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R E Coffman

Publications and source records attributed to R E Coffman.

10 recordsLinked to original sources
PubMed1996

Self-association of nicotinamide in aqueous solution: light-scattering and vapor pressure osmometry studies.

Nicotinamide is a hydrotropic agent that has been reported to self-associate in aqueous solution. The objective of this study is to characterize the self-association of nicotinamide with regard to the extent of self-association as well as association constants using light-scattering and vapor pressure osmometry. Both methods allow calculation of association constants; however, while light-scattering measurements depend on the size of particles in solution, vapor pressure osmometry depends on the number of particles in solution. Using light-scattering, nicotinamide was found to associate primarily as dimers and trimers. Higher order aggregates can be characterized by an average aggregation number of 4.37. The association constants were 9.99 L/mol and 13.1 L/mol for dimerization and trimerization, respectively. From vapor pressure osmometry data were calculated a dimerization constant of 0.203 L/mol and a trimerization constant of 14.1 L/mol. In comparison, the trimerization constants are in good agreement, while the dimerization constants differ by an order of magnitude. Since light-scattering measurements are less reliable for small molecules like nicotinamide at low concentrations, it is felt that the dimerization constant calculated from vapor pressure osmometry is the more accurate.

Light↗
PubMed1996

Effect of nicotinamide and urea on the solubility of riboflavin in various solvents.

Hydrotropy is a solubilization process whereby addition of large amounts of a second solute results in an increase in the aqueous solubility of another solute. Past investigations have focused on the potential interaction of the hydrotropic agent with the solubilized solute. Conversely, this study proposes that at least some hydrotropic agents exert their solubilizing effect predominately by interacting with the solvent. To that end, the effect of two hydrotropic agents, nicotinamide and urea, on riboflavin solubility in aqueous and nonaqueous systems was examined. The term "solutropy" is introduced to describe solubilization by addition of large amounts of a second solute in any solvent. The nonaqueous solvents used included methanol, N-methylformamide, dimethyl sulfoxide, and acetone. In water, methanol, and N-methylformamide, riboflavin solubility was found to increase with increasing nicotinamide concentration; however, riboflavin solubility decreased with increasing nicotinamide concentration in dimethyl sulfoxide and acetone, thus establishing the solvent-dependent nature of solutropy. An examination of solvent properties revealed that the solvent's ability to be both a proton donor and acceptor is important mechanistically, while dielectric constant and polarity are not. The same solvent-dependency was observed with urea, although urea is a poorer solutrope than nicotinamide. This study proposes that some solutropic agents act by changing the nature of the solvent, specifically by altering the solvent's ability to participate in structure formation via intermolecular hydrogen bonding.

Niacinamide↗
PubMed1996

Hydrotropic solubilization--mechanistic studies.

PURPOSE: This study examines the mechanism of hydrotropic solubilization using the riboflavin-nicotinamide system. The most commonly proposed mechanism for hydrotropic solubilization is complexation, and therefore, is investigated. Additionally, since nicotinamide and several other hydrotropic agents self-associate in aqueous solution, the possibility that self-association of the hydrotropic agent is important mechanistically is examined by studying the effect of temperature on hydrotropic ability. Researchers have shown that the degree of self association decreases with increasing temperature. Therefore, if temperature affects the solubilizing capacity of nicotinamide, self-association must be mechanistically significant. METHODS: The complexation hypothesis is tested by looking at nicotinamide's ability to quench riboflavin fluorescence and by examining changes in the UV/Vis spectrum of riboflavin upon addition of nicotinamide. The solubility of riboflavin in nicotinamide solutions as a function of temperature is determined to assess the impact of self-association on hydrotropicity. RESULTS: Nicotinamide does not alter the intrinsic fluorescence of riboflavin nor are changes indicative of complexation observed in the UV/Vis spectrum Temperature does have an effect on the hydrotropic ability of nicotinamide. Specifically, as temperature increases, the solubilizing capacity of nicotinamide decreases. CONCLUSIONS: Because nicotinamide is unable to quench riboflavin fluorescence, and does not produce significant spectral changes, complexation of hicotinamide and riboflavin does not occur. However, since increasing temperature causes a decrease in the hydrotropic ability of nicotinamide and in its degree of self-association, it is proposed here that the self-association of nicotinamide impacts the hydrotropic mechanism.

