Sketch the titration curve that would result if the fully unprotonated form of cysteine were titrated with HCl. Label midpoints, equivalence points and indicate which part of the titration corresponds to each of the specific pKa values of cysteine
The structure of cysteine (SH-CH2-CH(NH3+)(COO-) at pH = 7 shows that the side group is protonated even though the pKa is 8.33, the sulfhydryl (−SH) is acting as an acid.
In alkaline solutions, 1.96 < pH < 8.33, the carboxylic acid sheds its proton in order to neutralize OH-
pH = pKa1 = acidic
pH = pI = netrual
pH = pKa2= basic
As shown in following two images:
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