Nuclear Magnetic Resonance Spectroscopy of Proteins - Dynamics

Dynamics

In addition to structures, nuclear magnetic resonance can yield information on the dynamics of various parts of the protein. This usually involves measuring relaxation times such as T₁ and T₂ to determine order parameters, correlation times, and chemical exchange rates. NMR relaxation is a consequence of local fluctuating magnetic fields within a molecule. Local fluctuating magnetic fields are generated by molecular motions. In this way measurements of relaxation times can provide information of motions within a molecule on the atomic level. In NMR studies of protein dynamics the nitrogen-15 isotope is the preferred nucleus to study because its relaxation times are relatively simple to relate to molecular motions This however requires isotope labeling of the protein. The T₁ and T₂ relaxation times can be measured using various types of HSQC based experiments. The types of motions which can be detected are motions that occur on a time-scale ranging from about 10 picoseconds to about 10 nanoseconds. In addition slower motions, which take place on a time-scale ranging from about 10 microseconds to 100 milliseconds, can also be studied. However, since nitrogen atoms are mainly found in the backbone of a protein, the results mainly reflect the motions of the backbone, which is the most rigid part of a protein molecule. Thus, the results obtained from nitrogen-15 relaxation measurements may not be representative for the whole protein. Therefore techniques utilising relaxation measurements of carbon-13 and deuterium have recently been developed, which enables systematic studies of motions of the amino acid side chains in proteins. A challenging and special case of study regarding dynamics and flexibility of peptides and full length proteins is represented by disordered structures. Nowadays it is an accepted concept that proteins can exhibit a more flexible behaviour known as disorder or lack of structure, however, it is possible to describe an ensemble of structures instead of a static picture representing a fully functional state of the protein. Many advances are represented in this field particularly in terms of new pulse sequences, technological improvement, and rigorous training of researchers in the field.