SPECIAL SECTION: CURRENT SCIENCE, VOL. 101, NO. 7, 10 OCTOBER 2011 900 *For correspondence. (e-mail: bbagchi@sscu.iisc.ernet.in) Dynamic and thermodynamic anomalies of water at low temperatures: from bulk water to reverse micelles and DNA hydration layer Biman Jana, Rakesh S. Singh, Rajib Biswas and Biman Bagchi* Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012, India Liquid water is known to exhibit remarkable thermo- dynamic and dynamic anomalies, ranging from solva- tion properties in supercritical state to an apparent divergence of the linear response functions at a low temperature. Anomalies in various dynamic proper- ties of water have also been observed in the hydration layer of proteins, DNA grooves and inside the nano- cavity, such as reverse micelles and nanotubes. Here we report studies on the molecular origin of these anomalies in supercooled water, in the grooves of DNA double helix and reverse micelles. The anomalies have been discussed in terms of growing correlation length and intermittent population fluctuation of 4- and 5-coordinated species. We establish correlation between thermodynamic response functions and mean squared species number fluctuation. Lifetime analysis of 4- and 5-coordinated species reveals interesting differences between the role of the two species in supercooled and constrained water. The nature and manifestations of the apparent and much discussed liquid–liquid transition under confinement are found to be markedly different from that in the bulk. We find an interesting ‘faster than bulk’ relaxation in re- verse micelles which we attribute to frustration effects created by competition between the correlations im- posed by surface interactions and that imposed by hy- drogen bond network of water. Keywords: Dynamic and thermodynamic anomalies, DNA hydration layer, reverse micelles. Introduction LIQUID water continues to draw immense attention from scientists working in all branches of natural sciences. Dif- ferent groups remain interested in different aspects of this liquid. Although some study ‘structured water’ in micelles and reverse micelles 1–3 , some study its properties in protein and DNA hydration layers 1,5–14 , there are still others who study it in carbon nanotubes 15 , while for some attention remains focused on electrolyte solutions 16 , and for many bulk water itself poses unlimited puzzles to keep them engaged over a lifetime. Many books and reviews have been devoted to water and their number seems to be increasing every year 17–21 . One can list the large number of anomalies that water exhibits in different environments. The list continues to grow. More recently a considerable amount of attention has been devoted to water at temperatures below the freezing temperature of 273 K 22,23 . Not only does water at such a low temperature exhibit certain amazing anoma- lies, it is also hoped that this can serve as a key to under- stand properties at higher temperatures. Notable anomalies are the divergent-like growth of the linear response func- tions (heat capacity, isothermal compressibility, etc.) 22,23 . Several explanations of these anomalies have been put forward. The explanations in terms of the re-crossing of the gas–liquid spinodal at lower temperature (spinodal retracing) 24 and in terms of the negative slope of the tem- perature maximum density line in the (P, T) plane (singu- larity-free scenario) 25 are only partly successful in explaining the experimental results. The most recent and promising approach in this area is in terms of liquid– liquid phase transition 26–29 . In this scenario, water is hypothesized to exhibit a phase transition in the region of deeply supercooled water, at higher than ambient pre- ssure, from high-density liquid (HDL) to low-density liquid (LDL). This liquid–liquid critical point in water is estimated to be located at a pressure P ≈ 100 MPa and temperature T ≈ 220 K 28 . Beyond this critical point, one can draw a line of the maximum density fluctuation in (P, T) plane owing to the remnant effect of the criticality and such a line is termed as Widom line 27,29 . The anoma- lous increase in the response functions upon cooling at the ambient pressure can be a manifestation of the cross- ing of the Widom line 29 . This hypothesis not only explains the response functions anomalies of water but also explains the polymorphism of glassy water and seems at present to be the most consistent with experiments 30–32 . Moreover, liquid–liquid (L–L) phase transition scenario is also consistent with the presence of a crossover in the dynamics found in recent experiments in the nano- confined water 33,34 . The molecular origin of anomalous behaviour near the Widom line is well-understood in several cases. For example, in the gas–liquid case, it is the correlated den-