Minor species. For an amino acid such as alanine, the major species in solution at pH 7 is the zwitterionic form. Assume a pKa value of 8 for the amino group and a pKa value of 3 for the carboxylic acid. Estimate the ratio of the concentration of the neutral amino acid spe- cies (with the carboxylic acid protonated and the amino group neutral) to that of the zwitterionic species at pH 7 (Section 1.3).

Explanation:

Using the Henderson-Hasselbach equation, we find the ratio of alanine-COOH to alanine-COO- at pH 7 to be 10power-4. The ratio of alanine-NH2 to alanine-NH3+, determined in the same fashion, is 10power -1. Thus, the ratio of neutral alanine to zwitterionic species is 10-4 × 10 power-1 = 10power-5.

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