Intrinsic thermal expansivity and hydrational properties of amyloid peptide Abeta42 in liquid water

Volumetric and conformational properties of the amyloid beta(1-42) peptide (Abeta(42)) are studied in relation to the properties of hydration water in a wide temperature range by computer simulations. The apparent volume of Abeta(42), which is the change in the total volume of the solution due to the presence of Abeta(42), shows a quite different temperature dependence below and above T approximately 320 K. The apparent thermal expansion coefficient alpha(app)(Abeta(42)) is about 1.5x10(-3) K(-1) at T320 K. By evaluation of the thermal expansivity of hydration water, the intrinsic expansivity of the biomolecule in liquid water was determined for the first time. The intrinsic thermal expansion coefficient of Abeta(42) is found to be negative: alpha(int)(Abeta(42)) approximately -0.8x10(-3) K(-1). The negative thermal expansion coefficient of Abeta(42) can be attributed to its rubberlike (entropic) elasticity and/or to a decreasing number of intrapeptide hydrogen bonds. Upon heating, Abeta(42) transforms from an extended chain with a significant content of alpha-helices to a compact coil with noticeable content of beta-structures. A hydrogen-bonded spanning network of hydration water envelops Abeta(42) homogeneously at low temperatures but breaks into an ensemble of small water clusters upon heating via a percolation transition, whose midpoint is close to the temperature, where the apparent volume of Abeta(42) changes its temperature behavior. The mutual relation between the volumetric properties of Abeta(42), its conformational properties, and the properties of the hydration water is discussed.