Hydration water and peptide dynamics – two sides of a coin. A neutron scattering and adiabatic calorimetry study at low hydration and cryogenic temperatures

Abstract

In the present work we bridge neutron scattering and calorimetry in the study of a low-hydration sample of a 15-residue hybrid peptide from cecropin and mellitin CA(1–7)M(2–9) of proven antimicrobial activity. Quasielastic and low-frequency inelastic neutron spectra were measured at defined hydration levels – a nominally ‘dry’ sample (specific residual hydration h = 0.060 g/g), a H₂O-hydrated (h = 0.49) and a D₂O-hydrated one (h = 0.51). Averaged mean square proton mobilities were derived over a large temperature range (50–300 K) and the vibrational density of states (VDOS) were evaluated for the hydrated samples. The heat capacity of the H₂O-hydrated CA(1–7)M(2–9) peptide was measured by adiabatic calorimetry in the temperature range 5–300 K, for different hydration levels. The glass transition and water crystallization temperatures were derived in each case. The existence of different types of water was inferred and their amounts calculated. The heat capacities as obtained from direct calorimetric measurements were compared to the values derived from the neutron spectroscopy by way of integrating appropriately normalized VDOS functions. While there is remarkable agreement with respect to both temperature dependence and glass transition temperatures, the results also show that the VDOS derived part represents only a fraction of the total heat capacity obtained from calorimetry. Finally our results indicate that both hydration water and the peptide are involved in the experimentally observed transitions.