Enthalpy, Entropy, and Structural Relaxation Behaviors of A- and B-DNA in Their Vitrified States and the Effect of Water on the Dynamics of B-DNA

Abstract

The A and B forms of NaDNA with hydration level of between 0.15 and 0.64 (g of water)/(g of NaDNA) have been vitrified by cooling at rates between 4 and ∼2700 K min-1, and their thermal behavior on reheating was studied from ∼120 to 300 K by differential scanning calorimetry (DSC). The effects of the annealing time, ta, for two different hydration levels at a fixed temperature and of the annealing temperature, Ta, for a fixed ta have been investigated, and the effects of various Ta and ta on the enthalpy and entropy relaxations and recovery were ascertained. From these effects we evaluate τa, the characteristic structural relaxation time, E*, the activation energy, τ0, the preexponential factor, and β < 1 as an empirical parameter for apparent distribution of relaxation times. No DSC features of significance that may be attributed to the onset of molecular motions are found for A-DNA or when the water content is low, but for B-DNA and high water content, endothermic features resembling the onset of molecular motions, or glass → liquid transition, are observed from ∼153 K to ∼263 K. This corresponds to a slower increase in the heat capacity with temperature than is observed for most glass → liquid transitions, and it is attributed to the sum of a large number of relaxation modes of different parts with closely spaced single relaxation times. This is also seen as equivalent to a very broad distribution of structural relaxation times or of energy barriers separating the conformational and other substates corresponding to the various modes of local motions in a picture of multiple energy barriers. These local modes of motions involve both DNA segments and the water attached to them. Annealing vitrified B-DNA at temperatures from ∼153 K to ∼263 K causes its structure's net energy or enthalpy (and by implication its entropy) to decrease. The magnitude of this decrease has been measured by using the DSC difference scans in which the enthalpy lost on annealing is recovered on reheating but at a temperature higher than that of annealing. This recovered enthalpy increases with ta according to the stretched exponential relation, ΔH(ta) ∝ 1 − exp[−(ta/τa)β]. τa seems to remain constant with ta but changes with Ta in much the same manner as for synthetic amorphous polymers. The peak temperature of the endotherm observed during the recovery of the lost enthalpy, Tp, also increases according to a relation, Tp2 ∝ 1 − exp[−(ta/τa)β], with the same values of β and τa as for the increase in ΔH(ta). It is concluded that the molecular segmental motions of B-DNA and of the water attached to it are attributable to a broad distribution of energy barriers between conformational substates.