Electron paramagnetic resonance properties of liver fluke (Dicrocoelium dendriticum) nitrosyl hemoglobin

The electron paramagnetic resonance properties of the nitric oxide derivative of liver fluke (Dicrocoelium dendriticum) hemoglobin (DD-Hb) have been investigated in the pH range from 4.8 to 7.8. In the neutral and alkaline regions the spectra have a rhombic shape, with gx = 2.09, gy = 1.99 and gz = 2.009, and a triplet hyperfine structure of 2.2 mT, due to the nitrogen of the bound NO molecule, in the center resonance. No superhyperfine lines in the gz region, related to the interaction of the iron with the proximal histidine, are detected, suggesting a large distance between the metal and the N epsilon of the imidazole. By lowering the pH the EPR spectrum undergoes a reversible change showing a 3-line pattern in the high-field region. Such a spectrum is fully formed at pH 4.8 and is interpreted in terms of a dissociation of the proximal histidine from the heme iron.