Biochemical reaction thermodynamics ionic strength

Effects of ionic strength on biochemical reaction thermodynamics... 

This is referred to as the extended Debye-Huckel equation. It is an approximation that gives a good fit of data at low ionic strengths (Goldberg and Tewari, 1991) when B= 1.6 L1/2 mol 1/2. Better fits can be obtained with more complicated equations with more parameters, but these parameters are not known for solutions involved in studying biochemical reactions. The way that thermodynamic properties vary with the ionic strength is discussed in more detail in Section 3.6. 

A long time ago chemists realized that the most efficient way to store thermodynamic data on chemical reactions is by making tables of standard thermodynamic properties of species. The NBS Tables of Chemical Thermodynamic Properties (4) gives AfG°, Af// and Sm° for species at 298.15 K at xero ionic strength. Since the standard molar entropy is not available for many species of biochemical interest, the standard entropies of formation Af S" are used. This property of a species is calculated by using... 

Now we need a program to calculate the function of pH and ionic strength that yields the change in a standard thermodynamic property for a biochemical reaction (3). 

Derives the function of pH and ionic strength that gives the thermodynamic properties of a biochemical reaction typed in the form atpG+h2oG+der= adpG+piG. Other suffixes can be used for H, S, and NH. ) function = Solve[eq, de] function [l, 1, 2]]]... 

When the pH is specified, each biochemical half reaction makes an independent contribution to the apparent equilibrium constant K for the reaction written in terms of reactants rather than species. The studies of electochemical cells have played an important role in the development of biochemical thermodynamics, as indicated by the outstanding studies by W. Mansfield Clarke (1). The main source of tables of ° values for biochemical half reactions has been those of Segel (2). Although standard apparent reduction potentials ° can be measured for some half reactions of biochemical interest, their direct determination is usually not feasible because of the lack of reversibility of the electrode reactions. However, standard apparent reduction potentials can be calculated from for oxidoreductase reactions. Goldberg and coworkers (3) have compiled and evaluated the experimental determinations of apparent equilibrium constants and standard transformed enthalpies of oxidoreductase reactions, and their tables have made it possible to calculate ° values for about 60 half reactions as functions of pH and ionic strength at 298.15 K (4-8). 

The calculation of Af G° and Af H° of species from experimental data on apparent equilibrium constants and transformed enthalpies of reaction is described in R. A. Alberty, Thermodynamics of Biochemical Reactions, Wiley, Hoboken, NJ (2003) and a number of places in the literature. That is not discussed here because this package is oriented toward the derivation of mathematical functions to calculate thermodynamic properties at specified T, pH, and ionic strength. There are two types of biochemical reactants in the database ... 

Because of the complexities of the equilibria involved in biochemical reactions Alberty and others and a Panel on Biochemical Thermodynamics have suggested that tables of thermodynamic properties for biochemical use be tabulated for the following conditions T = 298.15 K, P = 1 bar (0.1 MP = 0.987 atm), pH = 7, pMg = 3 ([Mg +] = 10 M), and ionic strength I (p in this chapter) = 0.25 M. There appears to be both advantages and disadvantages to this. The proponents suggest that the symbols AG, K , etc. be used for this... 

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