I am trying to calculate the net charge on an amino acid, but I seem to be doing it wrong. The question I am trying to answer is "In a pH 7.0 buffer, what will be the charge of Methionine?" We are referred to a table that gives us the following information:
pKa1 (COOH) = 2.28
pKa2 (NH3+) = 9.21
pI = 5.74
From what I understand, if pH > pI, (which it is here), the amino acid should have a negative charge because even though the pH is high enough to deprotonate all the carboxyl ends and not enough to deprotonate all the amino ends, since it is higher than the pI some of the amino ends have been deprotonated (in addition to all the carboxyl ends).
Yet my answer book says that the net charge would be zero! Are 5.74 and 7.0 "close enough" that we can consider net charge to be zero still? If so, what is the standard permissible 'fuzzy' range? If not, then what is going on here?