According to my chemistry knowledge, The only thing that changes an equilibrium constant of a specific reaction is a change of temperature. However, I have seen in some literature which is reported that pKa of side chains of some amino acids has been changed only due to change in microenvironment, like pH or alteration in position of close residues.
Hi Zaiddodine,
The equilibrium constant is related with the stability of reactants and products (in this case, acid and base)
If you put an acid (neutral system) in toluene, the conjugated base would be less stabilized by the solvent (due to the low polarity) and the acid dissociation constant would be lower than in water, where the anion is able to form favorable interactions.
The effect of the closest residues can be explained in the same way: stabilization of acid of base. Think in trifluoroacetic acid and acetic acid. The first one is a stronger acid. Why? Because the negative charge in the anion is delocalized due to the fluorine. The more delocalized charge, the more stable. In the case of amino acid is exactly the same. Some residues can take or give electronic density, which change the relative stability (and pKa)
Hope it helps,
Fernando
Electric fields also (de)stabilise charged species, which shifts the protonation equilibria of side chains. A classic example is http://www.pnas.org/content/88/5/2026.abstract.
...and such electric fields can arise from other side chains, so two aspartates next to each other would be slightly more basic than if separated.
Please, have a look to Leghninger book. All the parameters affecting the properties of the amino acids (including side chains) are described into detail.
Hi all! In fact the pKa of a titratable residue is just an equilibrium constant governed by the Le Chatelier's principle. In a microscopic viewpoint, and depending of the surrounding environment, like a buried cavity or a solvent exposed environment with a acid (or basic) residue in the neighborhood, the pKa could have large fluctuations even when the macroscopic pKa is fixed at some value that can be (or not) close to the corresponding value in a fully solvated environment (which is normally called the standard pKa)
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