Biomedical subjects

Peter J Skrdla

Publications and source records attributed to Peter J Skrdla.

4 recordsLinked to original sources

PubMed2006

A collision theory-based derivation of semiempirical equations for modeling dispersive kinetics and their application to a mixed-phase crystal decomposition.

In recent works, the author has shown the utility of new, semiempirical kinetic model equations for treating dispersive chemical processes ranging from slow (minute/hour time scale) solid-state phase transformations to ultrafast (femtosecond) reactions in the gas phase. These two fundamental models (one for homogeneous/deceleratory sigmoidal conversion kinetics and the other for heterogeneous/acceleratory sigmoidal kinetics; isothermal conditions), based on the assumption of a "Maxwell-Boltzmann-like" distribution of molecular activation energies, provide a novel, quantum-based interpretation of the kinetics. As an extension to previous work, it is shown here that the derivation of these dispersive kinetic equations is supported by classical collision theory (i.e., for gas-phase applications). Furthermore, the successful application of the approach to the kinetic modeling of the solid-state decomposition of a binary system, CO2.C2H2, is demonstrated. Finally, the models derived appear to explain some of the (solid-state) kinetic data collected using isoconversional techniques such as those often reported in the thermal analysis literature.

Journal Article↗

PubMed2006

An HPLC chromatographic reactor approach for investigating the hydrolytic stability of a pharmaceutical compound.

The solution-phase hydrolysis kinetics of the Aprepitant (Emend) prodrug, Fosaprepitant Dimeglumine, were investigated using an HPLC chromatographic reactor approach. The term 'chromatographic reactor' refers to the use of an analytical-scale column as both a flow-through reactor and, simultaneously, as separation medium for the reactant(s) and product(s). Recently, we reported a novel mathematical treatment for the kinetic data obtained from chromatographic reactors, which we believe is superior to other treatments in terms of its accuracy, robustness and ease of implementation. In this work, we demonstrate that our treatment may be applied equally well to HPLC reactors, as previously we studied only GC reactors. It is found that the hydrolysis of Fosaprepitant Dimeglumine (FD) has an apparent activation energy of 107 kJ/mol when the reaction is investigated on-column, using the gradient elution conditions of the validated HPLC impurity profile method for this compound. For comparison, the activation energy determined for the same reaction occurring in a quiescent solution consisting of a fixed ratio of acetonitrile-0.1% v/v aqueous H3PO4 (50:50, v/v) is 91 kJ/mol, calculated using direct application of the Arrhenius equation. The data presented show that, when used as a screening tool, chromatographic reactors may be feasible for use in the pharmaceutical industry to quickly gauge the relative stabilities of various compounds with similar degradation pathways.

Chromatography, High Pressure Liquid↗

PubMed2005

Statistical kinetic approach for modeling lifespan.

Lifespan regulation through gene expression involves complex biochemical processes. Unfortunately, current mathematical models for treating lifespan data afford little insight into the mechanisms that control longevity. In this work, we demonstrate the use of a novel kinetic model to successfully fit the lifespan curves of the nematode, Caenorhabditis elegans. Our findings show that population aging may be treated analogously to a dispersive chemical process [P.J. Skrdla, R.T. Robertson., J. Phys. Chem. B 109 10611 (2005)]. Much like the Gompertz model, only two fit parameters, alpha and beta, are needed to adequately describe the entire data set for each nematode population. These parameters relate a 'global first-order time constant' and a 'global second-order rate constant', with units of (time) and (time)(-2), respectively. In C. elegans, the increased longevity resulting from DAF-16 (a transcription factor) activity in the intestinal tissue correlates with a larger alpha value and a smaller beta value; the opposite is true for animals with shorter lifespans. A basic physical interpretation of the two parameters is provided.

Aging↗

PubMed2005

Semiempirical equations for modeling solid-state kinetics based on a Maxwell-Boltzmann distribution of activation energies: applications to a polymorphic transformation under crystallization slurry conditions and to the thermal decomposition of AgMnO4 crystals.

Many solid-state reactions and phase transformations performed under isothermal conditions give rise to asymmetric, sigmoidally shaped conversion-time (x-t) profiles. The mathematical treatment of such curves, as well as their physical interpretation, is often challenging. In this work, the functional form of a Maxwell-Boltzmann (M-B) distribution is used to describe the distribution of activation energies for the reagent solids, which, when coupled with an integrated first-order rate expression, yields a novel semiempirical equation that may offer better success in the modeling of solid-state kinetics. In this approach, the Arrhenius equation is used to relate the distribution of activation energies to a corresponding distribution of rate constants for the individual molecules in the reagent solids. This distribution of molecular rate constants is then correlated to the (observable) reaction time in the derivation of the model equation. In addition to providing a versatile treatment for asymmetric, sigmoidal reaction curves, another key advantage of our equation over other models is that the start time of conversion is uniquely defined at t = 0. We demonstrate the ability of our simple, two-parameter equation to successfully model the experimental x-t data for the polymorphic transformation of a pharmaceutical compound under crystallization slurry (i.e., heterogeneous) conditions. Additionally, we use a modification of this equation to model the kinetics of a historically significant, homogeneous solid-state reaction: the thermal decomposition of AgMnO4 crystals. The potential broad applicability of our statistical (i.e., dispersive) kinetic approach makes it a potentially attractive alternative to existing models/approaches.

Chemistry↗