Biochemistry - Chemical Reactions in the Living Cell, Volume 1 - D. Metzler 1980

Bioenergetics of Biochemical Reactions
Tables of ∆G⁰ Values for Biochemical Compounds
Standard Free Energy of Formation

Table 3-3 lists in its first Column The values of — the Standard Free energy of formation from elements for A number of pure solid, liquid, and gaseous substances, as well as analogous values for compounds in solution at a hypothetical molal activity. Let us examine the latter case in somewhat greater detail.

The value of ∆G_f⁰ for pure liquid acetic acid is —389.1 kJ∙mol^-1. The equation for The formation of this compound from its elements is

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To obtain the Free energy of formation in aqueous solution, we must know the solubility and activity coefficient of acetic acid at various concentrations. From these data, we need to calculate the free energy change associated with dissolving liquid acetic acid in Water to yield a hypothetical molal aqueous solution of acetic acid in its standard state:

From equations (3-38) and (3-39) we obtain

In many calculations, it is convenient to have ∆G values for individual ionic species, such as acetate. The value of ∆G⁰_f for acetate^- (aq) can be

Table 3-3 Free energies of formation and oxidation at 25°C for a series of compounds^a,b

^a The values given in the table represent: ∆G⁰_f, the standard free energies of formation of compounds from elements; ∆G⁰_c, the standard free energies of combustion; ∆G⁰_ox, the standard free energies of oxidation mediated by NAD⁺ yielding NADH+H⁺, CO₂, H₂O, N₂, HPO^2-₄, and SO^3-; ∆G'_ox (pH7), the apparent Free Energy Changes at pH 7. All values are expressed in kJ∙mol^-1 and, unless otherwise specified, refer to aqueous solution at 25 °C. The symbol (g) indicates that the substance is in the gaseous state at a pressure of 1 atm. The final column indicates the number of electrons involved in the Complete oxidation of the substance to CO₂, H₂O, N₂, and H₂SO₄. If this value is negative, it means that the substance must be reduced to yield the specified products (e.g., 2NO^-₃ + 10e^- + 12H⁺ —> N₂ + 6H₂O). Data for phosphate esters pertain to compounds with fully dissociated phosphate groups (—O—PO^2-₃). The ∆G⁰_f values for many of these compounds were calculated using the formula: ∆G⁰_f (nonphosphorylated compound) — ∆G⁰ of Hydrolysis (to HPO^2-₄, Table 3-5) — ∆G⁰_f (one molecule of water is formed per molecule of phosphate ester) + ∆G⁰_f of HPO^2-₄ (taken from this table). Data from Bassham and Krause are used in the table without modification. For acyl-CoA derivatives, the CoA group (—SH) is treated as an "element";

in this case, the ∆G⁰_f value is marked with an asterisk and refers to the Formation of the compound from the corresponding elements and free CoA, with the ∆G⁰_c and ∆G_ox values corresponding to oxidation yielding the usual products plus CoA. The ∆G⁰_f values for each of these compounds were calculated from the ∆G⁰_f of the corresponding alcohol or carboxylate anion using the ∆G⁰_f values of hydrolysis (Table 3-5). Another very comprehensive table of free energy values can be found in: Biochemical Microcalorimetry (H. D. Brown, ed.), pp. 305–317, Academic Press, New York, 1969.

^b Principal source of data: C. Long, ed., Biochemists' Handbook, pp. 90–92. Van Nostrand Reinhold, Princeton, New Jersey, 1961. Much of the data is taken from K. Burton, Ergeb. Physiol. Biol. Chem. Exp. Pharmakol., 49, 275–298, 1957.

^c D. R. Stull, E. F. Westrum Jr., and G. C. Sinke, The Chemical Thermodynamics of Organic compounds, Wiley, New York, 1969.

^d J. A. Bassham and G. H. Krause, BBA, 189, 207–221, 1969.

^e J. R. Van Wazer, Phosphorus and Its Compounds, Vol. I, p. 889, Wiley (Interscience), New York, 1958.

be obtained from the ∆G⁰_f of aqueous acetic acid by incorporating the ∆G⁰ of dissociation:

The free energy of formation of H⁺ ions is taken to be zero. Summing equations (3-40) and (3-41), we obtain ∆G⁰_f (acetate^-) = —369.2 kJ∙mol^-1.