ESR‐Vis/NIR Spectroelectrochemical Study of C70(CF3)2−. and C70(C2F5)2−. Radical Anions

A spectroelectrochemical study of the two isostructural asymmetric perfluoroalkyl derivatives C₁‐7,24‐C₇₀(CF₃)₂ and C₁‐7,24‐C₇₀(C₂F₅)₂ is presented. Reversible formation of their stable monoanion radicals is monitored by cyclic voltammetry and by in situ ESR‐Vis‐NIR spectroelectrochemistry. The ESR spectrum of the C₇₀(CF₃)₂^?. radical is a 1:3:3:1 quartet with a ¹⁹F hyperfine coupling constant (a(F)) of 0.323(4) G, demonstrating that the unpaired spin is coupled to only one of the two CF₃ groups. The ¹³C satellites are assigned to specific carbon atoms. The ESR spectrum of the C₇₀(C₂F₅)₂^?. radical is an apparent octet with an apparent a(F) value of 0.83(2) G. DFT calculations suggest that this pattern is due to the superposition of spectra for four nearly isoenergetic C₇₀(C₂F₅)₂^?. conformers. Time‐dependent DFT calculations suggest that the NIR band at 1090 nm exhibited by both C₇₀(R_f)₂^?. radical anions is assigned to the SOMO→LUMO+3 transition. The analogous NIR band exhibited by the closed‐shell C₇₀(CF₃)₂^2? dianion was blue‐shifted to 1000 nm.     

Chemical Imaging of Monolayers on Metal Surfaces: Applications in Corrosion,Catalysis, and Self‐Assembled Monolayers

In situ techniques are indispensable to understanding many topics in surface chemistry. As a consequence, several spectroscopic methods have been developed to provide molecular‐level information that only spectroscopy can supply. However, as important as this information is, it is just as critical to realize that nearly all surfaces under investigation have spatial heterogeneities of the order of nanometers to millimeters; thus, spatial analysis is very important to the overall interpretation. This Minireview focuses on a few of the recent developments in spectroscopic techniques that can provide spatial, spectroscopic, and in situ information. These techniques include photo‐electron microscopy, infrared and Raman imaging, and nonlinear optical imaging vibrational spectroscopy as applied to topics in corrosion, catalysis and self‐assembled monolayers.     

Monitoring FET flow control and wall adsorption of charged fluorescent dye molecules in nanochannels integrated into a multiple internal reflection infrared waveguide

Using Si as the substrate, we have fabricated multiple internal reflection infrared waveguides embedded with a parallel array of nanofluidic channels. The channel width is maintained substantially below the mid-infrared wavelength to minimize infrared scattering from the channel structure and to ensure total internal reflection at the channel bottom. A Pyrex slide is anodically bonded to the top of the waveguide to seal the nanochannels, while simultaneously enabling optical access in the visible range from the top. The Si channel bottom and sidewalls are thermally oxidized to provide an electrically insulating barrier, and the Si substrate surrounding the insulating SiO(2) layer is selectively doped to function as a gate. For fluidic field effect transistor (FET) control, a DC potential is applied to the gate to manipulate the surface charge on SiO(2) channel bottom and sidewalls and therefore their zeta-potential. Depending on the polarity and magnitude, the gate potential can accelerate, decelerate, or reverse the flow. Here, we demonstrate that this nanofluidic infrared waveguide can be used to monitor the FET flow control of charged, fluorescent dye molecules during electroosmosis by multiple internal reflection Fourier transform infrared spectroscopy. Laser scanning confocal fluorescence microscopy is simultaneously used to provide a comparison and verification of the IR analysis. Using the infrared technique, we probe the vibrational modes of dye molecules, as well as those of the solvent. The observed infrared absorbance accounts for the amount of dye molecules advancing or retracting in the nanochannels, as well as adsorbing to and desorbing from the channel bottom and sidewalls.     

Substrate specificity of the acyl transferase domains of EpoC from the epothilone polyketide synthase

The production of epothilone mixtures is a direct consequence of the substrate tolerance of the module 3 acyltransferase (AT) domain of the epothilone polyketide synthase (PKS) which utilises both malonyl- and methylmalonyl-CoA extender units. Particular amino acid motifs in the active site of AT domains influence substrate selection for methylmalonyl-CoA (YASH) or malonyl-CoA (HAFH). This motif appears in hybrid form (HASH) in epoAT3 and may represent the molecular basis for the relaxed specificity of the domain. To investigate this possibility the AT domains from modules 2 and 3 of the epothilone PKS were examined in the heterologous DEBS1-TE model PKS. Substitution of AT1 of DEBS1-TE by epoAT2 and epoAT3 both resulted in functional PKSs, although lower yields of total products were observed when compared to DEBS1-TE (2% and 11.5% respectively). As expected, epoAT3 was significantly more promiscuous in keeping with its nature during epothilone biosynthesis. When the mixed motif (HASH) of epoAT3 within the hybrid PKS was mutated to HAFH (indicative of malonyl-CoA selection) it resulted in a non-productive PKS. When this mixed motif was converted to YASH (indicative of methylmalonyl-CoA selection) the selectivity of the hybrid PKS for methylmalonyl-CoA showed no statistically significant increase, and was associated with a loss of productivity.     

