A cross linking reagent suitable for forming an acid cleavable cross link between 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 {HEAD ~ R1} and R2 are independently selected from {TAIL ~ 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 an acid cleavable cross link between 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 { HEAD ~ R1 } and R2 are independently selected from { TAIL ~ 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
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