The bacterial photoreceptor PYP has provided unique opportunities to study a biological sensing process in atomic detail. The structure of PYP in the ground state and the structure of two intermediates in the light cycle have been determined by time-resolved protein crystallography. While the similarity of the light cycle kinetics in the crystal and in solution has been established and the crystal structures of the intermediates explain many spectroscopic results from solution studies there has been a pesistent concern about the relevance of crystallographic results for processes in solution. Recent biochemical experiments have lead to the proposal that the structural changes associated with the light cycle are substantially larger in solution than those observed in the crystal. One study suggests that as much as 40% of the protein is tansiently unfolded during the light cycle. To gain direct evidence for the presence or absence of large scale protein structural changes during the PYP light cycle in solution we are carrying out millisecond time-resolve small angle solution X-ray scattering experiments of laser flash activated PYP samples at SSRl.
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