Crowding effect on helix-coil transition: beyond entropic stabilization

We report circular dichroism measurements on the helix-coil transition of poly(L-glutamic acid) in solution with polyethylene glycol (PEG) as a crowding agent. Using small angle neutron scattering, PEG solutions have been characterized and found to be well described by the picture of a transient network of mesh size $\xi$, usual for semi-diluted chains in good solvent. We show that the increase of PEG concentration stabilizes the helices and increases the transition temperature. But more unexpectedly we also notice that the increase of crowding agent concentration reduces the mean helix extent at the transition, or in other words reduces its cooperative feature. This result cannot be accounted for by an entropic stabilization mechanism. Comparing the mean length of helices at the transition and the mesh size of the PEG network, our results strongly suggest two regimes: helices shorter or longer than the mesh size

Quantitative interpretation of anomalous adsorption effects in a critical binary mixture

By performing a numerical simulation of a previous experiment showing the existence of a critical anomaly of wetting layers in a binary mixture, we have shown that the data are consistent with the profile predicted by Fisher and de Gennes with an exponent in the range 0.3-0.6, as well as an exponential profile with an amplitude varying as tβ.Par simulation numérique d'une précédente expérience montrant l'existence d'une anomalie critique de la couche de mouillage dans un mélange binaire, nous avons montré que les données sont compatibles avec le profil prédit par Fisher et de Gennes avec un exposant compris entre 0.3 et 0.6, et aussi avec un profil exponentiel dont l'amplitude varie comme tβ

Coil-helix transition in poly(L-glutamic acid): Evidence for a 3-state non-cooperative process

A careful analysis of measurements of circular dichroism of poly(L-glutamic acid) (PGA) shows that the data can be very accurately described by introducing a third state for the PGA configuration, in addition to the helix and coil ones, and considering a simple equilibrium between these three states, without cooperativity. The third state is more conspicuous when high molecular weight polyethyleneglycol (PEG) is added. Excluded-volume effects shown by differences in the presence of short and long PEG chains suggest a direct interaction of PEG and PGA rather than an osmotic effect

SPECTRAL MEASUREMENTS OF THE DEPOLARIZED SCATTERED LIGHT FROM A BINARY MIXTURE NEAR ITS CRITICAL POINT

Nous présentons ici des mesures spectrales de la lumière dépolarisée diffusée par un mélange binaire (nitrobenzène et n-hexane) près de son point critique de démixion. La lumière diffusée dépolarisée est due principalement aux fluctuations d'orientation et aux rediffusions multiples. Un Fabry-Pérot de 60 GHz d'intervalle spectral libre permet de séparer spectralement ces deux contributions. La largeur de la raie de réorientation se comporte comme la viscosité. Suffisamment loin du point critique, la variation spatiale et la dépendance en température de l'intensité de diffusion multiple sont bien expliquées par un calcul invoquant un processus de double diffusion, en bon accord avec la récente théorie d'Oxtoby et Gelbart, à condition de prendre en compte le phénomène de turbidité.We report spectral measurements of the depolarized light scattered from a binary mixture (nitrobenzene and n-hexane) near its critical mixing point. The depolarized scattered light is mainly related to orientation fluctuations and multiple scattering. A spectral analysis performed with a 60 GHz free spectral range Fabry-Pérot spectrometer allows a rigorous separation of these two contributions to be made. The temperature dependance of the width of the reorientational spectrum is seen to behave in the same manner as the viscosity. Both the spatial and the temperature dependence of the multiple scattered light are well explained by a double scattering calculation when T is not too close to Tc, and are in good agreement with a recent theory given by Oxtoby and Gelbart if however one takes the turbidity into account

Test of the universality of the critical adsorption profile by neutron reflection

cited By 13International audienceUsing neutron reflectivity, we directly and self-consistently obtained all the parameters of the critical adsorption profile of three critical mixtures (n-hexane+perfluorohexane, deuterated methanol+cyclohexane and methanol+deuterated cydohexane) assuming only a Liu-Fisher form for the profile. The characteristic exponent and the amplitude ratio which can be deduced from the neutron spectra alone are in good agreement with one another, with ellipsometric data and with theoretical values. Owing to auxiliary measurements of coexistence curves and correlation lengths, we could also check the amplitude of the intermediate and distal parts of the profile with good agreements for the methanol cyclohexane mixtures only. The adsorbed amount is much greater in the n-hexane+perfluorohexane mixture where both the power-law amplitude and the characteristic length of the profile are affected