XAFS Research on Biological Systems

X-ray absorption fine structure (XAFS) studies were performed on various protein systems, including concanavalin A, myoglobin and cytochrome c. The XAFS data were measured at beamline X11 at the National Synchrotron light Source. The UW XAFS programs AUTOBK and FEFFIT were employed for the data analysis. The results were compared with X-ray crystal diffraction results when they were available. The good agreement confirmed the reliability of the XAFS analysis. We discovered that the structure of some proteins can be dramatically changed on crystallization, for example, ZnCa concanavalin A; other proteins are more stable on crystallization, for example, NiCa concanavalin A and myoglobin. The local structure around heme in denatured cytochrome c has also been characterized by XAFS. We found that there is no significant difference in the Fe ligands between the native and the denatured ferrous cytochrome c. In the case of ferric cytochrome c, we found the S ligand of Fe in denatured ferric cytochrome c is replaced by either a N or an O. XAFS results were also used to estimated the entropy change of the reversible bonding of O_2 to hemerithrin; the results were close to the experiment results, which shows that the entropy change was mostly contributed from the thermal vibrations.