It has been reported in literature that when the methionine is followed by
a serine or threonine (Met-Thr), one does observe a small quantity of
uncleaved peptide in which the -S-CH3 group of the methionine is replaced
by an -OH group resulting in a homoserine residue in place of methionine.
This is the result of intramolecular attack of the -OH group of threonine
(or serine) on the homoserine lactone ring via a 5 membered ring.
Now the peptide that I have has a Thr-Met-Asp sequence and through mass
spec (and after CNBr digestion) I am observing a peptide of molecular
weight corresponding to the the intact peptide with Met replaced by
homoserine residue.
My question is this: Has there been any report in the literature
regarding the formation of uncleaved homoserine peptide, via CNBr
cleavage, in which the threonine PRECEDES methionine. Mechanistically this
reaction also seems possible as the reaction now occurs via a 6 membered
ring rather than the five membered one in Met-Thr case.
Thanks
Khurram Mehmud Sunasara
95, 25th Street
Troy, NY 12180
phone: 518-273-4861
email: sunask@rpi.edu