorbates to semiconductor nanoparticles. Photoexcitation of molecular adsorbates to their excited states is followed by electron injection into semiconductor nanoparticles. The products of electron injection (oxidized adsorbate and electrons in semiconductor) are monitored by their electronic and vibrational spectra, allowing direct measurement of injection rate. The dependence of injection rate on the properties of semiconductor nanoparticle, molecular adsorbate, intervening bridging and anchoring group, and interfacial environment are discussed and compared with Marcus theory of interfacial electron transfer.

Electron injection dynamics of Ru polypyridyl complexes on SnO2 nanocrystalline thin films.

Ultrafast infrared spectroscopy was utilized to investigate the electron-transfer dynamics from Ru(dcbpy)(2)(X)(2) complexes (dcbpy = 4,4'-dicarboxy-2,2'-bipyridine; X(2) = SCN(-), 2CN(-), and dcbpy; referenced as RuN3, Ru505, and Ru470, respectively) to nanocrystalline SnO(2) films. For both films exposed to air (dry) and submerged in a pH 2 buffer solution, all traces show biphasic dynamics with a small ultrafast component (less than 10%) and nonexponential slow component, indicating that most injection occurs from thermalized excited state of the dye. In the dry film, the injection rate becomes slower, comparing RuN3, Ru505, and Ru470, correlating with decreasing excited-state oxidation potentials in these dyes. However, the variation of injection rate with dye potential is less noticeable at pH 2. The possible reason for the different injection dynamics in these dyes and under different environments are discussed. These injection dynamics are also compared with those on TiO(2) and ZnO.

pH-dependent electron transfer from re-bipyridyl complexes to metal oxide nanocrystalline thin films.

Photoinduced interfacial electron transfer (ET) from molecular adsorbates to semiconductor nanoparticles has been a subject of intense recent interest. Unlike intramolecular ET, the existence of a quasicontinuum of electronic states in the solid leads to a dependence of ET rate on the density of accepting states in the semiconductor, which varies with the position of the adsorbate excited-state oxidation potential relative to the conduction band edge. For metal oxide semiconductors, their conduction band edge position varies with the pH of the solution, leading to pH-dependent interfacial ET rates in these materials. In this work we examine this dependence in Re(L(P))(CO)3Cl (or ReC1P) [L(P) = 2,2'-bipyridine-4,4'-bis-CH2PO(OH)2] and Re(L(A))(CO)3Cl (or ReC1A) [L(A) = 2,2'-bipyridine-4,4'-bis-CH2COOH] sensitized TiO2 and ReC1P sensitized SnO2 nanocrystalline thin films using femtosecond transient IR spectroscopy. ET rates are measured as a function of pH by monitoring the CO stretching modes of the adsorbates and mid-IR absorption of the injected electrons. The injection rate to TiO2 was found to decrease by 1000-fold from pH 0-9, while it reduced by only a factor of a few to SnO2 over a similar pH range. Comparison with the theoretical predictions based on Marcus' theory of nonadiabatic interfacial ET suggests that the observed pH-dependent ET rate can be qualitatively accounted for by considering the change of density of electron-accepting states caused by the pH-dependent conduction band edge position.

Biological Control of Septoria Leaf Spot Disease of Hybrid Poplar in the Field.

Biological control of Septoria leaf spot of hybrid poplars was investigated using disease-suppressive Streptomyces strains. Field experiments were conducted in 1998 and 1999 on potted trees placed in a hybrid poplar plantation near Rosemount, MN, and on field-planted trees in 1998 at St. Paul. At both locations, one resistant and three susceptible hybrid poplar clones were sprayed with Streptomyces spore suspensions and exposed to natural field inoculum of Septoria musiva. In the 1998 potted-tree experiment, strains GS-93-3, 93, and Mycostop in Tergitol or Triton X-100 solutions applied every 7 days significantly reduced leaf disease by 29 to 83% compared with the controls. In the 1999 potted-tree experiment, Streptomyces strain mixtures in Tergitol solution applied every 5 days significantly reduced leaf disease by 50 to 87% compared with the controls. In the 1998 plantation experiment, strains GS-93-3, 93, or Mycostop in Tergitol solution applied weekly, bi-monthly, or monthly significantly reduced leaf disease in all treatments by 64 to 78% compared with the controls.

Time-resolved spectroscopic studies of B(12) coenzymes: a comparison of the primary photolysis mechanism in methyl-, ethyl-, n-propyl-, and 5'-deoxyadenosylcobalamin.

An ultrafast transient absorption study of the primary photolysis of ethyl- and n-propylcobalamin in water is presented. Data have been obtained for two distinct excitation wavelengths, 400 nm at the edge of the UV gamma-band absorption, and 520 nm in the strong visible alphabeta-band absorption. These data are compared with results reported earlier for the B(12) coenzymes, methyl- and adenosylcobalamin. The data obtained for ethylcobalamin and n-propylcobalamin following excitation at 400 nm demonstrate the formation of one major photoproduct on a picosecond time scale. This photoproduct is spectroscopically identifiable as a cob(II)alamin species. Excitation of methyl-, ethyl-, and n-propylcobalamin at 520 nm in the low-lying alphabeta absorption band results in bond homolysis proceeding via a bound cob(III)alamin MLCT state. For all of the cobalamins studied here competition between geminate recombination of caged radical pairs and cage escape occurs on a time scale of 500 to 700 ps. The rate constants for geminate recombination in aqueous solution fall within a factor of 2 between 0.76 and 1.4 ns(-1). Intrinsic cage escape occurs on time scales ranging from