Question 28

[fraserss]

Wouldn't the best answer actually be A, since at the pKa of the basic substituent, the concentration of the zwitterion and the negatively charged species are 1:1? It doesn't say how basic the solution is, so are we supposed to assume its so basic as for the negatively charged species to predominate over the zwitterion? Thanks.

[jellywing_2058]

Just remember that the MCAT is not out to trick you. In general if the pH is higher than the isoelectric point of an amino acid, then the amino acid loses a proton and becomes negatively charged.

[calena7178147]

Consider the following: For the net charge to be neutral, the pH is equal to the pI (that’s the definition of isoelectric point). The question does not state that the medium is necessarily basic, it just says that the medium is above the isoelectric point. Some amino acids have a pI of 3, others have a higher value, for example 10. Remember that each functional group has its own pKa also. See the following titration curve for alanine:



The isoelectric point is 6.01, therefore in a medium of pH=6, the net charge is zero.
The molecule is positively charged below the pI, at pH 2. The molecule has a net negative charge at a pH above the second pKa.

You can also think of how isoelectric focusing works. AA’s with a pI = pH of the medium will have a net charge of zero and will not move in the gel when current is applied. Those where the pH is greater than the pI will tend to have a net negative charge and will move toward the positive electrode. Those whose pI is lower than the pH of the medium will be positively charged and will move toward the negative electrode.