[Molarman]

Hi!  I'm being asked to draw a structural formula for this reaction, and I don't know which form I should draw each amino acid in, since they can exist in two forms:   

a) NH3+ (amino group) & COO- (carboxyl group)
b) NH2  (amino group) & COOH (carboxyl group)

The course materials (ILC) show them in both forms in two separate locations within the lesson, which is very confusing!

Also, does it matter whether the OH is taken from glycine and the H from valine, or could it be the other way around?

Thanks!

[Babcock_Hall]

In water solution the zwitterionic form of amino acids is the predominant form (a).  I am not sure about the rest of your question.  Are you being asked to draw a dipeptide?

[Molarman]

The question asks to show how a peptide bond forms between two amino acids: glycine and valine.  I'm tempted to draw the zwitterionic forms since amino acids are more commonly found in that form.  However, the course book has spent so much time talking about OH being removed from one molecule and H leaving the other that I'm worried I had better show them in that form....?

As for my second question: in all of the examples I have seen of peptide bonds between these two molecules, it is always the amino group from glycine that bonds to the carboxyl group from valine, but could it be the other way around? (since both molecules have both functional groups attached to them, how do I know which functional group from each molecule takes part in the dipeptide bond?)

Thanks for your help with this!

[Babcock_Hall]

The two possible dipeptides are glycyl valine (Gly-Val) and valyl glycine, and they are isomers of each other.  Without more information, either or both might be acceptable answers.  I don't have anything against drawing the two amino acids in their neutral forms.  It is then fairly easy to see the loss of water to form the peptide bond.  However, as written this reaction is very unfavorabie (it has a very small equilibrium constant).  The actual formation of a peptide bond works through intermediates and energy is spent along the way.  Whether your equation uses the neutral forms or the zwitterionic forms, it is more of a conceptual equation; it does not represent how an actual peptide bond is made.