### Gibbs free energy

The Gibbs free energy is the maximum amount of non-expansion work that can be extracted from a closed system; this maximum can be attained only in a completely reversible process. When a system changes from a well-defined initial state to a well-defined final state, the Gibbs free energy ΔG equals the work exchanged by the system with its surroundings, less the work of the pressure forces, during a reversible transformation of the system from the same initial state to the same final state.

Gibbs energy is also the chemical potential that is minimized when a system reaches equilibrium at constant pressure and temperature. As such, it is a convenient criterion of spontaneity for processes with constant pressure and temperature.

Gibbs defined what he called the "available energy" of a body as such: The greatest amount of mechanical work which can be obtained from a given quantity of a certain substance in a given initial state, without increasing its total volume or allowing heat to pass to or from external bodies, except such as at the close of the processes are left in their initial condition.

Gibbs energy is useful because it incorporates both enthalpy and entropy. In some reactions, the enthalpy and entropy contribuitions reinforce each orther. If ΔG is negative, the process is said to be exorgonic (work producing and the process is spontaneous); if positive, the process is endergonic (work consuming and the process is not spontaneous); and if ΔG is equal zero, the system is at equilibrium.

The outcome is shown as a picture in the PNG format. The associated file name is given by a sequence of random numbers. The X axis presents distance values in angstroms and the Y axis presents the Gibbs free energy at each distance.

We have only use the primary sequence to realize these calculations.

After the calculation process, the user can download a text file (ASCII format) which contains a report with the main information about the calculations.

### Second cross virial coefficient

The second cross virial coefficient is a basic parameter representing the integral of the intermolecular potential over distance [see Hunter, R. J. (1989) Foundations of Colloid Science, Vol. II. Calderon Press, Oxford. 675-873].

Precise calculation of the interaction energies of protein molecules is a hard task computational, mainly because theses systems have a complex geometry and the uneven charge distribution.

The second cross virial coefficient, B23, is a useful indicator of the overall interactions between two molecules and it is very important to describe protein aggregation, therefore comprises the predominant effect of the interaction between them. If B23 is positive, then there is a net repulsion; and if negative, the net interaction is attrative.

The outcome is shown as a picture in the PNG format. The associated file name is given by a sequence of random numbers. The X axis presents pH values and the Y axis presents the second virial coefficient overall distances between the two proteins, chosen by user, at each pH.

We have only use the primary sequence to realize these calculations.

After the calculation process, the user can download a text file (ASCII format) which contains a report with the main information about the calculations.

Web developers: Tulio Marcus and Rodrigo Faccioli.