Fast changes in the structure of a protein on photoexcitation have implications for the elucidation of many important processes in nature, such as photosynthesis and other photo-induced chemical reactions in proteins. Changes accompanying protein crystallization can be experimentally followed by the technique of time-resolved X-ray crystallography, which involves the measurement and indexing of Bragg spots. However, not all proteins (particularly membrane proteins) involved in many of the crucial processes of life, form crystals. In addition, reactions undergone by proteins in a crystal may not mirror those of proteins in living systems. As such, an ability to study experimentally photo-induced chemical reactions of proteins in solution would constitute a significant breakthrough. The objective of this project is to develop a valid theoretical method for extracting structural information from a pump-probe experiment designed to follow structural changes of single molecules immediately after photoexcitation. This project will demonstrate this capability through measurements of diffuse diffraction patterns from an X-ray free electron laser (XFEL) at the Linac Coherent Light Source (LCLS). The diffraction pattern will be obtained from a sample of liquid droplets, containing ensembles of identical proteins, before and after photoexcitation. The photoexciting beam will be a laser, synchronized with the XFEL pulses, which will excite the molecules a short time before they are interrogated by the XFEL beam. The ability to take advantage of larger signals from ensembles of molecules will be developed, for ensembles of randomly oriented molecules, by the evaluation of angular correlations of the diffraction data. The idea will be demonstrated initially with data from computer simulations of such diffraction patterns and followed by demonstrations with measured experimental diffraction patterns. The latter work will involve collaboration with Prof. John Spence of Arizona State University, who has experience of protein structure work with the XFEL at the Linac Coherent Light Source (LCLS) in Stanford, CA.
Broader Impacts
Undergraduates recruited with the help of the UW-Milwaukee's undergraduate research opportunity program (UROP) will be integrated into the research efforts. Cutting-edge techniques of structural biology will be introduced into high-school classrooms with the help of K-12 teachers recruited to the campus by the UW-Milwaukee's NSF-funded research experience for teachers (RET) program. Teachers will first participate in research efforts with the principal investigators and transfer what they learn into their classrooms. A newly designed web-page will increase the visibility and impact of the research efforts.
