The amino acid glycine NH2 CH2 COOH contains both a carboxyl group (COOH) and an

Question

The amino acid glycine NH2 CH2 COOH contains both a carboxyl group (COOH) and an amine group (NH2). The carboxyl group can act as a Brønsted acid (pKa = 2.34), while the amine group (-NH2) group can act as a Brønsted base (pKb = 4.4). At a pH of 6, what is the major species in an aqueous, highly diluted solution of glycine? Hint: Think about this problem as if the protonated form of glycine was a diprotic acid.

(a) NH2 CH2 COOH

(b) NH+ 3 CH2 COOH

(c) NH+ 3 CH2 COO

(d) NH2 CH2 COO

(e) NH+ 3 CH COO

The answer should be c, but I'm not sure how to get to that answer.

Explanation / Answer

Answer:- (C) NH+3-CH2-COO-

This is a neutral form (zwitterion form) of glycine . At isoelectric point (pl) the amino acid exist in a neutral form.

The isoeletric point is the pH at which the amino acid exists in the zwitterion form.

This pH is calculated by calculating the average of pKa 1 & pKa2

here pKa1 is 2.34 & pKa 2 is 9.6 (converting pKb 4.4 to pKa)

The average of this is 5.97 which is approximately 6.

Hence pH 6 is a isoelectric point of glycine & at this pH it will exist in zwitterion form.

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