Polarized X-ray absorption near-edge structure spectroscopy of neuroglobin and myoglobin single crystals.

Polarized Fe K-edge X-ray absorption near-edge structure (XANES) spectra of murine carbonmonoxy-neuroglobin (NgbCO) single crystals have been collected and compared with a number of derivatives of sperm whale myoglobin (Mb), that is, the nitrosyl (MbNO) and deoxy (Mb) derivatives, the previously reported cyanomet (MbCN) and carbonmonoxy (MbCO) derivatives, and the cryogenic photoproduct of MbCO (Mb·CO). The single crystals under study exhibit a strong XANES angular dichroism which allows the heme geometry of each sample to be analyzed with extremely high accuracy via the full multiple scattering (MS) approach. The results of two alternative methods to undergo the MS analysis have been compared with high resolution X-ray diffraction (XRD) data and with X-ray absorption spectroscopy (XAS) data in solution. As a result of the present analysis, the Fe-heme structure in solution and in the cryo-trapped NgbCO single crystal (which cracks at room temperature) are the same. Accordingly, the residual energy involved in the protein relaxation responsible of crystal cracking at room temperature after CO binding does not reside in the heme pocket. A combined approach (polarized XANES and XRD) is suggested to be applied on the same single crystals of metalloproteins at opportunely equipped synchrotron beamlines.