Question

At what pH would cysteine be a good buffer?

1) pH 2
2) pH 7
3) pH 10
4) pH 14

Answer 1

Cysteine acts as a good buffer at pH levels near its pKa values. With pKa values around 1.71 and 8.33, cysteine would be a good buffer at pH 7, not at pH 2, 10, or 14.

Step-by-step explanation:

The student asked at what pH cysteine would be a good buffer. To determine this, one needs to consider the pKa values of cysteine.

Cysteine has two pKa values, one around 1.71, corresponding to the carboxylic acid group (-COOH), and another around 8.33, corresponding to the ammonium group (-NH3+). A substance acts as a good buffer at a pH near its pKa values because at this pH, the compound can effectively donate and accept protons.

Considering that the pH scale varies from 0 to 14, where values below 7 are acidic and above 7 are alkaline, and that buffers function best plus or minus one pH unit around the pKa, cysteine would work well as a buffer at pH 7 (close to its second pKa value of 8.33) rather than at pH 2, 10, or 14, which are too far from its pKa values for optimal buffering capacity.