X-RAY ABSORPTION SPECTRAL STUDY OF FERRIC HIGH-SPIN HEMOPROTEINS - XANES EVIDENCES FOR COORDINATION STRUCTURE OF THE HEME IRON

The Fe XANES (X-ray absorption near-edge structure) spectra were measured at 80 K for hemoproteins in a ferric high-spin state. As ferric hemoproteins, we chose sperm whale myoglobin (Mb), its derivative modified at the distal histidine (His) by cyanogen bromide (BrCN-Mb), Aplysia Mb, and horseradish peroxidase (HRP), which have different structures at the heme distal side. In comparison of the spectra among the three Mbs, we found that the spectral features, in particular the intensities of the preedge P peak and the fine structure C1 at the K-edge absorption, serve as a sensitive probe for the iron six-coordination structure; intense P and weak C1 correspond to five-coordinated iron, while weak P and intense C1 to six-coordination. Comparing the XANES spectrum of HRP with those of three Mbs, we suggested that the water molecule is absent at the sixth site of HRP, with pentacoordination of the heme iron. © 1990, American Chemical Society. All rights reserved.