A cross linking reagent suitable for forming {HEAD ~ 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 reagent suitable for forming { HEAD ~ 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 OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL 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