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Ligand binding kinetics to heme-proteins between 40 and 300 K point to a regulatory role of protein dynamics. A protein-specific susceptibility of the heme-iron reactivity to dynamic fluctuations emerges from the distribution of reaction enthalpies derived from flash-photolysis measurements below ca. 180 K; we quantify it in terms of 'intramolecular viscosity', postulating that narrow low-temperature enthalpy distributions correspond to low internal viscosity and vice versa. The thermal evolution of ligand binding kinetics suggests, with other results, an interplay between high-frequency transitions of the amino acid side chains and low-frequency collective motions as a possible regulatory mechanism of protein dynamics.

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.

The replacement of O2 with CO was studied on cobalt-iron hemoglobin hybrids. Both proto- and mesocobalt hemes were used for the reconstitution. In the oxy quaternary conformation no difference is observed between alpha- and beta-subunits when only proto hemes are present in the hybrid (k4 = 30 s-1, k'4/l'4 = 2.5). If Co-meso heme is present on the beta-chains the binding properties of the partner subunit are modified (k'4/l'4 = 4).