horbowiec.7527 wrote:Re ORG CHM Chapter 12 Question 3 (ORG-279):
Worked solution mention Aspartic Acid (D) and Glutaminic Acid (E) but none of the answer choices have that, is it a typo in answer choice A and it should be D-E or E-D? And that is correct answer?
Those were quite reasonable suppositions! However, it is just that the worked solution was not clear enough, here is the improvement:
In a basic (alkaline) solution (e.g., pH of 12), acidic protons will be removed. Glycine (G) is neutral (i.e. according to Figure 2, G does not have an acidic side chain); however, Aspartic Acid (D) has a COOH group as a side chain (Figure 2) which can be deprotonated and thus the dipeptide would be negatively charge. Note that the only 2 amino acids with -COOH side groups are D and E (Glutamic Acid); however, E is not among the answer choices so the answer must be answer choice A. Also note that D and E are the only amino acids with the word "acid" in their names.
From Figure 2:
W: tryptophan
K: lysine
Q: glutamine
N: asparagine
P: proline
