Amino acid analysis results

Angela c. Murphy acmurphy at
Wed Sep 20 12:27:27 EST 1995

The only way I know to get a truly accurate value for cysteine is to oxidize
it with performic acid to cysteic acid, or to derivatize it with tetra-
thionate to make S-sulfo cysteine or with another reagent that converts it
to pyridylethyl cysteine.  You could convert it to carboxymethyl cysteine,
but unless you have a really well-evacuated tube or system, this derivative
tends to break down under acid hydrolysis.  Cysteine itself overlaps proline
on an ion-exchange column.  If it oxidizes to cystine, which can happen 
during hydrolysis, there will be a peak for half-cystine on the ion-exhange
trace, but it will not be anywhere near quantitative.  To see cysteine with
a reverse phase system using pre-column derivatization, it must be converted
to either the S-pyridylethyl or S-carboxymethyl form first.  Hope this helps.
Angela C. Murphy

On 19 Sep 1995, Daniela Sciaky wrote:

> We had a protein expressed in a pET vector with the his tag at the C
> terminus. Amino acid analysis shows that the amount of each amino acid is
> as expected for cysteine. We don't understand what would account for the
> lack of cysteines. Is cysteine particularly labile?
> Daniela

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