Photochemically induced reaction of 1, N4-dimethylcytosinewith isopropanol

Photoexcited 1, $\text{N}$⁴-dimethylcytosine (I) adds isopropanol to form 5,6-dihydro-1, $\text{N}$⁴-dimethyl-6-(2-hydroxy-2-propyl)cytosine (II) as the major product; a small amount of 5,6-dihydro-1-methyluracil (III) is formed as well.     

Structure of the DNA binding cleft of the gene 5 protein from bacteriophage fd.

The structure of the gene 5 DNA unwinding protein from bacteriophage fd has been solved to 2.3-A resolution by X-ray diffraction techniques. The molecule contains an extensive cleft region that we have identified as the DNA binding site on the basis of the residues that comprise its surface. The interior of the groove has a rather large number of basic amino acid residues that serve to draw the polynucleotide backbone into the cleft. Arrayed along the external edges of the groove are a number of aromatic amino acid side groups that are in position to stack upon the bases of the DNA and fix it in place. The structure and binding mechanism as we visualize it appear to be fully consistent with evidence provided by physical-chemical studies of the protein in solution.     

The synthesis of ω-dialkylaminoalkylaminopyrazino[2,3-d]-, pyrido[2,3-d]-, imidazo[4,5-c]- and imidazo[4,5-d]pyridazines

The synthesis of 5-chloro-8-(ω-dialkylaminoalkylamino)pyrazino[2,3-d]pyridazine (II) proceeded smoothly when 5,8-dichloropyrazino[2,3-d]pyridazine (I) was allowed to react with ω-dialkylaminoalkylamines. Similarly, the reaction of 5,8-dichloropyrido[2,3-d]pyridazine (IV) with ω-dialkylaminoalkylamines gave the two expected products 8-chloro-5-(ω-dialkylaminoalkylamino)pyrido[2,3-d]pyridazine (V) and 5-chloro-8-(ω-dialkylaminoalkylamino)pyrido[2,3-d]pyridazine (VI) in a 2:3 ratio. 4,7-Dichloroimidazo[4,5-d]pyridazine (XII) was found to be much less reactive towards nucleophilic substitutions and more vigorous conditions resulted in disubstituted products (XIII). 7-Chloroimidazo[4,5-c]pyridazine (XVIII) was also found to be much less reactive towards nucleophilic substitution. In both of these cases one of the imidazole nitrogen atoms was blocked by a tetrahydropyranyl group which increased the reactivities and led to the desired monosubstituted products XVII from XII and in the latter case the expected products (XIX).     

Thermodynamics of the As(III)-thiol interaction: arsenite and monomethylarsenite complexes with glutathione, dihydrolipoic acid, and other thiol ligands

Colorimetric (near-UV absorption spectroscopy) and calorimetric (isothermal titration calorimetry) methods have been used to quantify the equilibrium and thermodynamics of arsenite and monomethylarsenite (MMA) coordinating to glutathione (GSH) and the dithiols dimercaptosuccinic acid (DMSA), dihydrolipoic acid (DHLA), and dithiothreitol (DTT). We found that both arsenite and MMA form moderately stable complexes (beta = 10(6)-10(7)) with GSH; that arsenite forms a particularly stable 2:3 complex (beta approximately 10(18)) with the biological cofactor DHLA; that MMA has a somewhat higher affinity than arsenite for thiol ligands; and that entropic factors modulate the overall stability of As(III) complexes with thiols, which are favored by the exothermic formation of As(III)-thiolate bonds. The implications of these results for arsenic toxicity are discussed.     

Determination of the structures of antiinflammatory copper(II) dimers of indomethacin by multiple-scattering analyses of X-ray absorption fine structure data

