What will net charge on cysteine be at pH of 10.5?

Question

Question

Answer 1

Answer #1

Cystein has three pKa values

pKa1 = 1.92 for carboxylic acid group

pKa2 = 10.70 for amino group

pKa3 = 8.33 for side chain -CH₂SH

if the pH is higher than the pKa, the site is deprotonated, and if the pH is lower than the pKa, the site is protonated.

so at pH= 10.5

-COOH group of cystein will be deprotaned -COO^- with -1 charge because pH 10.75 is higher than its pKa 1.92

--CH₂SH group of cystein will be deprotaned --CH₂S^- with -1 charge because pH 10.75 is higher than its pKa 8.33.

--NH₂ group of cystein will be protaned -NH₃⁺with +1 charge because pH 10.75 is higher than its pKa 8.33.

so overall at pH 10.5 cystein wil have 2 negative and 1 positive charge so net charge will be -2+1= -1 at pH=10.5.

Answer 2

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