MCAT Organic Chemistry Practice Question

In our last orgo question, we talked about how organic chem is the least important subject on the MCAT and you’re better off focusing on a few very high-yield topics rather than trying to memorize tons and tons of reaction mechanisms.

For this question, we’ll stick with the theme of really important orgo topics and switch to a perennial MCAT favorite: stereochemistry. Try your hand at this question:

Item 41

How many different dipeptides may exist that include the two amino acids listed below?

A) 1

B) 2

C) 4

D) 8

Explanation

This question looks a little daunting at first, but ultimately it’s testing two basic areas of MCAT orgo knowledge: amino acids and stereocenters.

First, you need to be comfortable with the general structure of amino acids and need to recognize the two amino acids presented as the two simplest ones: glycine and alanine.

 (click for full-size image)

When two amino acids hook up to form a peptide, the carboxylic acid part of one molecule forms an amide bond with the amino group of another. That means when two amino acids join, there are two possible results:

But we’re not done there! We also have to check for different possible isomers. Remember that to have different enantiomers, you must first have a chiral carbon. Any carbon bonded to four different things is chiral. In our problem, the central carbon for alanine is chiral (in fact, ALL amino acids except glycine are chiral!).

So we have two different possible dipeptides, and each of those two dipeptides can have the alanine in either an L or D configuration. Thus (C) is the correct answer because we have four possible molecules:

(l)Ala-Gly

(d)Ala-Gly

Gly-(l)Ala

Gly-(d)Ala