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Why use thioglycollic acid? (2 of 2)

Typical results for the reagent ammonium thioglycollate are shown in Table 1. The concentrations of free amino acids produced from the hair protein by the reaction were measured and it was found that 63% of S-S bonds had been broken after 30 minutes of treatment with 1 mol dm^-3 ammonium thioglycollate at pH 7.5, for example.

Looking at experimental results enabled cosmetic chemists to choose the best reagent and conditions – thioglycollic acid in relatively alkaline conditions has proved suitable.

Explaining the results

These results have been rationalised as follows

At a relatively low pH (i.e. fairly acidic conditions), thioglycollic acid loses a proton from its carboxylic acid group:

H-SCH₂COO-H +OH^- H-SCH₂COO^- + H₂O

This reacts with hydroxide ions (contained in the product as potassium hydroxide) to form a dianion

H-SCH₂COO^- + OH^- -SCH₂COO^-+ H₂O

It is the S^- end of the dianion that is active in breaking the disulfide bridges. As we are only interested in this part of the molecule, we can simplify it to R’S^- to save us having to write out the full structure each time. The reaction occurs in 4 stages;

a) Nucleophilic attack by S in RS^- on one of the S atoms in cystine. This breaks the disulfide bridge in cystine and generates a new one between a cysteine molecule and the thioglycollate ion.

b) The anion generated in step (a) strips a proton off water to generate a cysteine molecule.

c) A second thioglycollate molecule attacks the first to break the just formed disulfide bridge and remake it between the thioglycollate molecules.

d) The anion generated in step (c) strips a proton off water to generate the second cysteine molecule.

The S^- on ^-SCH₂COO^- acts as an even better nucleophile than the O^- on H-SCH₂COO^- for two reasons:

• The ion has a double negative charge

• The lone pair electrons on sulfur are more available for donation than those on oxygen.