Final answer:
The isoelectric point of the new amino acid, which has an alpha-amine, an alpha-carboxylic acid, and an R-group carboxylic acid, is calculated to be 3.75. This is obtained by averaging the two lowest pKa values (3 and 4.5) of the acid groups.
Step-by-step explanation:
Isoelectric Point (pl) Calculation of a New Amino Acid
The isoelectric point (pl) of an amino acid is the pH at which the amino acid has no net charge. When solving for the isoelectric point of an amino acid with more than two ionizable groups, the relevant pKa values for the groups around the isoelectric point are used. Since this new amino acid has an additional carboxylic acid group in the side chain, it has three pKa values: the alpha-amine group (pKa = 7.5), the alpha-carboxylic acid group (pKa = 3), and the R-group, which is another carboxylic acid (pKa = 4.5).
To find the pl, we need to identify the two pKa values that are around the point where the amino acid has no net charge. As the amino acid is acidic due to the two carboxylic groups, the average pKa of the two lowest pKa values (those of the carboxylic acids) will give the pl:
pKa of alpha-carboxylic acid = 3
pKa of R-group carboxylic acid = 4.5
The pl is calculated as the average of these two pKa values:
pl = (pKa1 + pKa2) / 2
Substituting the values:
pl = (3 + 4.5) / 2
= 3.75
Therefore, the isoelectric point of the new amino acid is 3.75.