Copper K-edge X-ray absorption spectroscopic (XAS) measurements were recorded for the veterinary antiinflammatory Cu(II) complexes of indomethacin (1-(4-chlorobenzoyl)-5-methoxy-2-methyl-1H-indole-3-acetic acid = IndoH), of the general formula [Cu(2)(Indo)(4)L(2)] (L = N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA), N-methylpyrrolidone (NMP), and water), and [Cu(2)(OAc)(4)(OH(2))(2)] at room temperature and 10 K. The bond lengths and bridging O-C-O angles of the dimeric Cu(II) cage (Cu(2)O(10)C(8)) obtained from the multiple-scattering (MS) fitting of the X-ray absorption fine structure (XAFS) using a centrosymmetric model of [Cu(2)(Indo)(4)(DMF)(2)] gave Cu.Cu = 2.62(2) A, mean Cu-O(Ac) = 1.95(2) A, Cu-O(L) = 2.15(2) A, bridging O-C-O = 125(1) degrees, Cu displacement from plane 0.19 A compared with the XRD data Cu.Cu = 2.630(1) A, mean Cu-O(Ac) = 1.959 A, Cu-O(L) = 2.143(5) A, bridging O-C-O angles = 123.2(5) degrees, Cu displacement from plane 0.20 A. The excellent agreement between the XAFS- and XRD-derived data allowed the structures of related [Cu(2)(Indo)(4)L(2)] (L = DMA, NMP) complexes to be determined. All display a similar Cu(2)O(10)C(8) coordination geometry, which is independent of the nature of the axial ligand. While XAFS analysis of [Cu(2)(Indo)(4)(OH(2))(2)] and [Cu(2)(OAc)(4)(OH(2))(2)] indicates a coordination geometry similar to that of [Cu(2)(Indo)(4)L(2)] (L = DMF, DMA, NMP), removal of symmetry restraints in the MS model is required to obtain axial bond lengths comparable to those derived in the XRD structures of the acetate complex. For the Indo complex, the fitted bond lengths with the lower symmetry model give a mean Cu-L(OH2) bond distance within experimental errors of the value for [Cu(2)(Indo)(4)(DMSO)(2)] (2.16(2) A) (XRD). The difficulty in refining the Cu-O(OH2) distance of [Cu(2)(OAc)(4)(OH(2))(2)] and [Cu(2)(Indo)(4)(OH(2))(2)] using a centrosymmetric MS model is attributed to a symmetry reduction due to hydrogen-bonding effects characteristic of the aqua adducts, as is observed in the XRD structure of the acetate complex.     

Improved model for improving the inter-instrument agreement of spectrocolorimeters

A proposal by Robertson slightly modified by Berns and Petersen, to use spectral differences to predict systematic errors in spectrophotometers has found limited success in practical application. Porter suggested a way to improve the level of agreement between standardizing laboratories based on the Berns and Petersen method but suggested using derivatives calculated from piecewise polynomial splines. He did not know it at the time, but such a model was already in use. That model now has over five years of successful field testing and this paper discloses how the model was developed, the efficiency with which it can reduce systematic errors and the kinds of errors that cannot presently be corrected by computational comparison of reflectance or transmittance factor readings. For instruments of the same basic design, this model will produce a reduction of the systematic errors in colorimetric coordinates on the order of factors of 2–3. The magnitude of the initial color differences appears to be irrelevant. The corrective power of the model is limited by the numerical noise generated by the process of simulating analytical derivatives. We show that instruments with average color differences of 1.0 CIELAB unit can be reduced to a level of 0.5–0.3 units. Our testing has included a large variety of material samples including textiles, plastics, inks, paints and ceramics. Over 400 samples have been measured in proving this method. In addition, the model has been in place in industrial environments where multiple instruments of different manufacturer have been made to operate successfully from the same set of laboratory standards at reproducibility levels that rival those of national standards laboratories.     

Size effects in electronic and catalytic properties of unsupported palladium nanoparticles in electrooxidation of formic acid

We report a combined X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), and chronoamperometry (CA) study of formic acid electrooxidation on unsupported palladium nanoparticle catalysts in the particle size range from 9 to 40 nm. The CV and CA measurements show that the most active catalyst is made of the smallest (9 and 11 nm) Pd nanoparticles. Besides the high reactivity, XPS data show that such nanoparticles display the highest core-level binding energy (BE) shift and the highest valence band (VB) center downshift with respect to the Fermi level. We believe therefore that we found a correlation between formic acid oxidation current and BE and VB center shifts, which, in turn, can directly be related to the electronic structure of palladium nanoparticles of different particle sizes. Clearly, such a trend using unsupported catalysts has never been reported. According to the density functional theory of heterogeneous catalysis, and mechanistic considerations, the observed shifts are caused by a weakening of the bond strength of the COOH intermediate adsorption on the catalyst surface. This, in turn, results in the increase in the formic acid oxidation rate to CO2 (and in the associated oxidation current). Overall, our measurements demonstrate the particle size effect on the electronic properties of palladium that yields different catalytic activity in the HCOOH oxidation reaction. Our work highlights the significance of the core-level binding energy and center of the d-band shifts in electrocatalysis and underlines the value of the theory that connects the center of the d-band shifts to catalytic reactivity.