Dynamical changes of hemoglobin and its surrounding water during thermal denaturation as studied by quasielastic neutron scattering and temperature modulated differential scanning calorimetry

Abstract

We have investigated the dynamical behavior of both the protein hemoglobin and its surrounding water during the denaturation process using modulated temperature differential scanning calorimetry, quasielastic neutron scattering, and frequency dependent conductivity measurements. To distinguish between the scattering from the protein and its surrounding water, neutron scattering measurements were performed on both a fully hydrogenated sample as well as a sample where the water and the exchangeable hydrogen atoms on the protein surface were deuterated. The experimental data show that the unfolding and aggregation processes are substantially overlapping in temperature. The unfolding process occurs in the approximate temperature range of 315-345 K, whereas the aggregation process starts around 330-335 K and is completed at 360 K. Furthermore, the results suggest that the secondary structure of the protein unfolds at about 325 K, and that this leads to an increased number of water molecule hydrogen bonded to the protein. Thus, the unfolding of the secondary structure reduces the number of mobile (on the experimental time scale of about 50-100 ps) water molecules. In contrast, the aggregation of protein molecules seems to have a minor effect on the dynamics of its surrounding water. In the case of the protein dynamics there are competing effects from unfolding and aggregation. The unfolding process increases the flexibility of the protein, whereas the initial aggregation reduces its dynamics. The conductivity seems to be negatively affected by both reduced water mobility and an aggregation of protein molecules.