S. Rudisser et al., ENTHALPY, ENTROPY, AND STRUCTURAL RELAXATION BEHAVIORS OF A-DNA AND B-DNA IN THEIR VITRIFIED STATES AND THE EFFECT OF WATER ON THE DYNAMICSOF B-DNA, JOURNAL OF PHYSICAL CHEMISTRY B, 101(2), 1997, pp. 266-277
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 b
etween 4 and similar to 2700 K min(-1), and their thermal behavior on
reheating was studied from similar to 120 to 300 K by differential sca
nning calorimetry (DSC). The effects of the annealing time, t(a), for
two different hydration levels at a fixed temperature and of the annea
ling temperature, T-a, for a fitted t(a) have been investigated, and t
he effects of various T-a and t(a) on the enthalpy and entropy relaxat
ions and recovery were ascertained. From these effects we evaluate tau
(a), the characteristic structural relaxation time, E, the activation
energy, tau(0), the preexponential factor, and beta < 1 as an empiric
al parameter for apparent distribution of relaxation times. No DSC fea
tures 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 ons
et of molecular motions, or glass --> liquid transition, are observed
from similar to 153 K to similar to 263 K. This corresponds to a slowe
r 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 spa
ced single relaxation times. This is also seen as equivalent to a very
broad distribution of structural relaxation times or of energy barrie
rs separating the conformational and other substates corresponding to
the various modes of local motions in a picture of multiple energy bar
riers, These local modes of motions involve both DNA segments and the
water attached to them. Annealing vitrified B-DNA at temperatures from
similar to 153 K to similar to 263 K causes its structure's net energ
y or enthalpy (and by implication its entropy) to decrease. The magnit
ude of this decrease has been measured by using the DSC difference sca
ns in which the enthalpy lost on annealing is recovered on reheating b
ut at a temperature higher than that of annealing. This recovered enth
alpy increases with t(a) according to the stretched exponential relati
on, Delta H(t(a)) proportional to 1 - exp[-(t(a)/tau(a))(beta)]. tau(a
) seems to remain constant with t(a) but changes with T-a in much the
same manner as for synthetic amorphous polymers. The peak temperature
of the endotherm observed during the recovery of the lost enthalpy, T-
p, also increases according to a relation, T-p(2) proportional to 1 -
exp[-(t(a) tau(a))(beta)], with the same values of beta and tau(a) as
for the increase in Delta H(t(a)). It is concluded that the molecular
segmental motions of B-DNA and of the water attached to it are attribu
table to a broad distribution of energy barriers between conformationa
l substates.