Absolute Protein-Ligand Binding Constants

It still remains that the pervasive use of the expression absolute free energy in the literature requires that this concept be clarified in the case of protein-ligand 

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One asset of mass spectrometry in protein science is that ESI and MALDI [11, 75] can introduce noncovalent complexes to the gas phase [12, 76, 77]. If one can assume that the gas-phase ion abundances (peak intensities) for the complex, apo protein, and ligand are directly related to their equilibrium concentrations in solution, the relative and absolute binding affinities can be deduced [78-81]. Extended methods are now available that also make use of the intensity of the complex and the protein at high ligand concentration to determine binding constants [78, 82-84]. 

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