For the amino acid Lysine, the nitrogen with the H attached to it is not basic (since you can draw out the resonance for it). But the other nitrogen that has a double bond is not basic. Why is this?? I can draw out the resonance for the double bonded nitrogen as well.. so shouldnt it be considered nonbasic as well. can someone please explain this to me? Thanks
I don't have an explanation for you. The site below shows the structure of lysine and I don't see any doubly bonded N atoms. Both amino groups appear to be primary to me.
http://en.wikipedia.org/wiki/Lysine
sorry i was referring to hisidine
Sure! I'd be happy to explain the basicity of Lysine's nitrogen atoms to you.
Lysine is an amino acid that contains a primary amino group (-NH2) and a side chain with a terminal amino group (-NH2). It has two nitrogen atoms, one in the amino group and the other in the side chain.
The basicity of an amine depends on the availability of its lone pair of electrons to accept a proton (H+). In the case of Lysine, the primary amino group (with the nitrogen and hydrogen attached) is more basic than the nitrogen in the side chain, and here's why:
The lone pair of electrons on the nitrogen atom in the primary amino group is relatively more available because it is not involved in any resonance interactions. Therefore, it can more readily accept a proton and act as a base. The lone pair can easily form a bond with a proton to give NH3+.
On the other hand, the nitrogen in the side chain of Lysine is involved in a resonance structure. When you draw out the resonance, you will see that the nitrogen forms a double bond with the adjacent carbon, and the electrons involved in the double bond can delocalize across the nitrogen and carbon atoms. This resonance delocalization disperses the electron density and makes the lone pair of electrons on the nitrogen less available to accept a proton.
So, even though you can draw resonance structures for both nitrogen atoms, the nitrogen in the side chain of Lysine is less basic because of the resonance stabilization.
To summarize, the basicity of Lysine's nitrogen atoms can be explained by the availability of their lone pairs of electrons. The nitrogen in the primary amino group is more basic because its lone pair is not involved in any resonance, while the nitrogen in the side chain is less basic due to resonance delocalization.