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MCAT Biology Question — Predominate Structures

Glycine is an achiral amino acid with pKa values of 2.34 and 9.60. Which of the following is the predominant structure of glycine at a physiological pH?

 

predominant structures

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This questions asks the examinee to determine the predominate structure of the amino acid glycine. Given the pKa value for the amine and carboxylic acid group, you can apply the Henderson-Hasselbalch equation to figure out whether the substituent exists as an acid or conjugate base at a physiological pH of 7.4.

COOH:

pH = pKa + log[COO]/[COOH]

7.4 = 2.34 + log[COO]/[COOH]

5.06 = log[COO]/[COOH]

105.06 = [COO]/[COOH]

[COO] = 105.06[COOH]
Thus, the conjugate base COO= is the predominate form.

NH3+:

pH = pKa + log[NH2]/[NH3+]

7.4 = 9.60 + log[NH2]/[NH3+]

 -2.2 = log[NH2]/[NH3+]

10-2.2 = [NH2]/[NH3+]

102.2[NH2] = [NH3+]
Thus, the acid NH3+ predominates.

Therefore, the structure in answer choice D is the most common form of glycine at a physiological pH.

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