Could anyone explain why -- (This is a biochemistry question)

[simpleton]

---

Members do not see this ad.

The R group of Arginine accepts only one proton on its three nitrogens under normal conditions. Thanks in advance for your help!!

 

[moo]

Look at the various pKa values of each of the groups. (Although this doesn't really answer your question...To truly answer it you have to delve deeper into the realms of quantum mechanics and all that orbital theory stuff)

 

[MorningLight210]

At first I was going to reiterate looking @ pKa's, and this still might help, but then I re-read your question and realized you're specifically talking about the guanidino group. If you're wondering why one N is protonated and the others are not...I think you might want to try drawing the resonance structures. You'll actually see that ONE nitrogen is not just protonated, but either one of two, depending upon where you draw your double bond (although resonance structures imply partial double bond character, so what I just said doesn't adhere to good chemical etiquette...I can just see my professors now, ready to beat me down, periodic tables in hand).

I hope this helps...I hope I haven't just confused you even more. 🙁

 

[simpleton]

Thanks guys for you insites. I was thinking it had something to do with negativity of the nitrogen which initially accepts the proton and then the resonance which stabilizes the charge.