Biological standard states

The next point takes the standard-state idea and makes it more suitable for biological processes by defining a new AG°, called AG°. This new standard state is one with a pH of 7. This is the standard state used most of the time for biochemical reactions and is the one we will use. Not only does it make a big difference in reactions in which is consumed or produced, it also requir es us to be aware of the form in which various species exist at a pH of 7. A reaction that is endergonic at [H" ] = 1 M can easily become exergonic at [H" ] = 10 M (pH = 7) and vice versa. 

For biochemical reactions in which hydrogen ions (H ) are consumed or produced, the usual definition of the standard state is awkward. Standard state for the ion is 1 M, which corresponds to pH 0. At this pH, nearly all enzymes would be denatured, and biological reactions could not occur. It makes more sense to use free energies and equilibrium constants determined at pH 7. Biochemists have thus adopted a modified standard state, designated with prime ( ) symbols, as in AG°, AH°, and so on. For values determined... 

In this chapter, we continue the discussion begun in the last chapter of applying thermodynamics to chemical processes. We will focus our discussion here on two examples of biological interest. The principles are the same — all of our thermodynamic relationships work. But biochemists have their own vocabulary, and sometimes apply unique conditions to their systems. For example, as we shall see, unusual standard states are sometimes chosen. There is nothing wrong with this, since the choice of standard states is completely arbitrary as long as we keep track of what is done. Standard states are usually chosen in a way that makes the results most useful. That is true in this case. 

Note that these are free energies using the biological standard state. That is, the activity of H+ is 1 at pH = 7, the activity of liquid water is taken as 1, and activities of other species are approximated by their molar concentrations. 

Many biological processes involve hydrogen ions the standard state of an H+ solution is (by definition) a 1 mol L"1 solution, which would have a pH of nearly 0, a condition incompatible with most forms of life. Hence, it is convenient to define the biochemical standard state for solutes, in which all components except H+ are at 1 mol L-1, and H+ is present at 10 7mol L (i.e., pH 7). Biochemical standard-state free energy changes are symbolized by AG0, and the other thermodynamic parameters are indicated analogously (AH0, AS0, etc.). 

Carbon-centered radicals play an important role in organic synthesis, biological chemistry, and polymer chemistry. The radical chemistry observed in these areas can, to a good part, be rationalized by the thermodynamic stability of the open shell species involved. Challenges associated with the experimental determination of homolytic bond dissociation energies (BDEs) have lead to the widespread use of theoretically calculated values. These can be presented either directly as the enthalpy for the C-H bond dissociation reaction described in Equation 5.1, the gas-phase thermodynamic values at the standard state of 298.15K and 1 bar pressure being the most commonly reported values. 

Conditions appropriate to the three conventions introduced for the standard state (solvent, solute, and gases) usually do not occur under biological situations. A solute is essentially never at a concentration of 1 m, neither is an important gas at a pressure of 1 atm, nor is a pure solvent present (except sometimes for water). Hence, care must be exercised when... 

The reduction potentials of many biologically important redox couples are known (Table 18.1). Table 18.1 is like those presented in chemistry texts except that a hydrogen ion concentration of 10 M (pH 7) instead of 1 M (pH 0) is the standard state adopted by biochemists. This difference is denoted by the prime in E q. Recall that the prime in A G° denotes a standard free-energy change at pH 7. 

Many reactions that occur in living cells are oxidation-reduction reactions. Appendix IX lists several compounds of biological importance and shows their relative tendencies to gain electrons ai 25°C and pH 7 under standard conditions. The numerical values of Ho reflect the reduction potentials relative to the 2H + 2e" H2 half-reaction which is taken as — 0.414 volt at pH 7. The value for the hydrogen half-reaction at pH 7 was calculated from the arbitrarily assigned value (Ho) of 0.00 volt under true standard-state conditions (1 M H and 1 atm Hs). For those few halfreactions of biological importance that do not involve as a reactant, the Ho and Ho values are essentially identical. 

The development of new reference standards for biologies and biotechnology is managed by the Division of Complex Activities Expert Committees. The process is predicated on establishing an initial set of expectations for testing and performance of the compound/protein. The chapters in the USP state what is expected with regard to purity, stability, and potency. Later in the development, an actual reference standard is provided and made available for use. This part of the process often takes several years to complete all the testing required. 

Macroaggregates are obtained by aggregation of human serum albumin (HSA). HSA is isolated from donor blood and complies with the purity standards stated in the European Pharmacopeia, monographs 255 and 853) (Council of Europe 2004a, b) in accordance with EU and WHO requirements for biological substances (Council of Europe 1992 World Health Organization 1994). 

The equations for the rate constant t and the equilibrium constant K were obtained under conditions corresponding to the biological standard state (pH = 7, p = 1 bar Section 7.2d). Thus the values of ArG calculated from the equation for K are ArG values which can differ significantly from ArG (pH — 1, p — 1 bar). Prebiotic conditions are more likely to be near pH = 7 than pH — 1 so we expect that the reaction will still be favorable (AT 3> 1) thermodynamically. 

For this and other biological reactions the standard state chosen depends on temperature, the ionic strength, pH of the solution, and the presence of other ions (especially magnesium ions for this reaction). At very low ionic strength, pH = 7, and in the absence of magnesium ions, the values of the standard-state Gibbs energy of reaction,... 

Recall How can you tell if the standard Gibbs free energy given for a reaction is for chemical standard states or biological standard states ... 

Statement (a) is true, but statement (b) is not. The standard state of solutes is normally defined as unit activity (1 Mfor all but the most careful work). In biological systems, the pH is frequently in the neutral range (i.e., H is close to 10" M) the modification is a matter of convenience. Water is the solvent, not a solute, and its standard state is the pure liquid. 

The designation AG° indicates a biological standard state. If the prime is omitted, then it is for chemical standard states. 

For a standard state, H+ concentration is set as 1M, which is equal to pH = 0. However, biological reactions do not occur at pH = 0, thus for standard state reactions, scientists adopted pH of 7 (10 M). For a generalized reaction involving H+ ... 

This table contains selected values for the >K, standard molar enthalpy of reaction and standard molar heat-capacity change A,Cp for the ionization reactions of 64 buffers many of which are relevant to biochemistry and to biology. The values pertain to the temperature T = 298.15 K and the pressure p = 0. MPa. The standard state is the hypothetical ideal solution of unit molality. These data permit one to calculate values of the pK and of A ° at temperatures in the vicinity T (274 K to 350 K) of the reference temperature 0 = 298.15 K by using the following equations ... 

In equation 7.11 molar solution concentrations are used with 1 M standard states. The standard state ( ) pH equals zero in contrast to the biological standard state (0) of pH 7. For the ATP hydrolysis... 

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