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 levelsa nominally 'dry' sample (specific residual hydration h = 0.060 g/g), a H 2 O-hydrated (h = 0.49) and a D 2 Ohydrated 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 H 2 O-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.

Inelastic and quasi-elastic neutron scattering. Application to soft-matter

“…In biomolecules, on average, half of the atoms are protons and their roughly homogeneous distribution reflects the collective dynamics. Also as neutron scattering originate from nuclear interactions, no selection rule applies, so that g(ω) can be considered as a genuine VDOS [13].…”

“…The vanadium being an incoherent scatterer, it also provides information on the detector efficiency. For relaxational or Gaussian processes, the time-energy equivalence 13 writes:…”

Vibrational dynamics changes of protein hydration water across the dynamic transition

“…Water molecules within a few Å of the protein surface are able to effectively perturb both the diffusive and vibrational dynamics of biomolecules, as shown by experimental [1][2][3][4][5][6][7][8][9][10][11][12][13] and theoretical [14][15][16][17][18] investigations. This kind of water, referred to as protein hydration water, has been shown to play a role in efficient enzymatic catalysis [19], in folding processes [20][21][22], as well as in molecular recognition and in mediating protein-protein interactions.…”