Page 178 - Thermodynamics of Biochemical Reactions

10.4 Effect of Temperature 177

Table 10.2 Standard Heat Capacities of Chemical Reactions in Dilute Aqueous
Solutions at 298.15 K

Chemical Reaction A,C:/J K-' mol-'

glucose 6-phosphate'- + H,O = glucose + HP0;- -48 20
mannose 6-phosphate'- + H,O = mannose + HP0;- -46 f 10
fructose 6-phosphate'- + H,O = fructose + HP0;- -28 40
ribose 5-phosphate'- + H,O = ribose + HP0;- -63 k 40
ribulose 5-phosphate'- + H,O = ribulose + HPOi- -84 f 30
glucose 6-phosphate' = fructose 6-phosphate' 44 * 10
mannose 6-phosphate' = fructose 6-phosphate' 38 30
glucose = fructose 76 k 30
xylose = xylulose 40 k 20
ATP4- + H,O = ADP3- + HP0;- + H+ -237 f 30

Source: With permission from R. N. Goldberg and Y, Tewari, J. Phys. Clzem Re$
Data 18, 809 (1989). Copyright American Institute of Physics.

In Chapter 4 the effects of temperature on A,G" and A,H" and on A,G" and
A,H'O are discussed on the basis of the assumption that A,Ho at zero ionic
strength is independent of temperature. Therefore the effects of heat capacities of
species were not treated. When a biochemical reactant contains two or more
species, the standard transformed molar heat capacity of the pseudoisomer group
is given by (Alberty, 1983a)
h,,,
C:p(iso) = 1 rJC:m(j) + r,(A,Hi0)2 - (AiH'o(iso))2
J= 1
The second term is always positive because as the system is heated, the acid
dissociations shift in such a way as to absorb heat, as predicted by the Le
Chatelier principle.
Calorimetric measurements yield enthalpy changes directly, and they also
yield information on heat capacities, as indicated by equation 10.4-1. Heat
capacity calorimeters can be used to determine CFm directly. It is almost
impossible to determine A, CF from measurements of apparent equilibrium
constants of biochemical reactions because the second derivative of In K' is
required. Data on heat capacities of species in dilute aqueous solutions is quite
limited, although the NBS Tables give this information for most of their entries.
Goldberg and Tewari (1989) have summarized some of the literature on molar
heat capacities of species of biochemical interest in their survey on carbohydrates
and their monophosphates. Table 10.1 give some standard molar heat capacities
at 298.15 K and their uncertainties. The changes in heat capacities in some
chemical reactions are given in Table 10.2.