[AcetylCoA]

I am studying about amino acids, I find that Tryptophan does not show basic properties though it has amino group. I have found that "the aromatic ring strongly decreases the basicity of the amine, depending on its substituents" in Wikipedia. I still can't figure out the reason and I don't find any further information about that. Can anyone help me about this? Thank you...

P.S. I post this in Organic Chemistry forum but actually I am a high-school student and I don't study Organic Chemistry as deeply as in the university.

[Rico]

Hey AcetylCoA

Aniline has a decreased basicity (pKaH = 4.6) compared to other amines (pKaH = 10-11) because the lone pair on nitrogen (which is responsible for the basicity of amines) is to some degree distributed into the aromatic ring. Try drawing resonance structures of aniline and this will give you a better picture of the electron distribution in aniline.

Rico

[AcetylCoA]

Thanks you very much, Rico.
I now understand it...

[AWK]

Note, tryptophan has two amino groups - only one is heterocyclic aromatic.

[lele-duan]

N in aniline is of sp2 hybridization.

[AWK]

N in aniline is of sp2 hybridization.

No aminoacid contains Ar=NH2 group like aniline

[sjb]

No aminoacid contains Ar=NH2 group like aniline

Perhaps more accurately, no endogenous amino acid has an aniline-like NH₂ group (at least of the 20 main ones)

S