Dynamical Transition of Protein-Hydration Water

Abstract

Thin layers of water on biomolecular and other nanostructured surfaces can be supercooled to temperatures not accessible with bulk water. Chen et al. [PNAS 103, 9012 (2006)] suggested that anomalies near 220 K observed by quasi-elastic neutron scattering can be explained by a hidden critical point of bulk water. Based on more sensitive measurements of water on perdeuterated phycocyanin, using the new neutron backscattering spectrometer SPHERES, and an improved data analysis, we present results that show no sign of such a fragile-to-strong transition. The inflection of the elastic intensity at 220 K has a dynamic origin that is compatible with a calorimetric glass transition at 170 K. The temperature dependence of the relaxation times is highly sensitive to data evaluation; it can be brought into perfect agreement with the results of other techniques, without any anomaly. In contrast to bulk water, protein hydration water can be supercooled down to a glass transition at T g ≃ 170 K. Near T g translational degrees of freedom arrest, which induces discontinuities in the specific heat and the thermal expansion coefficient of the hydration water [1][2][3][4]. Due to the dynamic nature of the glass transition, freezing of microscopic degrees of freedom can already be observed far above T g .The protein dynamic transition is an abrupt onset of atomic displacements on the microscopic length and time scale probed by quasielastic neutron scattering (QENS). First observed twenty years ago in hydrated myoglobin and lysozyme at T ∆ ≃ 240 K [5], it is now known to be a generic property of hydrated proteins, while it is absent in dehydrated systems. It is therefore related to the dynamics of the hydration shell.In QENS, mean squared dispacements δx 2 are deduced from the elastic scattering intensity S(q, 0). Due to the finite spectrometer resolution (fwhm 2∆ω), one actually measures S(q, |ω| ∆ω). Full spectral measurements show that the anomalous decrease of the central peak is compensated by increasing inelastic wings [5,6]. These effects have been interpreted as precursors of the glass transition. Since QENS probes structural relaxation at ∆ω −1 ≃ 100 ps, while the calorimetric T g refers to a time scale of 100 s, it is natural that T ∆ is located far above T g : the protein dynamic transition is the microscopic manifestation of the glass transition in the hydration shell. The time-scale dependence of T ∆ also explains its variation with viscosity [7][8][9]. * Electronic address: wdoster@ph.tum.de Recently, these views have been challenged by the suggestion that there might be a time-scale independent transition. Support came mainly from QENS experiments on the backscattering spectrometer HFBS at NIST. In hydrated lysozyme, DNA and RNA, a kink was found not only in S(q, |ω| ∆ω), but also in the α relaxation time τ deduced from full spectra S(q, ω) [10][11][12][13]. This change of τ (T ) from high-T super-Arrhenius to low-T Arrhenius behavior at T L ≃ 220 K has been interpreted as a fragile-to-strong-transition (FST) from the...

Hydration-dependent dynamic crossover phenomenon in protein hydration water

“…The mean characteristic relaxation time τ is calculated from τ and β by τ = β −1 (β −1 )τ . In this study, β is fixed to 0.5, just as in the analysis of Doster et al [17] and a previous work by our group [12]. In addition, we have taken the elastic fraction p to be of the form p(Q) = p 0 A(Q), with p 0 constant.…”

“…Figure 1 in the sequence of E to guarantee that the Riemann sum of the normalized S th (Q,E) converges to 1; thus the numerical problem pointed out by Doster et al in Ref. [17] will not appear. Therefore, a direct comparison between the results in this study and in Ref.…”

“…To date, this phenomenon and its physical implications are still largely debated [14][15][16][17][18][19]. Doster et al [17] state that the crossover observed in Ref. [12] is due to numerical errors in the data analysis protocol and can be ruled out with an improved analysis method.…”

“…This phenomenon is of particular interest, partially because (i) the crossover temperature T X of the hydration water is close to the transition temperature of the so-called protein dynamic transition (PDT) [13] T D and thus it is conjectured that the PDT is induced by the dynamic crossover of the hydration water [12] and (ii) this crossover is interpreted as an anomalous feature of the structural relaxation of the hydration water and is ascribed to the existence of the high-density liquid and low-density liquid phases in the supercooled water [12]. To date, this phenomenon and its physical implications are still largely debated [14][15][16][17][18][19]. Doster et al [17] state that the crossover observed in Ref.…”

Anharmonic transitions in nearly dry l-cysteine I

“…This scenario was based on studies that advocated a second critical point of water (liquid-liquid critical point) in confined supercooled water [4]. However, a recent high resolution QENS experiment performed by Dosteret al [5] in fully deuterated C-phycocyanin protein, showed no evidence of such fragile-to-strong character in the dynamical transition at T D . As pointed out by these authors, their experimental findings are consistent with a glass transition of the hydration shell.…”