Amino acids having more number of amino groups than carboxyl groups are basic amino acids. Histidine has a 'NH' attached to 2 carbons and it is called basic amino acid. However, tryptophan (having >NH) is called neutral amino acid. Why is it so?

  • $\begingroup$ related chemistry.stackexchange.com/questions/48499/… $\endgroup$ – Mithoron Nov 25 '17 at 16:20
  • $\begingroup$ Not my area of expertise, but isn't it a function of which groups are protonated/deprotonated at physiological pH? I.e. the imidazole group of histidine is partially protonated whereas the indole side chain of tryptophan is not? $\endgroup$ – airhuff Nov 25 '17 at 16:24
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    $\begingroup$ @airhuff Indeed it is. OP doesn't get how and when lone pairs are delocalised and loose most of their basicity. $\endgroup$ – Mithoron Nov 25 '17 at 16:34
  • $\begingroup$ @airhuff,sorry, I didn't get you. $\endgroup$ – user166465 Nov 26 '17 at 3:19
  • $\begingroup$ Only one nitrogen of histidine’s side chain is basic. Hint: it is not the protonated one. For more, please go back to your books. $\endgroup$ – Jan Nov 26 '17 at 15:27

According to PubChem, indole (the nitrogen containing ring system of tryptophan) has a pKa value of -2.4. That means that it is a very weak base: at pH 0 (strong acid) it will be less than 1% protonated, and at neutral pH (physiological conditions) it will be neutral for all practical purposes. Notice that the indole nitrogen lone electron pair is conjugated as part of the pi system of the indole ring--hence it is unable to act as a base. The imidazole ring of histidine, which superficially resembles the indole group in tryptophan, has two nitrogens--one is similarly conjugated, but the other has a lone pair in the plane of the ring and is available to be protonated.

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