Chemistry, Physical↗
PubMed1977

EPR determination of the Co(II)-free radical magnetic geometry of the "doublet" species arising in a coenzyme B-12-enzyme reaction.

The physical significance of the observed structure of the EPR signal, commonly known as the "doublet" spectrum, is that it contains information not only about the exchange coupling but also about the geometry of the magnetic dipole-dipole spin-spin coupling. We can show this because we have developed a general method of analysis applicable to this type of system and because we demand a quantitative fit of theory to experiment at two microwave frequencies. We have chosen the "doublet" free radical signal, which arises in the ribonucleotide reductase-5'-deoxyadenosylcobalamin system (from Lactobacillus leichmannii, see Hamilton et al., Biochemistry 11, 4696--4705 (1972)), for study, for the particular reason that the 35 GHz "doublet" spectrum has three components (in this case) rather than two, which provides an important test of the recently proposed model of isotropic exchange coupling by Schepler et al. ((1975) Biochim. Biophys. Acta 397, 510--518). We find that a quantitative fit to the EPR "doublet" lineshape can be obtained with a model of isotropic exchange, and a "point" magnetic dipole-dipole interaction acting over a distance of 9.9 A with the radical located approx. 34 degrees off the principal gzz axis and less than 1 degree off the principal gxx axis of the Co(II) in the corrin ring. Quantitative fits of the doublet portion of the observed lineshape at both 9 and 35 GHz were achieved with this model, assuming an axially symmetric free radical signal and a Gaussian spin-packet lineshape with isotropic linewidth.

Cobalt↗
PubMed1976

Analysis of the electron paramagnetic resonance spectrum of the cobalamin intermediate in ribonucleotide reduction.

EPR absorption-derivative lineshapes have been computed and least-squares fitted to the spectrum of the intermediate derived from 5'-deoxy-5'-adenosylcobalamin in the ribonucleotide reductase reaction. A Gaussian-type intrinsic lineshape was assumed and the effects of inhomogenous broadening, rotation of coordinate axes of the A-tensor relative to the g-tensor, angular dependence of transition probability and ligand hyperfine splitting have also been investigated. When the overall spectrum was computed as the sum of the lineshapes corresponding to two distinct Co(II) species, A and B, each having rhombic symmetry, the least squares procedure converged to a much better fit than with a single species, and matched almost all of the features of the experimental spectrum. The magnetic properties of A and B were compared with those of a series of other Co(II) complexes by a plot of g - g versus A - A. The results eliminate cobalt with 5-coordination to nitrogen for A and B, and suggest low-spin cobalt complexes having strongly distorted 6-fold coordination. The possibility that the sixth, symmetry-decreasing ligand is the oxygen molecule is excluded by the chemistry of the system and by the EPR properties of previously reported cob(II)alamins. It is suggested that the sixth ligand is a carbonyl, amide or sulfhydryl group of an enzyme sidechain which is inserted off-axis into the coordination position so as to exert the observed symmetry-lowering effect.

Binding Sites↗
PubMed1975

An investigation of the role of the copper in galactose oxidase.

Galactose oxidase is a metalloenzyme containing a single copper atom per molecule. The mechanism of action of galactose oxidase is studied in this paper by investigating substrate specificity and activation by peroxidase, and probing the copper site by electron spin resonance (ESR) spectroscopy. Line-shape simulation of ESR spectra are also reported and a comparison is made between observed and simulated spectra for galactose oxidase. A comparison is also reported for the enzyme from various commercial sources and enzyme isolated from a fungus in this laboratory. The results of this investigation suggest that the copper is in an environment of four in-plane nitrogens with axial symmetry.

Alcohol Oxidoreductases↗