The hydration structure studied by TSDC. Myoglobin and haemoglobin

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) technique is a sensitive tool to study water molecules bound to biological macromolecules and characterized by mobility and order degree, quite different from those of bulk water. TSDC spectra for both proteins were recorded in the temperature range 100 to 300 K, at very low hydration levels (h=0.01 to 0.62) obtained with different procedures. Different classes of water molecules were found: the preferred hydration sites and the water dipole activation energies are discussed. In particular, the existence of a critical hydration level which induces a partially irreversible structural transition was monitored in myoglobin. The technique allowed estimation of the average number of hydrogen bonds established by the water molecules in the inner cages of haemoglobin at different hydration levels.