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Rate Constant Values (rate + constant_value)

Distribution by Scientific Domains

Chemistry	50%

Selected Abstracts

Vitamin B2 -sensitized Photo-oxidation of Dopamine

PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 5 2008
Walter A. Massad
Kinetics and mechanism of the photo-oxidation of the natural catecholamine-type neurotransmitter dopamine (DA) has been studied in aqueous solution, under aerobic conditions, in the presence of riboflavin (Rf, vitamin B2) as a photosensitizer. Results indicate the formation of a weak dark complex Rf,DA, with a mean apparent association constant Kass = 30 m,1, only detectable at DA concentrations much higher than those employed in photochemical experiments. An intricate mechanism of competitive reactions operates upon photoirradiation. DA quenches excited singlet and triplet states of Rf, with rate constants of 4.2 × 109 and 2.2 × 109 m,1 s,1, respectively. With the catecholamine in a concentration similar to that of dissolved molecular oxygen in air-saturated water, DA and oxygen competitively quench the triplet excited state of Rf, generating superoxide radical anion (O2,,) and singlet molecular oxygen (O2(1,g)) by processes initiated by electron and energy-transfer mechanisms, respectively. Rate constants values of 1.9 × 108 and 6.6 × 106 m,1 s,1 have been obtained for the overall and reactive (chemical) interaction of DA with O2(1,g). The presence of superoxide dismutase increases both the observed rates of aerobic DA photo-oxidation and oxygen uptake, due to its known catalytic scavenging of O2,,, a species that could revert the overall photo-oxidation effect, according to the proposed reaction mechanism. As in most of the catecholamine oxidative processes described in the literature, aminochrome is the DA oxidation product upon visible light irradiation in the presence of Rf. It is generated with a quantum yield of 0.05. [source]

The role of Ni(II) in the oxidation of glycylglycine dipeptide by peroxomonosulfatex

INTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 1 2009
P. Thendral
The kinetics of oxidation of the dipeptide glycylglycine by peroxomonosulfate (PMS) was studied in the pH range of 3.42,5.89. The rate is first order in [PMS], glycylglycine concentration, and inverse first order in [H+]. The kinetic data suggest that SO2,5reacts, with glycylglycine, faster than HSO,5 by five orders of magnitude. The observed kinetics can be explained as due to the formation of an intermediate by the nucleophilic interaction of peroxide with the terminal protonated amine of glycylglycine, which then decomposes in the rate-limiting step to the product aldehyde. The thermodynamic parameters are evaluated. The reaction is catalyzed by Ni(II) ions only when pH , 4.63, and above this pH the rate is zero order with respect to [Ni(II)]. Perusal of the enhanced rate constant values suggests that the Ni-peroxide intermediate also reacts with glycylglycine. The Ni-dipeptide complex is not oxidized by PMS, and this complex in fact inhibits the formation of Ni-peroxide intermediate. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 41: 18,26, 2009 [source]

Evaluation of heat and oxidative damage during storage of processed tomato products.

JOURNAL OF THE SCIENCE OF FOOD AND AGRICULTURE, Issue 11 2002

Abstract The evolution of heat damage during storage of tomato pulp, puree and paste was studied by accelerated aging tests. Heat damage indices,5-hydroxymethyl-2-furfural (HMF), furosine and colour changes (,E),were evaluated for tomato products stored at 30, 40 and 50,°C for up to 90 days. Furosine and ,E values increased following pseudo-zero-order kinetics, and the higher the solid content of the products, the higher were the rate constant values. HMF formation followed pseudo-first-order kinetics in tomato pulp and pseudo-zero-order kinetics in puree and paste samples. Data show that heat damage reactions in tomato products proceed even at room temperature, and the kinetic model provided can be used to predict changes occurring during shelf-life. © 2002 Society of Chemical Industry [source]

Surface plasmon resonance study on binding interactions of multivalent cyclophane hosts with immobilized guests

POLYMERS FOR ADVANCED TECHNOLOGIES, Issue 2 2010
Osamu Hayashida
Abstract Guest-binding affinities of water-soluble cyclophane heptadecamer (1) and pentamer (2) with immobilized guests such as 1-pyrenylmethylamine (PMA) and 2-(1- naphthyl)ethylamine (NEA) were investigated by surface plasmon resonance (SPR) measurements. As a typical example, the binding constants (K) for 1 and 2 with the immobilized PMA as a guest were evaluated to be 2.5,×,107 and 2.7,×,106,M,1, respectively, and were much larger than that of a monocyclic reference cyclophane (K, 2.5,×,104,M,1). Interestingly, in the complexation of 1 and 2 with the immobilized guests, more favorable association and dissociation rate constant values (ka and kd, respectively) were observed in comparison with those for the monocyclic cyclophane, reflecting multivalent effects in macrocycles. The multivalent effects in macrocycles as well as molecular recognition abilities of the cyclophane oligomers were confirmed even when the guest molecules were immobilized on SPR sensor chip surfaces. Copyright © 2009 John Wiley & Sons, Ltd. [source]