Pre-transition State Research Articles

Migration decision making and migration behavior in rural Kenya

Most micro theories of migration assume that all migrants have decided to move and all nonmigrants have decided not to move. The data for this study consisting of 1st round interviews of 1077 and 2nd round interviews (after 9 months) from 972 youths between 13 and 21 in 4 sublocations in rural Kenya call this assumption into question. They suggest that at a given time only about 35% of young people are contemplating migration and 1/3 of these ultimately decide to move. Also over the interval studied here the nondecision-processor migrants actually outnumbered decision processing migrants (defined as those cognitively aware of migration as an alternative who develop an expectation to migrate or not) so that only about 41% of migrants showed a clear indication of having made a decision. Similarly among the nonmigrants the data suggest that a decision not to move was made by close to 1/3. Neither a majority of migrants nor nonmigrants made the decision customarily attributed to them. A model was therefore developed to identify factors leading to migration decision making assuming that in the pretransitional state decisions are made for youth primarily by male family heads. As children become more exposed to extra-family influences they become increasingly aware of migration as an alternative. This model explained 26% of the variance in the decision-making variable. The data make it clear that understanding of migration as a process could be enhanced for those who decide and those who do not.

The three states criterion of nucleophilic reactivity in the conjugate acid of an anionic nucleophile and some problems of proton partitioning: implications for the reactions of 2,2′-dipyridyl disulphide with papain and with L-ergothioneine

The circumstances in which observation of three reactive protonic states of a reaction, involving an electrophile that increases its reactivity consequent on protonation, may be regarded as compelling evidence of nucleophilic character in the conjugate acid of an anionic nucleophile (”three states criterion“) are delineated. Aspects of the reaction of papain with 2,2′-dipyridyl disulphide not previously reported suggest that this reaction at pH 4 is best described as an intracomplex thiol-disulphide interchange involving the unionized thiol group of the cysteine-25-histidine-159-asparagine-175 hydrogen bonded system of papain with 2,2′-dipyridyl disulphide hydrogen bonded at one nitrogen atom to the carboxyl group of aspartic acid-158. The reaction appears to involve pre-transition state proton transfer in the thiol-imidazole hydrogen bond; the protonation of the side chains of aspartate-158 and histidine-159 may be positively cooperative. Rate equations for reactions involving up to three reactive protonic states are presented in an appendix.

Solute and Temperature Induced Pitch Changes and Pretransitional Effects in Cholesteric Liquid Crystals

The effect of non-chiralic solute molecules on the pitch of cholesteric mixtures of cholesteryl chloride and cholesteryl nonanoate has been studied as a function of the composition and the temperature. The occurrence of a pretransition to the smectic state at low CC content is established. In the pretransitional state the pitch decreases exponentially with increasing temperature and is extremely sensitive against the addition of most organic molecules. In the normal cholesteric state the pitch increases with increasing temperature. In this state only a very limited number of nonchiralic molecules exert an appreciable influence on the pitch.

Influence of Leaving-Group Electronic Effect on α-Chymotrypsin: Catalytic Constants of Specific Substrates

Rate constants and binding constants for the alpha-chymotrypsin-catalyzed hydrolysis of N-acetyltyrosine, tryptophan, and phenylalanine anilides are presented. Both k(cat) and K(m) are independent of electronic effects in the substrate over a range of 9.8 orders of magnitude (as measured by pK of the leaving group). Similarly, K(m) is independent of charge and orientation about the alpha-carbon for various substrates and pseudo-substrates. These results are not consistent with the pretransition state protonation hypothesis; instead, they are discussed in terms of a tetrahedral intermediate that is thermodynamically less stable than the Michaelis complex.

Streptococcal Proteinase-catalyzed Hydrolysis of Some Ester and Amide Substrates

Abstract Streptococcal proteinase was shown to belong to a group of enzymes such as trypsin, chymotrypsin, and subtilisin that exhibit a high ratio of esterase to peptidase activity. In this respect, streptococcal proteinase is different from other sulfhydryl enzymes, such as papain and ficin, and also different from pepsin. These enzymes hydrolyze a specific ester and amide substrate at comparable rates. A spectrophotometric method is described for following the streptococcal proteinase-catalyzed hydrolysis of substrates of the general type benzyloxycarbonyl-X-Y, where X is lysine, norleucine, and glutamic acid, and Y is phenyl, p-nitrophenyl, aniline, and p-nitroaniline. This method was used to demonstrate that streptococcal proteinase is an efficient esterase in the cleavage of phenyl and p-nitrophenyl esters, and that in the cleavage of these esters the enzyme is favored by the presence of a cationic group in X. Phenyl esters were found to be more versatile substrates than p-nitrophenyl esters for the proteinase, since autolysis of phenyl ester substrates was not observed even at higher pH values (pH 9.0). The effect of pH on the proteinase-catalyzed hydrolyses of benzyloxycarbonylysyl-p-nitrophenyl esters was examined. The data revealed that the ratio of log kcat/Km for the enzyme depends on an ionizable group with a pK of 4.8. When the leaving group, Y, in A-X-Y is changed from phenyl to p-nitrophenyl, kcat/Km values increase 5-fold. However, when Y is changed from aniline to p-nitroaniline, the kcat/Km value unexpectedly decreases 36-fold. Wang's theory of pretransition state protonation was used to interpret these data, and it is proposed that streptococcal proteinase hydrolyzes specific ester or amide substrates by different mechanisms. Tosylnorleucylphenylalanine and tosyllysylphenyl ester were found to be resistant to cleavage by streptococcal proteinase. These two derivatives are competitive inhibitors of streptococcal proteinase, with Ki values near the kinetically determined value of Km(app) for the corresponding benzyloxycarbonyl derivatives.

On the Mechanism of Action at the Acylation Step of the α-Chymotrypsin-catalyzed Hydrolysis of Anilides

Abstract The hydrolysis of a series of substituted anilides of N-acetyl-l-tyrosine by α-chymotrypsin has been investigated. Values of the kinetic parameters have been obtained in water and deuterium oxide solvent systems. It has been shown that the isotope effect decreases as the basicity of the substrate is increased. pH dependence studies have shown that the rate of hydrolysis, k2, is independent of pH above pH 7. The Michaelis-Menten constant, Km increases at both low and high pH values. The relative basicities of the substrates have been measured by titration of a series of model compounds, substituted acetanilides, in protonated and deuterated glacial acetic acid. In this way, it has been possible to examine quantitatively the relationship between the basicity of the substrate and the corresponding rate of α-chymotrypsin-catalyzed hydrolysis of the substrates. It has been shown that k2 increases with the basicity of the substrate in both water and deuterium oxide systems. This, together with the observation that the hydrogen isotope effect decreases with increasing basicity, suggests that the substrate is protonated before the transition state. The experimental data are consistent with a mechanism involving a pretransition state protonation equilibrium. The limitations of the present approximate treatment are discussed. It appears to be valid for the anilides studied and for peptides, but not for esters or substituted nitroanilides.

Some properties of liquid crystals

The present paper contains the results of measuring the velocity and absorption of ultrasonic waves in materials of the homologous series of dialkoxyazoxybenzene. The existence of pretransition states in liquid-mesophase and mesophase-mesophase temperature transitions was detected for the first time. Regular fluctuations of the velocity and absorption were noted. The ferroelectric properties of the nematic and smectic phases of para-normal octyloxybenzoic acid (domains, hysteresis effects) were discovered. Experimental data on the scattering of light by cholesteric-type liquid crystals and their mixtures were obtained. It is shown that the sequence and rate of change of specimen color depended strictly on temperature.