Time-dependent analysis of K2PtBr6 binding to lysozyme studied by protein powder and single crystal X-ray analysis

Multi heavy atom cluster compounds like K2PtBr6 offer a way forward to solve, de novo, unknown protein structures by powder diffraction involving dispersive (measured at two X-ray wavelengths) and isomorphous intensity differences as a complement to micro-crystallography or by using both approaches in combination. Towards this end, using the ESRF high resolution synchrotron X-ray powder diffraction beamline ID31, we have recorded high quality protein powder diffractograms at the platinum LIII and bromine K absorption edges, as well as reference wavelengths, for K2PtBr6 bound to lysozyme. These experiments were conducted at 80K to protect the sample against X-radiation damage as much as possible and also to fix the K2PtBr6 bound state, which seemed to show instability at room temperature. With multiple powder pattern analysis we extracted intensities and showed the PtBr6 bound in lysozyme using ‘omit electron density maps’. In addition the wavelength dispersive Fourier around the Br K edge shows up the bromine signal at PtBr6 binding site 1 in one of the six samples tested. To better understand the chemical properties of this heavy atom compound we have elucidated the detailed binding behaviour using single crystal analyses with time-resolved freeze quenching after soak times of 10, 90 and 170 minutes. Whilst the quick soaking of 10 to 30 minutes, used at ESRF ID31 shows clear binding there is increasing binding strength with increasing soak time. Thus, these time-resolved analytical chemistry results show that further heavy atom signal optimizations are possible. Prospects for extending our approach to the yet larger isomorphous and wavelength dispersive signal case of Ta6Br12 bound to lysozyme are also described.