Martyn C.R. Symons

Nonelectrolyte Solvation in Aqueous Dimethyl Sulfoxide—A Calorimetric and Infrared Spectroscopic Study

We report enthalpies of solution of formamide, N,N-dimethylformamide,N,N-dimethylacetamide, acetic acid, methyl acetate, and acetonitrile in water +dimethylsulfoxide mixed solvents. These, along with literature data for additional solutes,are analyzed in terms of the extended coordination model of solvation. We alsoanalyze infrared data for several of these solutes. These analyses show thatN,N-dimethylformamide and N,N-dimethylacetamide are preferentially hydrated,while the other solutes appear to be preferentially solvated by dimethylsulfoxide.In all cases, the extent of preferential solvation is relatively small. It is also foundthat the degrees of preferential solvation recovered from analyses of the enthalpydata correspond closely to those recovered from the infrared data, although thelatter refer only to the polar chromophores on the solute molecules. It is foundthat the extent to which the solutes disrupt the solvent-solvent interactions variessystematically with the area of the nonpolar surfaces of the solute molecules.

Possible identification of the dihydrogen radical anion in irradiated solid hydrogen

Abstract It has recently been shown that when solid para-hydrogen is exposed to ionising radiation at 4.2 K a narrow, 1:1 doublet, centered on g = 2.007 with a splitting of 12.4 G grows in the ESR spectrum. This was discussed in terms of a zero-field splitting of a triplet state comprising (H 2 +· −e t − ) pairs. The aim of the present study is to suggest that these features are the central doublet for H 2 −· radical anions. If this is correct, the real g -value becomes 2.0033, and the 1 H hyperfine splitting, estimated from the 12.4 G splitting, is 203 G for two equivalent protons. The outer, M I = ± 1 features were not detected. This is not surprising since there should be a large anisotropic component for the outer features, which should greatly reduce their intensities relative to the very narrow central components. (These have now been detected by Professor T. Miyazaki and co-workers.)

Electron Movement Through Proteins and DNA

Nature utilizes the phenomenon of single electron transfer very widely, especially in metallo-proteins. In systems when the metal donor (D) is well separated from the acceptor (A) by polypeptide chains, the transferring electron is presumed to be bonded, in part, by these chains, which may influence the pathway taken. This situation can be probed by radiolytic injection of electrons into proteins at low temperatures. One aim of this brief review is to consider how information derived from such radiolysis studies, and followed by ESR spectroscopy, may possibly impinge on studies of D-A systems. Electrons can also be injected into duplex DNA in this way, and the results are compared with those for proteins. They are also considered in the light of recent studies of D-A electron-transfer via a polynucleotide strand. It seems that such transfers are very efficient, and it is tentatively suggested that Nature may also use this conductivity in some as yet undiscovered systems.

Diaquabis(3,6-dioxaheptanoato)copper(II): crystal structure and EPR characteristics

Abstract The structure of the title compound has been determined by X-ray diffraction at 190 K. The complex has an all trans configuration with an elongated tetragonally distorted octahedral CuO 6 chromophore. The elongated axis corresponding to the trans -Cu–O(ether) bonds. The ligand molecules are bidentate via the carboxyl and the 3-ether O atoms; the 6-ether O atoms are not coordinated and are remote from the Cu centres. The bond lengths to the Cu centres are Cu–O(ether) 2.355 A, Cu–O(Carboxyl) 1.933 A and Cu–OH 2 1.995 A. The EPR spectrum of both the powder and frozen solution forms is typical of a rhombic system with a d x 2 − y 2 1 electronic configuration. There were no significant differences in spectra recorded over the temperature range 77 K to room temperature. These results are discussed in relation to earlier published results on closely related oxa-carboxyl complexes.

Polymeric ternary metal thiols I. Products from reaction of Cu(II) with MoS42

Abstract The black, polymeric, product isolated from the reaction of Cu(II) salts with MoS42− has been shown to have some unexpected and unusual properties, i.e. a stoichiometry of Cu1.6MOS4Xy (X = Cl−, Br−, y ≈ 1; X = SO42−, y ≈ 0.5), reduction of the metal centres to Cu(I) and Mo(V), the presence of S radical centres, and the absorption of ca. 2 mol O2. EXAFS spectra have also been recorded. The W anion WS42− behaves identically.

