Electronic structure contributions to electron-transfer reactivity in iron-sulfur active sites: 2. Reduction potentials

This study utilizes photoelectron spectroscopy (PES) combined with theoretical methods to determine the electronic structure contributions to the large reduction potential difference between [FeCl(4)](2)(-)(,1)(-) and [Fe(SR)(4)](2)(-)(,1)(-) (DeltaE(0) approximately 1 V). Valence PES data confirm that this effect results from electronic structure differences because there is a similarly large shift in the onset of valence ionization between the two reduced species (DeltaI(vert) = 1.4 +/- 0.3 eV). Specific electronic contributions to DeltaI(vert) have been investigated and defined. Ligand field effects, which are often considered to be of great importance, contribute very little to DeltaI(vert) (DeltaE(LF) < -0.05 eV). By contrast, electronic relaxation, a factor that is often neglected in the analysis of chemical reactivity, strongly affects the valence ionization energies of both species. The larger electronic relaxation in the tetrathiolate allows it to more effectively stabilize the oxidized state and lowers its I(vert) relative to that of the chloride (DeltaE(rlx) = 0.2 eV). The largest contribution to the difference in redox potentials is the much lower effective charge () of the tetrathiolate in the reduced state, which results in a large difference in the energy of the Fe 3d manifold between the two redox couples (DeltaE(Fe)( )(3d) = 1.2 eV). This difference derives from the significantly higher covalency of the iron-thiolate bond, which decreases and significantly lowers its redox potential.     

The Effects of UVA-I (340-400 nm), UVA-II (320-340 nm) and UVA-I+II on the Photoisomerization of Urocanic Acid in vivo

Abstract— Ultraviolet B radiation (280-320 nm) can systemically suppress contact hypersensitivity (CHS), delayed type hypersensitivity (DTH) and tumor rejection responses in mice. Several models have been postulated for the initiation of this UVB-induced immune suppression and, although the complete mechanism is unclear, our early studies suggested that initiation is via the activation of a photoreceptor in the skin, identified as urocanic acid (UCA). Recent preliminary data from our laboratory and others indicated that UVA (320-400 nm)-emitting broadband sunlamps can also isomerize UCA but may not lead to immune suppression, in contrast to UVB-emitting sunlamps, which cause both effects. Although the reason for this inconsistency is unknown, the emission spectra of UVA lamps contain differing amounts of UVB, UVA-I (340-400 nm) and UVA-II (320-340 nm) from those of UVB sources. In this study we determined a detailed dose-response for the isomerization of UCA in mouse skin using the UVA-I, UVA-II and UVA-I+II wavelength ranges. The dose-response curves obtained were put on an equal energy basis by quantum correction and the possibility of wavelength interaction for this effect investigated. A simple additive wavelength interaction between UVA-I, UVA-II, and UVA-I+II was observed for trans-UCA photoisomerization. This result indicates that the failure of UVA-I, UVA-II or UVA-I+II radiation to induce immune suppression of the CHS response in an animal model is not due to complex wavelength interactions and/or the presence of an in vivo endogenous photosensitizer of UCA isomerization. Other factors, such as downstream blocking by UVA of the cis -UCA generated signal, may be involved.     

An Alternative Procedure for the Aluminum-Mediated Conversion of Esters to Amides

A convenient method has been developed for directly converting esters to amides using reagents derived from the reaction of trimethylaluminum with ammonium chloride, methylamine hydrochloride or dimethylamine hydrochloride.     

The not-so-peculiar case of calcium oxide: a weakness in atomic natural orbital basis sets for calcium

Atomic natural orbital (ANO) basis sets for calcium may produce surprisingly poor atomic and molecular properties and energetics. The weaknesses in these basis sets may be traced primarily to deficiencies within the sets of d functions which are incapable of effectively correlating the 3s and 3p electrons. Examples are given which show that addition of tight d functions to the ANO basis is required to achieve qualitatively correct energetics and structures for conventionally bonded calcium compounds.