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I was looking at the Kjeldahl digestion of proteins, and I very much do not understand the mechanism by which it works. The reaction scheme is as follows:

$$\ce{Protein-N + H_2SO_4 ->[K_2SO_4, CuSO_4][380^0C] (NH_4)_2SO_4 + CO_2 + H_2O}$$

I read that the sulphate salts are used in order to raise the boiling point of the sulphuric acid (I'm not particularly concerned with how this works). What confuses me is that the reagents used are anhydrous, so to me this takes acid-catalysed hydrolysis off of the table.

Even if the amide linkage was cleaved, the only way I could think of by which ammonia could be released is an acid-promoted E1 elimination. The amine at a secondary carbon centre would be protonated so is a possible leaving group under the harsh conditions, and the absence of a nucleophile (since water is driven off by evaporation) but presence of sulphate ions lead me to theorise an E1 style elimination of ammonia. This would then be protonated in the acidic conditions to form the salt. The intense heating also lead me to believe elimination would be favoured:

enter image description here

This looks like a rather unconventional mechanism but I assumed the high temperature would allow for something like this to occur. I suspect this mechanism is going in the complete wrong direction considering carbon dioxide is not produced in it, but I'm unable to think of or find anything else.

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  • $\begingroup$ Well I doubt it's really known, but it sure doesn't look like that. $\endgroup$
    – Mithoron
    Jun 12, 2020 at 21:29
  • $\begingroup$ Kjeldahl digestion includes heating to 400 celsium. Oxidation of peptide isn't really all that out place at those conditions. $\endgroup$
    – permeakra
    Jun 12, 2020 at 23:40

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