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Chu , Joan G. Schellinger , Michael J. Bocek , Russell N. It can be demonstrated mathematically as well as chemically that, at the point where the concentration of protons actually hydronium ions in solution is equal to the Ka of a compound, that compound can act as a buffer. Just as pH is defined as the negative logarithm of the proton concentration, one can take the negative log of the Ka of a compound and refer to it as the pKa.
Since a compound will act as a buffer when the proton concentration is equal to its Ka, we can rewrite this to say that a compound will buffer a solution if the pH is the same as its pKa. Most compounds can actually act as reasonably good buffers across a whole range of pH's centered around their pKa's and not just at their pKa's.
The pKa of histidine is 6. The acidic amino acids have pKa's below histidine's, and the basic amino acids have pKa's far above histidine's, such that the pKa of histidine is the closest to pH 7. That is to say that histidine is the only amino acid with pH 7. It is certainly not the best biological buffer under these conditions, and it is not the best amino acid buffer under either very acidic or very basic conditions, but it is the best amino acid buffer within its buffering range.
As an aside: you can calculate the amount of charged histidine or whatever at a given pH. First subtract the pH from the pKa, and then take ten to this difference.
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