Nucleotide Hydrogen donor and acceptor

[jeep1010]

I have read that Guanine has 2 hydrogen bond donors (and 1 acceptor), as shown below. My question is: why is N9 (#9 - NH) not also considered a hydrogen donor, and why is N7 (#7) and N3 (#3) not considered hydrogen acceptors, as these will also have lone pairs?

So shouldn't Guanine have 3 donors, and 3 acceptors? I see that only the left side of the molecule is used for interaction, so is the reasoning that only 2 donors, and 1 acceptor because the "right" side of this molecule is attached to the phosphate backbone, while the "left" side participates in hydrogen bonding?

---

Members don't see this ad.

 

[aldol16]

So shouldn't Guanine have 3 donors, and 3 acceptors? I see that only the left side of the molecule is used for interaction, so is the reasoning that only 2 donors, and 1 acceptor because the "right" side of this molecule is attached to the phosphate backbone, while the "left" side participates in hydrogen bonding?

Exactly. They're not talking about donation in a strictly chemical sense because nucleotides don't do chemistry in a purely chemical context. Their chemistry has biological significance and so when biochemists talk about the H-bond abilities of nucleotides, they're talking about the H-bonds that can be involved in base pairing.

Also, N9 is where the nucleotide attaches to the sugar backbone and the lone pair on that nitrogen can delocalize, which makes it not a particularly good H-bond acceptor.