One of his observations was that if the two halves of the cystine were
released at the same cycle of the peptide sequence, a relatively higher yield
of PTH-Cys was observed, located within a few hundredths of a minute of PTH-
Tyr. If the two halves were released in different cycles, nothing was seen for
the first cys, and a lower yield was observed when the second cys was
encountered (an argument that the PTH peak was PTH-cystine and not
dehydroalanine or some other degradation product). In later studies he used
this observation to advantage by selecting protease digestion in combinations
that would place the two halves of a suspected disulfide bond as near as
possible the same cycle in the disulfide bonded fragments. The presence of a
real tyrosine complicates the analysis.
Although it is my understanding that ABI R3 does not currently contain DTT, R4
has in the past, and may still. Perhaps someone from ABI can comment on the
presence or absence of additives that might reduce disulfides. Obviously, the
age and relative amounts of reducing agent and disulfide peptides can make a
difference in eventual yield of PTH-Cys.
Mike Rohde