Welcome Guest Search | Active Topics |

Tag as favorite
Basic amino acids
Options
Previous Topic Next Topic
Kaylene_5146
#1 Posted : Friday, June 25, 2021 5:09:16 PM
Rank: Newbie

Groups: Registered
Joined: 5/4/2020
Posts: 7

Thanks: 0 times
Was thanked: 0 time(s) in 0 post(s)
To my understanding: the three basic amino acids (Arginine, Histidine, and Lysine) are all positively charged at pH = 7, whereas the two acidic amino acids are negatively charged at pH = 7.

I am just confused on this though because I was under the impression that typically negatively charged molecules act as Lewis bases- ie. due to this negative charge they can act as electron donors (nucleophiles = positive charge loving). So if this negative charge is usually associated with a bases, how come the positive amino acids and basic and the negative amino acids are acidic? Wouldn't it be the reverse where the basic are (-) and the acidic are (+)?

I'm assuming that it's because we are talking about the Bronsted-Lowry definition of acid/ base but I am still finding this concept hard to fully grasp.
Back to top
Report?
INSTR_Molly_129
#2 Posted : Saturday, June 26, 2021 3:47:05 PM
Rank: Advanced Member

Groups: Registered
Joined: 5/23/2021
Posts: 74

Thanks: 0 times
Was thanked: 0 time(s) in 0 post(s)
Hi Kaylene,

That's a good question! The amino acids are termed "acidic" or "basic" due to their interactions with water after the fact. So, when you see them as positive or negatively charged, they have already been protonated or deprotonated.

Let's now look at lysine: at a pH of 7 which is lower than the pKa of the amine group of lysine (about 8-9), the amine group of the side-chain will act as a base, donating its lone pair on the nitrogen to a proton in the solution, resulting in an overall positive charge.

I hope that helps! Another way to think of basic vs. acidic amino acids is to think of the pKas that their side-chains would generate. If it is a -COOH group, its generally going to be about 4-5, while -NH2 groups are generally going to be 8-9. As we learn from ChemOChem, the lower the pKa, the more acidic the material.

Cheers, and happy studying!

Molly
Back to top
Report?
Users browsing this topic
Guest
Tag as favorite
You cannot post new topics in this forum.
You cannot reply to topics in this forum.
You cannot delete your posts in this forum.
You cannot edit your posts in this forum.
You cannot create polls in this forum.
You cannot vote in polls in this forum.

Clean Slate theme by Jaben Cargman (Tiny Gecko)
Powered by YAF | YAF © 2003-2009, Yet Another Forum.NET
This page was generated in 0.090 seconds.