Intrinsic thermal expansivity and hydrational properties of amyloid peptide Aβ42 in liquid water

Volumetric and conformational properties of the amyloid beta(1-42) peptide (A beta(42)) are studied in relation to the properties of hydration water in a wide temperature range by computer simulations. The apparent volume of A beta(42), which is the change in the total volume of the solution due to the presence of A beta(42), shows a quite different temperature dependence below and above T approximate to 320 K. The apparent thermal expansion coefficient alpha(app)(A beta(42)) is about 1.5x10(-3) K-1 at T <= 320 K and about 0.6x10(-3) K-1 at T>320 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 A beta(42) is found to be negative: alpha(int)(A beta(42))approximate to-0.8x10(-3) K-1. The negative thermal expansion coefficient of A beta(42) can be attributed to its rubberlike (entropic) elasticity and/or to a decreasing number of intrapeptide hydrogen bonds. Upon heating, A beta(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 A beta(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 A beta(42) changes its temperature behavior. The mutual relation between the volumetric properties of A beta(42), its conformational properties, and the properties of the hydration water is discussed.