Question

Some textbooks assert that histidine can provide buffering capacity at the physiological pH of 7.4.

a) Is this assertion correct?

b) What are the unprotonated (base) and protonated (acid) forms of histidine that would contribute to the buffering?

c) Is there another amino acid that could potentially provide buffering at physiological pH? If so, which one and what are the unprotonated and protonated forms of this amino acid that would ocntribute to the buffering?

Answer 1

a) Yes, histidine can provide buffering capacity at the physiological pH of 7.4 because histine has a net positive charge under physiological conditions (around pH 7.4), most of the histidine under physiological conditions is uncharged.

b) The ability of a compound to act as a buffer at a given pH is determined by how readily it will accept and donate protons at that pH. base and acid forms of histidine is

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.

C) 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 6.0, so histidine is best at buffering at pH 6.0. 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 7.4 of any of the amino acids. That is to say that histidine is the only amino acid with pH 7.4 within its buffering range, such that histidine is the best amino acid buffer under physiological conditions.

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.