In a polypeptide at physiologic pH, hydrogen bonding may form between which of the following?

a) side chains of leucine residue and lysine residue
b) side chains of aspartyl residue and glutamyl residue
c) terminal amino acid group and terminal carboxyl group
d) amide group in peptide bond and aspartyl group

This is my thinking. H+ is positive so it would bind to something negative. Lysine is positive so a) would be wrong. Aspartate and glutamate are negative so I'm not too sure about that one. For c) terminal amino acid group has carbon which has a positive change so H wouldn't bind there. And then d) I have no idea.

I agree with you on c.

Apparently the correct answer is in fact d) howevere I don't even understand what d) is means.

Amide:

o=c-nh

Aspartyl:

CH2-COO-

I understand that it may bind to the O- on aspartyl but why would hydrogen binding occur b/t the c-nh when carbon is already bonded to the nitrogen?

It's the H attached to the N that is hydrogen bonded to a negative part; in this case to the CH2COO^-

But c has NH2 on one end and COOH on the other.That's why I picked c.

I'm going to have to ask my prof about this because in lecture she apparently tricked us because we all had chosen C as the answer.

Thank you very much.

To determine the correct answer, let's analyze the options one by one:

a) Side chains of leucine residue and lysine residue:
You correctly stated that lysine has a positive charge due to its amino group (-NH3+), while leucine does not have any charged group, which means it is uncharged at physiological pH. With this information, we can conclude that the side chains of leucine and lysine cannot form hydrogen bonds with each other.

b) Side chains of aspartyl residue and glutamyl residue:
Aspartate and glutamate both have negatively charged carboxylate groups (-COO-) in their side chains. Since hydrogen bonds typically form between a hydrogen atom and an electronegative atom (usually oxygen or nitrogen), it is possible for the side chains of aspartate and glutamate to form hydrogen bonds with each other.

c) Terminal amino acid group and terminal carboxyl group:
The terminal amino acid group has a positively charged amino group (-NH3+), and the terminal carboxyl group has a negatively charged carboxylate group (-COO-) at physiological pH. Opposite charges can form ionic bonds but not hydrogen bonds.

d) Amide group in peptide bond and aspartyl group:
The amide group in the peptide bond (-CONH-) does not contain any hydrogen atoms capable of forming hydrogen bonds. On the other hand, the side chain of aspartate is negatively charged. Therefore, the amide group in the peptide bond and the aspartyl group do not form hydrogen bonds with each other.

Based on this analysis, the correct answer would be option b) side chains of aspartyl residue and glutamyl residue. These two side chains can form hydrogen bonds due to the negatively charged carboxylate groups present in both amino acid side chains.