A cross linking {TAIL ~ reagent} suitable for forming {HEAD ~ 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 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 { TAIL ~ reagent } suitable for forming { HEAD ~ 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 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
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