Solvation dynamics of Hoechst 33258 in water: an equilibrium and nonequilibrium molecular dynamics study

Integrated within an appropriate theoretical framework, molecular dynamics (MD) simulations are a powerful tool to complement experimental studies of solvation dynamics. Together, experiment, theory, and simulation have provided substantial insight into the dynamic behavior of polar solvents. MD investigations of solvation dynamics are especially valuable when applied to the heterogeneous environments found in biological systems, where the calculated response of the environment to the electrostatic perturbation of the probe molecule can easily be decomposed by component (e.g., aqueous solvent, biomolecule, ions), greatly aiding the molecular-level interpretation of experiments. A comprehensive equilibrium and nonequilibrium MD study of the solvation dynamics of the fluorescent dye Hoechst 33258 (H33258) in aqueous solution is presented. Many fluorescent probes employed in experimental studies of solvation dynamics in biological systems, such as the DNA minor groove binder H33258, have inherently more conformational flexibility than prototypical fused-ring chromophores. The role of solute flexibility was investigated by developing a fully flexible force-field for the H33258 molecule and by simulating its solvation response. While the timescales for the total solvation response calculated using both rigid (0.16 and 1.3 ps) and flexible (0.17 and 1.4 ps) models of the probe closely matched the experimentally measured solvation response (0.2 and 1.2 ps), there were subtle differences in the response profiles, including the presence of significant oscillations for the flexible probe. A decomposition of the total response of the flexible probe revealed that the aqueous solvent was responsible for the overall decay, while the oscillations result from fluctuations in the electrostatic terms in the solute intramolecular potential energy. A comparison of equilibrium and nonequilibrium approaches for the calculation of the solvation response confirmed that the solvation dynamics of H33258 in water is well-described by linear response theory for both rigid and flexible models of the probe.     

Patterned immobilization of antibodies in mechanically induced cracks

A new approach of chemically immobilizing antibody within a pattern based on thin-film cracking is presented. An adjustable pattern width is achieved with resolutions varied from nano- to microscale by using loading stress on thin-film coated elastomer substrate in both one and two dimensions. By introduction of solution or chemical vapor deposition approaches, antibodies were covalently immobilized in the channels. To demonstrate the bioactivity, specificity, and response rate of antibody patterned structure, scanning electron microscopy was used to enumerating bacteria. The chemically coupled antibody is found to retain its specificity when incubated with different bacteria solutions. Trichloro(1H,1H,2H,2H-perfluoroctyl)silane coating on nonsensing regions exhibits a distinguished bacteria-resistant function that is beneficial for providing a low intrinsic background signal in detection. This technique shows a great potential for applications in the fields of sensing and tissue engineering.     

Action spectrum conversion factors that change erythemally weighted to previtamin D3-weighted UV doses

Abstract Many solar UV measurements, either terrestrial or personal, weight the raw data by the erythemal action spectrum. However, a problem arises when one tries to estimate the benefit of vitamin D(3) production based on erythemally weighted outdoor doses, like those measured by calibrated R-B meters or polysulphone badges, because the differences between action spectra give dissimilar values. While both action spectra peak in the UVB region, the erythemal action spectrum continues throughout the UVA region while the previtamin D(3) action spectrum stops near that boundary. When one uses the previtamin D(3) action spectrum to weight the solar spectra (D(eff)), one gets a different contribution in W m(-2) than what the erythemally weighted data predicts (E(eff)). Thus, to do proper benefit assessments, one must incorporate action spectrum conversion factors (ASCF) into the calculations to change erythemally weighted to previtamin D(3)-weighted doses. To date, all benefit assessments for vitamin D(3) production in human skin from outdoor exposures are overestimates because they did not account for the different contributions of each action spectrum with changing solar zenith angle and ozone and they did not account for body geometry. Here we describe how to normalize the ratios of the effective irradiances (D(eff)/E(eff)) to get ASCF that change erythemally weighted to previtamin D(3)-weighted doses. We also give the ASCF for each season of the year in the northern hemisphere every 5 degrees from 30 degrees N to 60 degrees N, based on ozone values. These ASCF, along with geometry conversion factors and other information, can give better vitamin D(3) estimates from erythemally weighted outdoor doses.