EPR spectra for high- and low-spin CoII encapsulated complexes

Abstract A study of high- and low-spin cobalt(II) derivatives having hexadentate encapsulated ligands known as ‘sar’ is reported (sar = 3,6,10,13,16,19-hexaazabicyclo(6.6.6)icosane). The chemically prepared derivatives have well defined EPR spectra at 4 K especially in the g = 9 region, and were clearly high-spin Co(II). However, when the Co II centres were prepared by radiation-induced electron addition to the Co(III) complex at 77 K, a low-spin species was initially formed with g values in the free-spin region. These were converted into the normal high-spin form on annealing to ∼ 115 K.

Cis- and trans-conformations for peroxynitrite anions.

The biochemically important anion, peroxynitrite, is commonly thought to have a cis-conformation which is stable. This is thought to have a cyclic structure, which blocks the formation of the trans-conformation. Furthermore, it is often suggested that the latter structure isomerises to the far more stable nitrate ion. Here I suggest that the cis-structure has comparable stability to the trans-structure, and they are in rapid equilibrium at room temperature. This is supported by the very large linewidths for certain vibrational bands at room temperature, which become narrow, well-separated bands at 4 K. Lifetimes are about 10(-12) s for each isomer.

The EPR spectrum for CuB in cytochrome c oxidase

Incubation of cytochrome c oxidase (CcO) in its resting state in saturated ammonium sulfate, at room temperature overnight, gave EPR signals characteristic of a single Cu(II) center. From the g// and A// values it is concluded that this is a square-planar type 2 copper center, and superhyperfine splitting shows the presence of three nearly equivalent 14N nuclei in the plane. It is suggested that this center, also formed by incubating the enzyme in 10% methanol followed by direct irradiation, must be the CuB center. This type 2 copper EPR spectrum is identical to the EPR spectrum of CuB reported for the isolated cytochrome bo3 complex from Escherichia coli; and to the EPR spectrum reported for the sulfobetaine 12 heat-treated cytochrome c oxidase complex. It is argued that a small perturbation in the system causes decoupling of the magnetic coupling of the heme a3-CuB binuclear center and the appearance of the type 2 EPR signal.

Effects of AQ4N and its reduction product on radiation-mediated DNA strand breakage

Supercoiled plasmid pBR322 DNA was irradiated in phosphate buffer by 60Co gamma-rays at a dose rate 19.26 Gy/min and total dose of 10 Gy in the presence of a bioreductive antitumour prodrug namely 1,4-bis [¿2-(dimethylamino-N-oxide)ethyl¿ amino] 5, 8-dihydroxyanthracene-9,10-dione (AQ4N) and its DNA affinic reduction product 1,4-bis[¿2(dimethylamino)ethyl¿ amino] 5,8-dihydroxyanthracene-9,10-dione (AQ4) under air and nitrogen. AQ4N and AQ4 were found to protect against radiation-induced plasmid single and double strand breakage as assessed by agarose gel electrophoresis. The differences between the two agents, and between atmospheres of air or nitrogen were negligible. It was also found that the protection efficiencies of the compounds were pH dependent and showed maximum protection at pH 6. These results indicate that protection of DNA by AQ4 and AQ4N against radiation damage is an indirect effect since both agents are equally effective despite major differences in their DNA affinity. It is likely that radiation-induced phosphate buffer radicals are intercepted by AQ4 and AQ4N in a pH-dependent process.

Paramagnetic species in IC engine top ring zone lubricant samples

Abstract Samples of formulated lubricant have been obtained from the top piston ring zone of a standard operating petrol test engine, the Petter W-1. ESR spectra of samples frozen immediately and also those allowed to stand at room temperature show a range of paramagnetic features, some of which interconvert with change in temperature and sample history. The ESR spectra of any organic radicals are absent but a ‘radical’ centre is identified as arising from amorphous carbon. In contrast, various metal centres are identified, Fe III high-spin centres decaying to low-spin Fe III centres. C II and Mn centres are also identified with hyperfine coupling to 14 N and to Cu II . It is argued that the hyperfine interactions from ligands to these centres may result from decomposition products of the additive pack components, for example 14 N hyperfine interaction to the Cu II from the nitrogen-functionalized dispersant and sulphur-centred ligands generated by the decomposition of the zinc dialkyl dithiophosphate antioxidant/anti-wear additives.