Laboratory diffracted x-ray blinking to monitor picometer motions of protein molecules and application to crystalline materials.

Abstract

In recent years, real-time observations of molecules have been required to understand their behavior and function. To date, we have reported two different time-resolved observation methods: diffracted x-ray tracking and diffracted x-ray blinking (DXB). The former monitors the motion of diffracted spots derived from nanocrystals labeled onto target molecules, and the latter measures the fluctuation of the diffraction intensity that is highly correlated with the target molecular motion. However, these reports use a synchrotron x-ray source because of its high average flux, resulting in a high time resolution. Here, we used a laboratory x-ray source and DXB to measure the internal molecular dynamics of three different systems. The samples studied were bovine serum albumin (BSA) pinned onto a substrate, antifreeze protein (AFP) crystallized as a single crystal, and poly{2-(perfluorooctyl)ethyl acrylate} (PC₈FA) polymer between polyimide sheets. It was found that not only BSA but also AFP and PC₈FA molecules move in the systems. In addition, the molecular motion of AFP molecules was observed to increase with decreasing temperature. The rotational diffusion coefficients (D_R) of BSA, AFP, and PC₈FA were estimated to be 0.73 pm²/s, 0.65 pm²/s, and 3.29 pm²/s, respectively. Surprisingly, the D_R of the PC₈FA polymer was found to be the highest among the three samples. This is the first report that measures the molecular motion of a single protein crystal and polymer by using DXB with a laboratory x-ray source. This technique can be applied to any kind of crystal and crystalline polymer and provides atomic-order molecular information.