New data on the dynamics of protein hydration water
A theoretical and experimental study led by the UB researcher Giancarlo Franzese, published in the Proceedings of the National Academy of Sciences of the United States of America, reveals new thermodynamic properties of protein hydration water at low temperatures. The study, which was contributed to by researchers from Boston University, TU Berlin, the University of the Basque Country and Roma Tre University, identifies two specific heat maxima at low temperatures and ambient pressure, due in one case to fluctuations in the formation of hydrogen bonds and in a second case to the cooperative reordering of the hydrogen-bond network.
A theoretical and experimental study led by the UB researcher Giancarlo Franzese, published in the Proceedings of the National Academy of Sciences of the United States of America, reveals new thermodynamic properties of protein hydration water at low temperatures. The study, which was contributed to by researchers from Boston University, TU Berlin, the University of the Basque Country and Roma Tre University, identifies two specific heat maxima at low temperatures and ambient pressure, due in one case to fluctuations in the formation of hydrogen bonds and in a second case to the cooperative reordering of the hydrogen-bond network.
The results of the study, which recreates the conditions used in the cryopreservation of biomaterials and food products, shed light on the relationship of protein dynamics with surrounding water. According to Franzese, a tenured professor from the Department of Fundamental Physics at the UB, “One of the questions that remains unanswered, and is keenly debated, is how the dynamics of water are related to its thermodynamic properties when water is adsorbed on the surface of a protein. This is also relevant to understanding how water dynamics and protein dynamics affect one another.”
Water has proved to be a complex puzzle to many researchers due to its anomalous properties. In normal conditions water freezes at approximately -38 ºC, but protein hydration water ʻavoidsʼ this crystallization because each molecule binds to a protein. This has made it possible to conduct experiments at temperatures as low as -123 ºC with hydration water for the protein lysozyme, an enzyme found in bodily fluids such as saliva and tears, with the identification of two unexpected changes in the dynamic behaviour of water protons, one at -23 ºC and another at close to -93 ºC. In the study it was shown that these two changes are caused by a thermodynamic property of water, reflected by two isobaric specific heat maxima in its temperature dependence. Specific heat measures the capacity of water for energy exchange with its surroundings in the event of temperature changes, and the maxima correspond to the two temperatures at which this energy exchange is most effective.
The researchers were also able to determine the origin of these maxima: the first was caused by fluctuations in the formation of hydrogen bonds, leading to a change in the diffusivity of the water; the second, observed at a lower temperature, was due to the cooperative reordering of the hydrogen-bond network. According to Franzese, “Thanks to the efficient water model and simulation technique, we were able to compare the results of our experiments with theoretical calculations and simulations at very low temperatures and over long time periods, a formidable task that would be impossible with other models and simulation techniques. The characteristic we observed is a thermodynamic property of water, provided that crystallization can be inhibited.”
Article:
Marco G. Mazza, Kevin Stokely, Sara E. Pagnotta, Fabio Bruni, H. Eugene Stanley, and Giancarlo Franzese, “More than one dynamic crossover in protein hydration water”, Proceedings of the National Academy of Sciences of the United States of America. 28th November 2011. doi:10.1073/pnas.1104299108 (2011).