Statistical mechanics of Monte Carlo sampling and the sign problem

Monte Carlo sampling of any system may be analyzed in terms of an associated glass model -- a variant of the Random Energy Model -- with, whenever there is a sign problem, complex fields. This model has three types of phases (liquid, frozen and `chaotic'), as is characteristic of glass models with complex parameters. Only the liquid one yields the correct answers for the original problem, and the task is to design the simulation to stay inside it. The statistical convergence of the sampling to the correct expectation values may be studied in these terms, yielding a general lower bound for the computer time as a function of the free energy difference between the true system, and a reference one. In this way, importance-sampling strategies may be optimized

Citrus flavanones enhance carotenoid uptake by human intestinal Caco-2 cells

The health benefit of a diet rich in fruits and vegetables could be attributed to the complex mixture of phytochemicals. It is now widely believed that the actions of the antioxidant microconstituents is a result of additive and/or synergistic effects of these phytochemicals present in whole food. Because citrus juices are considered as a rich source of antioxidants including ascorbic acid (or vitamin C), phenolics compounds, and carotenoids, these different molecules may affect bioavailability or intestinal absorption of each other microconstituent. For these reasons, our study focused on effects of flavonoids and acid ascorbic on intestinal carotenoid uptake. This study was conducted using the differentiated Caco-2 cellline as experimental in vitro model and interactions of different flavanones such as hesperidin (HES-G) and hesperetin (HES) with carotenoid uptake were examined. Effect of ascorbic acid (AA) added to HES-G was also investigated. The data showed an enhancing effect of HES-G and HES on ?-carotene (b-C) and ?-cryptoxanthin (b-CX) uptake. For instance, at 5h incubation in presence of a mixture b-C:b-CX, HES-G and HES significantly increased total carotenoid uptake by 1.7 and 1.6-fold, respectively. Moreover, AA was able to cancel the enhancing effect of HES-G by decreasing significantly the cellular uptake of carotenoids from 48.2 to 39.8 % (P<0.05). In order to attribute the enhancing effect of HES-G to its already known iron-chelating effect, another experiment was conducted by incubating cells with b-CX in presence of either iron or a metal chelator (deferoxamine). b-CX uptake decreases in presence of iron and increases in presence of deferoxamine In sum, the results indicate that citrus flavanones enhance the carotenoid uptake by intestinal cells and that iron inhibits this process. Thus, the present data suggest that the citrus polyphenols could act through their iron-chelating properties. (Texte intégral