Legendre transforms constant derivation

The fractional saturation of tetramer YT and the fractional saturation of dimer YD are functions only of [02] at specified T, P, pH, etc., as shown by equations 7.1-18 and 7.3-6. However, since the tetramer form is partially dissociated into dimers, the fractional saturation of heme Y is a function of both [02] and [heme]. Ackers and Halvorson (1974) derived an expression for the function Y([02], [heme]). When Legendre transforms are used, a simpler form of this function is obtained, and it can be used to derive limiting forms at high and low [heme]. These limiting forms are of interest because they show that if data can be obtained in regions where Y is linear in some function of [heme], extrapolations can be made to obtain YT and YD. These fractional saturations can be analyzed separately to obtain the Adair constants for the tetramer and the dimmer (Alberty, 1997a). 

The equilibrium relations of the preceding section were derived on the assumption that the charge transferred Q can be held constant, but that is not really practical from an experimental point of view. It is better to consider the potential difference between the phases to be a natural variable. That is accomplished by use of the Legendre transform (Alberty, 1995c Alberty, Barthel, Cohen, Ewing, Goldberg, and Wilhelm, 2001)... 

Equation 8.5-3 indicates that the number of natural variables for the system is 6, D = 6. Thus the number D of natural variables is the same for G and G, as expected, since the Legendre transform interchanges conjugate variables. The criterion for equilibrium is dG 0 at constant T,P,ncAoi, ncA(3, /icC, and The Gibbs-Duhem equations are the same as equations 8.4-8 and 8.4-9, and so the number of independent intensive variables is not changed. Equation 8.5-3 yields the same membrane equations (8.4-13 and 8.4-14) derived in the preceding section. 

We have postulated the existence of a function of state, the entropy, which achieves a maximum at constant U and V for a system at equilibrium. Using Legendre transforms we have derived the additional functions of state from the following fundamental equation ... 

Note that in Eq. (1,187), X and X2 must remain constant. In. Eq. (1.187), the order of the Jacobian has been reduced from (c + 2) to c. The above process can be reversed, going from order c to c + 2, which is also true from Eq. (1.187). The transformation properties of Jacobians is useful in the manipulation of Legendre transformation derivatives and can be utilized by the introduction of a new set of independent variables... 

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