Variable pH kinetics: an easy determination of pH-rate profile

Reversible photonic hydrogel sensors via holographic interference lithography

Continuous monitoring of physiological conditions and biomarkers via optical holographic sensors is an area of growing interest to facilitate the expansion of personalised medicine. Here, a facile laser-induced dual polymerization method is developed to fabricate holographic hydrogel sensors for the continuous and reversible colorimetric determination of pH variations over a physiological range in serum (pH 7-9). Readout parameters simulated through a Finite-difference time-domain Yee's algorithm retrieve the spectral response through expansion. Laser lithography of holographic hydrogel sensor fabrication is achieved via a single 355 nm laser pulse to initiate polymerization of ultrafine hydrogel fringes. Eliminating the requirement for complex processing of toxic components and streamlining the synthetic procedure provides a simpler route to mass production. Optimised pH-responsive hydrogels contain amine bearing functional co-monomers demonstrating reversible Bragg wavelength shifts of 172 nm across the entire visible wavelength range with pH variation from 7.0 to 9.0 upon illumination with broadband light. Photolithographic recording of information shows the ability to convey detailed information to users for qualitative identification of pH. Holographic sensor reversibility over 20 cycles showed minimal variation in replay wavelength supporting reliable and consistent readout, with optimised sensors showing rapid response times of <5 min. The developed sensors demonstrate the application to continuous monitoring in biological fluids, withstanding interference from electrolytes, saccharides, and proteins colorimetrically identifying bovine serum pH over a physiological range. The holographic sensors benefit point-of-care pH analysis of biological analytes which could be applied to the identification of blood gas disorders and wound regeneration monitoring through colorimetric readouts.

Unimolecular, Bimolecular, and Intramolecular Hydrolysis Mechanisms of 4-Nitrophenyl β-d-Glucopyranoside

1,2-trans-Glycosides hydrolyze through different mechanisms at different pH values, but systematic studies are lacking. Here, we report the pH-rate constant profile for the hydrolysis of 4-nitrophenyl β-D-glucoside. An inverse kinetic isotope effect of k(H₃O⁺)/k(D₃O⁺) = 0.65 in the acidic region indicates that the mechanism requires the formation of the conjugate acid of the substrate for the reaction to proceed, with the heterolytic cleavage of the glycosidic C-O bond. Reactions in the pH-independent region exhibit general catalysis with a single proton in flight, a normal solvent isotope effect of k_H/k_D = 1.5, and when extrapolated to zero buffer concentration show a small solvent isotope effect of k(H₂O)/k(D₂O) = 1.1, consistent with water attack through a dissociative mechanism. In the basic region, solvolysis in ¹⁸O-labeled water and H₂O/MeOH mixtures allowed the detection of bimolecular hydrolysis and neighboring group participation, with a minor contribution of nucleophilic aromatic substitution. Under mildly basic conditions, a bimolecular concerted mechanism is implicated through an inverse solvent isotope effect of k(HO^-)/k(DO^-) = 0.5 and a strongly negative entropy of activation (ΔS^‡ = -13.6 cal mol^-1 K^-1). Finally, at high pH, an inverse solvent isotope effect of k(HO^-)/k(DO^-) = 0.5 indicates that the formation of 1,2-anhydrosugar is the rate-determining step.

Kinetic buffers

This paper proposes a new type of molecular device that is able to act as an inverse proton sponge to slowly decrease the pH inside a reaction vessel. This makes the automatic monitoring of the concentration of pH-sensitive systems possible. The device is a composite formed of an alkyl chloride, which kinetically produces acidity, and a buffer that thermodynamically modulates the variation in pH value. Profiles of pH versus time (pH-t plots) have been generated under various experimental conditions by computer simulation, and the device has been tested by carrying out automatic spectrophotometric titrations, without using an autoburette. To underline the wide variety of possible applications, this new system has been used to realize and monitor HCl uptake by a di-copper(II) bistren complex in a single run, in a completely automatic experiment.

The influence of pH on the stability of antazoline: kinetic analysis

Degradation of the drug antazoline was studied in aqueous solutions by means of pH-rate profiling (pH 0–7.4).

β-Hydrogen Kinetic Effect

A combined kinetic and DFT study of the uncatalyzed isomerization of cationic solvent complexes of the type cis-[Pt(R')(S)(PR3)2]+ (R' = linear and branched alkyls or aryls and S = solvents) to their trans isomers has shown that the reaction goes through the rate-determining dissociative loss of the weakly bonded molecule of the solvent and the interconversion of two geometrically distinct T-shaped 14-electron three-coordinate intermediates. The Pt-S dissociation energy is strongly dependent on the coordinating properties of S and independent of the nature of R'. The energy barrier for the fluxional motion of [Pt(R')(PR3)2]+ is comparatively much lower ( approximately 8-21 kJ mol-1). The presence of beta-hydrogens on the alkyl chain (R' = Et, Prn, and Bun) produces a great acceleration of the reaction rate. This accelerating effect has been defined as the beta-hydrogen kinetic effect, and it is a consequence of the stabilization of the transition state and of the cis-like three-coordinate [Pt(R')(PR3)2]+ intermediate through an incipient agostic interaction. The DFT optimization of [Pt(R')(PMe3)2]+ (R' = Et, Prn, and Bun) reproduces a classical dihapto Pt....eta2-HC agostic mode between the unsaturated metal and a dangling C-H bond. The value of the agostic stabilization energy (in the range of approximately 21-33 kJ mol-1) was estimated by both kinetic and computational data and resulted in being independent of the length of the hydrocarbon chain of the organic moiety. A better understanding of such interactions in elusive reaction intermediates is of primary importance in the control of reaction pathways, especially for alkane activation by metal complexes.

Fast drug stability determination by LC variable-parameter kinetic experiments

Variable-parameter kinetic experiments were carried out using HPLC as analytical instrument. The hydrolysis of aspirin was followed both at variable-temperature and at variable-pH conditions. The peak areas relative to salicylic acid were processed by direct fit to a mathematical model and/or by differential method obtaining, by single experiments, the values of the apparent rate constant in the whole range of temperature and pH studied. The results, although the discontinuity of this kind of analysis, are in agreement with those obtained by constant-parameter kinetics but saving experimental time.

Variable-ionic strength kinetic experiments in drug stability studies

The dependence of the pseudo-first-order rate constant on the ionic strength for the alkaline hydrolysis of indomethacin has been obtained, for the first time, in a single kinetic experiment carried out by varying with time the salt concentration inside the reaction vessel. The kinetic profile obtained was processed using as a mathematical model the variable-parameter kinetic equation containing the Bronsted-Bjerrum equation as the dependence function. The results are in good agreement with those obtained by the traditional method but the experimental time is reduced to about one-tenth.

Temperature-rate profiles by polarimetric variable-temperature kinetic experiments to study racemization reactions

The racemization of (-)-adrenaline was followed by polarimetric variable-temperature kinetic experiments obtaining activation parameters and k(obs)(T) profile in one tenth of the time usually spent for traditional kinetic runs. A polarimeter connected to a computer for the acquisition and processing of the analytical data was used. The kinetic profiles were processed by both an integral method and a differential method. The results are in good agreement with each other and with those obtained by constant-temperature kinetics.