Kinetic Modeling of the X-ray-Induced Damage to a Metalloprotein

It is well-known that biological samples undergo X-ray-induced degradation. One of the fastest occurring X-ray-induced processes involves redox modifications (reduction or oxidation) of redox-active cofactors in proteins. Here we analyze room-temperature data on the photoreduction of Mn ions in the oxygen-evolving complex (OEC) of photosystem II, one of the most radiation damage-sensitive proteins and a key constituent of natural photosynthesis in plants, green algae, and cyanobacteria. Time-resolved X-ray emission spectroscopy with wavelength-dispersive detection was used to collect data on the progression of X-ray-induced damage. A kinetic model was developed to fit experimental results, and the rate constant for the reduction of OEC Mn-III and Mn-IV ions by solvated electrons was determined. From this model, the possible kinetics of X-ray-induced damage at a variety of experimental conditions, such as different rates of dose deposition as well as different excitation wavelengths, can be inferred. We observed a trend of increasing dosage threshold prior to the onset of X-ray-induced damage with increasing rates of dose deposition. This trend suggests that experimentation with higher rates of dose deposition is beneficial for measurements of biological samples sensitive to radiation damage, particularly at pink beam and X-ray free electron laser sources.