Reports

Dynamics is an essential aspect of protein structure and function. This was recognized early on when the first pro-tein crystal structure, of the oxygen-storage protein myoglo-bin (Mb), was solved. The active site was found to be shielded from solvent and no path for a ligand to enter or leave the binding site at the heme iron was apparent. Thus, structural dynamics of the protein matrix must control ac-cess to the active site as well as heme chemistry. Ligand binding to and dissociation from myoglobin have been stud-ied in great detail using numerous approaches, exploiting the fact that the Fe-carbon monoxide (CO) bond is light sen-sitive and can be cleaved with very high quantum yield (1– 7). In synchrotron-based time-resolved x-ray crystallography, the time-resolution was however limited by the 100 ps dura-tion of the x-ray pulse (8) and the structural changes ema