In various chemical systems enthalpy-entropy compensation (EEC) is a well-known rule of behavior, although the physical roots of it are still not completely understood. It has been frequently questioned whether EEC is a truly physical phenomenon or a coincidence due to trivial mathematical connections between statistical-mechanical parameters or even simpler, a phantom effect resulting from the misinterpretation of experimental data. Here, we review EEC from a new standpoint and conclude that it may be rationalized in terms of hidden but physically real factors implying a Carnot-cycle model in which a micro-phase transition (MPT) plays a crucial role. Examples of such MPTs underlying physically valid EEC should be typically cooperative processes in supramolecular aggregates, like changes of structured water at hydrophobic surfaces, conformational transitions upon ligand-biopolymer binding, and so forth. The MPT notion could help rationalize the occurrence of EEC in connection with hydration and folding of proteins, functioning of molecular motors, and similar phenomena.

We present theoretical work on the electronic states in a model DNA double helix of poly(dA)−poly(dT) (10 base pairs) as the molecule undergoes thermal fluctuations at room temperature. We couple state-of-the art empirical force field molecular dynamics (MD) simulations with an ab initio tight-binding formalism based on density-functional theory [Lewis et al. Phys. Rev. B 2001, 64, 195103−1]. The dynamical features of the charge density distributions and the electronic structure are presented. The periodic structure exhibits extended HOMO−LUMO electronic states; however, equivalent states are quite localized in the aperiodic structures generated as snapshots from the MD simulation. Our results show strong Anderson localization in DNA as a result of the disorder due to structural changes promoted by the thermal fluctuations.

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