Charge of indole in Asp-His dipeptide bond

[soby10]

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Title typo "charge of imidazole in asp-his dipeptide bond"

Why is the histidine imidizole group deprotonated at a ph of 7 in a aspartic-histidine dipeptide bond? Is this bc the histidine imidizole group is basic and it's pka is 6.0 which is close to a neutral pka of 7 and also it transfers protons? But the overall charge is negative bc the amine is deprotonated. Is this still a zwitterion? Aren't all amino acids zwitterions at a ph of 7?

 

[sleepy425]

first of all, it's not an indole group (an indole group is what you see on tryptophan). it's an imidazole group.

anyway, so in an asp-his dipeptide, what are the ionizable groups? (it doesn't really matter the order in which the two amino acids are linked, so for simplicity we'll do N-asp-his-C)

Ionizable groups:
free amino group on one end (pKa around 9.5ish)
free carboxyl group on other end (pKa around 2ish)
asp side chain carboxyl group (pKa around 4ish)
his imidazole group (pKa around 6)

Ok, at pH 7, what are the protonation states and respective charges of the ionizable groups:

free amino group on one end (nearly 100% protonated, so positively charged)
free carboxyl group on other end (nearly 100% deprotonated, so negatively charged)
asp side chain carboxyl group (nearly 100% deprotonated, so negatively charged)
his imidazole group (90% deprotonated, so basically no charge)

So adding up the charges:
1-1-1+0=-1

So it's overall negatively charged.

In reality, you'll have about 90% of species negatively charged at this pH, and 10% neutral at this pH.

 

[soby10]

Thanks yeah ment imidizole. But is this still a zwitterion bc it's not neutral? Also is this unique to histidine?

 

[sleepy425]

Thanks yeah ment imidizole. But is this still a zwitterion bc it's not neutral? Also is this unique to histidine?

well, no, most species in solution will not be zwitterionic, since they hold a net charge. However, 10% of the species will be zwitterionic, since they will be neutral.