Which of the following correctly illustrates a dipeptide and an amino acid in the optimal position to form a tripeptide?

Question

For this question, I would like to know exactly why A is the right answer, and why the remaining answers are wrong. I originally picked answer C. 

 

I had to take a picture and post it on another website because I cannot recreate it on WyzAnt.

 

 

http://s1195.photobucket.com/user/BirdySweet/media/IMG_6747_zpss5pd92es.jpg.html?sort=3&o=0

Answer 1

First of all remember that we are talking about amino acids, that means compounds that have 2 kinds of termini.

1)an amino group NH₂

2)an acid group COOH 

 

 all amino acids have this structure. the right answer is "a" because a COOH is facing a NH2. when these 2 compounds react a dehydration reaction (removal of water) will occur, forming a new high energy bond and the new tripeptide.

 

           NH₂-CRH-CO-NH-CRH-CO-OH   +    H-HN-CRH-COOH  → NH₂-CRH-CO-NH-CRH-CO-NH-CRH-COOH  + H₂O

 

answer B the structures have acid groups in all termini, no peptide bonds to be form.

answer C

There are no acid group in either of the compounds for the dehydration reaction to occur.

answer D

there is no amimo group in the second structure to react with the acid group of the first compound.