Tracking Transient Conformational States of T4 Lysozyme at Room Temperature Combining X-ray Crystallography and Site-Directed Spin Labeling.

Proteins are dynamic molecules that can transiently adopt different conformational states. As the function of the system often depends critically on its conformational state a rigorous understanding of the correlation between structure, energetics and dynamics of the different accessible states is crucial. The biophysical characterization of such processes is, however, challenging as the excited states are often only marginally populated. We show that a combination of X-ray crystallography performed at 100 K as well as at room temperature and EPR spectroscopy on a spin-labeled single crystal allows to correlate the structures of the ground state and a thermally excited state with their thermodynamics using the variant 118R1 of T4 lysozyme as an example. In addition, it is shown that the surrounding solvent can significantly alter the energetic as well as the entropic contribution to the Gibbs free energy without major impact on the structure of both states.