First-order Rate Process Research Articles

Folding of staphylococcal nuclease: Kinetic studies of two processes in acid renaturation

The folding of acidified staphylococcal nuclease, upon neutralization, was studied in a stopped-flow spectrofluorometer by measuring the increase in tryptophanyl fluorescence during renaturation. At 25 °C and 0.1 ionic strength, the fluorescence change may be described by two first-order rate processes with half-times of 55 and 350 msec. No significant change in either rate was effected on varying the initial pH from 3.2 to 3.8 (corresponding to 0 to 50% levels of folding) or the initial ionic strength from 0.001 to 0.1. In contrast, the over-all rate of folding is dependent upon temperature. The half-time of the slower rate process decreases from 600 msec at 13 °C to 150 msec at 38 °C; the half-time of the faster rate process does not change significantly over this temperature range. These results suggest that the two processes may correspond to a sequence involving nucleation of ordered structure followed by the formation of hydrophobic interactions. Possible structural correlates of these processes are discussed with respect to the crystallographic model of this protein.

Hemolysis induced by Prymnesium parvum toxin kinetics and binding

Rates of hemolysis of rabbit erythrocytes induced by Prymnesium parvum toxin (prymnesin) have been measured colorimetrically at 25.5°. The data have been treated as consecutive first-order rate processes associated with the prolytic and lytic period for which two specific rate constants were obtained ( k′ and k ψ , respectively). Both are a function of concentration and temperature, though only the first constant has a linear Arrhenius relationship. The variation of the second constant with concentration of prymnesin follows Michaelis-Menten kinetics. Both constants are decreased by inhibitors (cholesterol and cephalin (approx. 10 μM)) at a given concentration of prymnesin, and the effect is ascribed to reduction in the effective concentration of prymnesin owing to the formation of a toxin-inhibitor complex. The binding of prymnesin to erythrocyte membranes during the prolytic period was investigated using tritium-labeled toxin, and it was found that about 10% of the labeled material is loosely bound and about 30% is more firmly bound.

Paramagnetic Resonance of Alkyl Radicals from Dissociative Electron Attachment in γ-Irradiated Organic Glass

The radicals CH3·, CH3CH2·, CH3CH2CH2·, (CH3)3C·, (CH3)3CCH2·, and C6H5ĊHCH3 have been identified by EPR in γ-irradiated dilute solutions of the corresponding halides in 3-methylpentane at 77°K. For 0.4 mole % CH3I, G(CH3·)=1.2. Methyl radicals disappear by some first-order rate process for which k=4.6×10−4 sec−1. Other radicals did not decay measurably under similar conditions. The results are consistent with the mechanism of dissociative thermal electron attachment, RX+e−→R·+X−, which is an efficient, selective process whenever the electron affinity of X exceeds the bond dissociation energy of R—X.

Drug Partitioning II: In Vitro Model for Drug Absorption

An in vitro model to simulate some factors involved in the absorption process is described. It consists of a tube containing two aqueous phases separated by an immiscible phase. A rocking apparatus agitates the fluids while causing the liquid interfaces to expand and contract. Rates of drug transfer and equilibrium drug distribution were determined under conditions where one aqueous phase was maintained at pH 7.4 and the other buffered at various pH values. Salicylic acid, barbital, antipyrine, aminopyrine, and tetracycline were studied in this manner. The initial drug transfer simulated a first-order rate process. Results of the equilibrium studies are in general agreement with predictions of the pH-partition theory. Tetracycline did not undergo transfer from one aqueous phase to the other at any pH condition of the study.

Influence of Longitudinal Dispersion on Tray Efficiency

Mass-transfer process on a tray has been discussed theoretically, together with the influence of longitudinal dispersion of vapor (gas) and liquid phases. Reducing the overall mass-transfer behavior of the system to that of a continuous flow system with a homogeneous first-order rate process (Eqs. 7-10), a general relation has been derived to give the rate of mass-transfer (Eq. 15) and the tray efficiency (Eq. 18). In the derivation, the concept of a "model" has been eliminated entirely. (Sect. 1).From this general standpoint the theories presented previously by various authors have been proved to differ from one another only in the process of approximating Ex(φ) included in Eq. 15, by an alternative function of φ obtained from the behavior of a proper model adopted. All these theories developed before have been generalized, applying suitable functions for Ex(φ). It is concluded that three kinds of figures as stated in 1-5-B are enough to determine the tray efficiency to include the combined effect of longitudinal dispersion of each phase and other variables. (Sect. 2).

The proton dissociation and hydrolysis in trinitratonitrosylruthenium in aqueous solutions

Electrical conductivity measurements on solutions of the trinitratonitrosylruthenium complex in water indicate that it is an acid for which the first dissociation constant has a value of 0·014 mole litre −1. The partition coefficient of the complex between solutions in water and in tri- n-butyl phosphate (TBP) has been calculated from its observed distribution between these solvents and the degree of dissociation of the complex in aqueous solutions. The hydrolysis of the complex in nitric acid solutions has been shown to occur initially by a first-order rate process with a rate constant k = 0·046 min −1 at 25°C.