A cross linking {HEAD ~ reagent} suitable for forming an acid cleavable cross link between {TAIL ~ the amino nitrogen} of an amino group containing substance and a substance containing a sulfhydryl function, said cross linking reagent comprising one of the following units: ##STR12## where R1 and R2 are independently selected from H and alkyl groups of C5 or less, and A comprises a bridge unit; and ##STR13## where R2 and A are as defined above and s=2 5 
A cross linking { HEAD ~ reagent } suitable for forming an acid cleavable cross link between { TAIL ~ the amino nitrogen } of an amino group containing substance and a substance containing a sulfhydryl function , said cross linking reagent comprising one of the following units : # # STR12 # # where R1 and R2 are independently selected from H and alkyl groups of C5 or less , and A comprises a bridge unit ; and # # STR13 # # where R2 and A are as defined above and s=2 5
OTH OTH OTH HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH