THE HYDRATION STRUCTURE STUDIED BY TSDC - MYOGLOBIN AND HEMOGLOBIN

The coordination scheme of water in myoglobin and haemoglobin, the two oxygen carrier proteins in vertebrates, is still far from being fully understood. The TSDC (thermally stimulated depolarization currents) t echnique is a sensitive tool to study water molecules bound to biologi cal macromolecules and characterized by mobility and order degree, qui te different from those of bulk water. TSDC spectra for both proteins were recorded in the temperature range 100 to 300 K, at very low hydra tion levels (h = 0.01 to 0.62) obtained with different procedures. Dif ferent classes of water molecules were found: the preferred hydration sites and the water dipole activation energies are discussed. In parti cular, the existence of a critical hydration level which induces a par tially irreversible structural transition was monitored in myoglobin. The technique allowed estimation of the average number of hydrogen bon ds established by the water molecules in the inner cages of haemoglobi n at different hydration levels.