Dynamics in Hydrogen-Bonded Systems

The relevance of hydrogen bonding was already well stated in 1939 by Linus Pauling in his famous book „The Nature of the Chemical Bond“: ‘...it will be found that the significance of the hydrogen bond for physiology is greater than that of any other single structural feature.’

A hydrogen bond is a simple structural motif that consists of a donor and at least one acceptor atom, X and Y, between which a hydrogen atom is located: X–H· · ·Y. The donor forms a covalent bond with the hydrogen atom whereas the interaction between the hydrogen atom and the acceptor is often considerably weaker. Despite its simplicity, the relevance of hydrogen bonds in nature can hardly be overestimated. It is a unique interaction that is directional and strong enough to create stable genetic code or rigid lever arms which allow a muscle to contract [3]. But it is also weak enough for the rigid double helix that contains the genetic code to open during cell division or a peptide to bend in the initial event of vision. And in combination with the simple tetrahedral charge structure of an H2O molecule, liquid water displays properties that are vital for life.

The two secondary structures in proteins that result from hydrogen bond formation – the α–helix (postulated by Pauling, Corey, and Branson in 1951), and the β–sheet (identified by Blake and coworkers in 1965) – are held together by hydrogen bonds between the C=O and the N–H groups of different amino acids and are essential for protein function. Another prominent example of hydrogen bonds in biology is the base pairing in deoxyribonucleic acid (DNA), the sequence that constitutes the genetic code. The double helical structure was predicted in 1953 by Watson and Crick from data taken by Rosalind Franklin. The multitude of such bonds along with stacking interactions and the helical geometry makes DNA and α-helices very rigid structures.

Hydrogen bonds are also relevant in protein-cofactor and enzyme-ligand interactions where they are broken and reformed in cyclic processes such as binding/unbinding events. Also, an important class of chemical reactions involve intra- and intermolecular proton and hydrogen transfer processes that are mediated by hydrogen bonds. Proton transfer occurs continuously in liquid water and is of great physiological importance in intra- and intercellular signalling pathways.

The Ultrafast Molecular Dynamics group seeks to understand hydrogen bonding with several static and time-resolved techniques capable of revealing various aspects of hydrogen bonding which differ in their spatial and temporal extent. While X-ray spectroscopy can probe the impact hydrogen bonding has on the valence charge distribution of individual atom species (such as Nitrogen and Oxygen), vibrational spectroscopy interrogates slower more extended correlated motions that extend over two atoms or several molecules. The combination of different techniques and excitation triggers will lead to a more comprehensive picture of this fascinating interaction which manifests in so many contexts.