Amino acid side chain: amino group basic and hydroxyl not?

[Watari]

Hi all

I don't really understand this... How come that if an amino group is attached to the amino acid side chain, like in arginine or lysine, the molecule is basic, but if an hydroxyl group is attached, like threonine, it is not basic?

How come the amino group can accept a H+ and a hydroxyl group cannot?

Thanks

 

[Quantum Defect]

Hi all

I don't really understand this... How come that if an amino group is attached to the amino acid side chain, like in arginine or lysine, the molecule is basic, but if an hydroxyl group is attached, like threonine, it is not basic?

How come the amino group can accept a H+ and a hydroxyl group cannot?

Thanks

Ammonia (NH3) is a weak (either Bronsted-Lowry or Lewis definition) base, water (H2O) is a very weak base.

Amines are weak bases (R-NH2), alcohols (R-OH) are very weak bases.

They both can act as bases, but the nitrogen-containing species are better bases. If you want a good explanation for why this is the case, you could say that it is due to the fact that oxygen (0) is more electronegative than nitrogen (0) and wants to hang onto its electrons (not form a bond to H). On the other hand, R-O- has a surplus of electrons (compared with R-OH) , and can share the extra two electrons with an H+ -- why hydroxides, alkoxides are stronger bases.