adduct fragmentation in tandem mass spectrometry (MS) experiments. Model pe
ptides contained cysteine and had a molecular mass of less than 2 kDa to fa
cilitate peptide fragmentation in tandem MS analyses. Peptides were adducte
d with an excess of benzoquinone, and the adducts were analyzed by LC/MS. A
dducts were identified by addition of 108 Da to the monoisotopic mass of th
e peptide, except in the case of oxytocin, which formed a bis adduct with a
ddition of 216 Da. Tandem MS experiments were performed on the [M + 2H](3+)
ions and/or the [M + H](+) ions. Sequence information obtained from modifi
ed peptides was comparable to that of their unmodified counterparts. A uniq
ue ion pair separated by 141 or 142 Da corresponding to P-elimination of be
nzoquinol-S or benzoquinol-SH from a b(n) or y(n) series ion indicated atta
chment at the sulfur of the cysteine residue. An alternate ion pair of 211
Da corresponded to fragmentation at the peptide bond on either side of the
adducted cysteine. Enzymatic digestion of BSA and a 2560 Da frog peptide wi
th trypsin yielded tryptic peptides, which were treated with benzoquinone.
In addition to ion pairs of 142 and 211 Da, singly and doubly charged trypt
ic peptide adducts showed a neutral loss of 142 Da from the precursor. Eith
er one or both ion pairs were present in more than half of all the peptides
that were examined. The neutral loss of 142 Da was present in all singly c
harged tryptic peptide adducts and in 11 out of 14 doubly charged tryptic p
eptide adducts. The data indicate that reliable detection of benzoquinone-c
ysteinyl peptide adducts requires monitoring of multiple spectral character
istics.