Proteins and water

Proteins and water

Simulated structure of hydrated collagen
Simulated structure of hydrated collagen

The biological functions of proteins require presence of a hydration shell – or, at least, of an appropriate solvation shell. We use NMR experiments and MD simulations to study the origin of this phenomenon. It is known that an interplay of protein and water dynamics is of great importance. We investigate this interplay for the proteins elastin and collagen, two main components of the connective tissue. The connective tissue adopts a large variety of biological functions in vertebrates. For example, in the form of cartilage, it is responsible for the dissipation of kinetic energy produced during the locomotion of a creature. To gain fundamental insights on a molecular level, it proved useful to analyze the temperature dependence of the protein and the water dynamics. Furthermore, we study in which way the properties of proteins change when water is replaced by another solvent. Such studies are of large relevance for, e.g., a cryopreservation of proteins.

NMR experiments on hydrated proteins

Temperature-dependent 13C MAS NMR spectra of dry and hydrated elastin
Temperature-dependent 13C MAS NMR spectra of dry and hydrated elastin

We exploit that, using D2O, protein and water dynamics can be characterized by means of 13C NMR and 2H NMR, respectively. The results of our 13C NMR study show that the presence of a hydration shell is indeed necessary for an activation of elastin dynamics. However, the enhanced mobility of the hydrated protein vanishes upon cooling in the vicinity of 200K. In the literature, it is vigorously discussed whether these changes of the protein dynamics are triggered by some changes of the water dynamics at such temperatures.

The hydration waters do not crystallize, enabling studies of the dynamical behaviors of water in a broad temperature range. In particular, it is possible to obtain insights into the dynamics of supercooled water in a temperature regime, which is not accessible for the bulk liquid due to interference of crystallization. The findings of our 2H NMR study demonstrate that the dynamics of supercooled protein hydration waters are governed by a broad distribution of correlation times, extending over several orders of magnitude. Thus, pronounced dynamical heterogeneities exist. Moreover, we find that, upon cooling, water motion develops some anisotropy in the vicinity of 200K. This is a hint that, near this temperature, long-range water diffusion, which is observed via field-gradient methods at higher temperatures, evolves into a localised water motion.

MD simulations on hydrated proteins

Probability distribution of the angular displacement of water during a comparable time interval at different temperatures
Probability distribution of the angular displacement of water during a comparable time interval at different temperatures

Our simulations show that a defined hydrogen-bond network develops on the surfaces of the proteins elastin and collagen upon cooling. Specifically, there is an increasing fraction of water molecules, which participate in exactly four hydrogen bonds. Simultaneously, the amplitudes of the protein dynamics decrease and the mechanism for the water dynamics changes from rotational diffusion to rotational jumps, which involve large angles. These observations demonstrate not only the interaction between local structure and local dynamics, but also the interplay between protein and water in the connective tissue.