Identification of two-step chemical mechanisms using small temperature oscillations and a single tagged species.

In order to identify two-step chemical mechanisms, we propose a method based on a small temperature modulation and on the analysis of the concentration oscillations of a single tagged species involved in the first step. The thermokinetic parameters of the first reaction step are first determined. Then, we build test functions that are constant only if the chemical system actually possesses some assumed two-step mechanism. Next, if the test functions plotted using experimental data are actually even, the mechanism is attributed and the obtained constant values provide the rate constants and enthalpy of reaction of the second step. The advantage of the protocol is to use the first step as a probe reaction to reveal the dynamics of the second step, which can hence be relieved of any tagging. The protocol is anticipated to apply to many mechanisms of biological relevance. As far as ligand binding is considered, our approach can address receptor conformational changes or dimerization as well as competition with or modulation by a second partner. The method can also be used to screen libraries of untagged compounds, relying on a tracer whose concentration can be spectroscopically monitored.

Electro-hydrodynamic instability of stressed viscoelastic polymer films.

We study the stability of a viscoelastic thin polymer film under two destabilization factors: the application of an electric field normal to the surface--as in typical electro-hydrodynamic destabilization experiments--and the presence of a frozen-in internal residual stress, stemming from the preparation process of the film, typically spin-coating. At the film-substrate interface we consider a general boundary condition, containing perfect gliding on slippery substrates, as well as perfect sticking of the film to the substrate as limiting cases. We show that the interplay of the two sources of stress, the viscoelasticity and the boundary condition, leads to a rich behavior, especially as far as the fastest growing wave number (or wavelength) is concerned. The latter determines the initial growth of the instability, and often also the final pattern obtained in small capacitor gaps, and is the main experimental observable.

Identification of two-step chemical mechanisms and determination of thermokinetic parameters using frequency responses to small temperature oscillations.

Increased focus on kinetic signatures in biology, coupled with the lack of simple tools for chemical dynamics characterization, lead us to develop an efficient method for mechanism identification. A small thermal modulation is used to reveal chemical dynamics, which makes the technique compatible with in cellulo imaging. Then, the detection of concentration oscillations in an appropriate frequency range followed by a judicious analytical treatment of the data is sufficient to determine the number of chemical characteristic times, the reaction mechanism, and the full set of associated rate constants and enthalpies of reaction. To illustrate the scope of the method, dimeric protein folding is chosen as a biologically relevant example of nonlinear mechanism with one or two characteristic times.