For example, {TAIL ~ a disulfide bond} is not reduced at {HEAD ~ the S—S bond} but is displaced to yield an R—S—S fragment ion 
For example , { TAIL ~ a disulfide bond } is not reduced at { HEAD ~ the S — S bond } but is displaced to yield an R — S — S fragment ion
OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH