oup of from 1 to 20 carbons and from 0 to 10 heteroatoms selected from the group consisting of nitrogen, oxygen, and sulfur, which may either be in the chain or substituted on the chain which may be straight or branched; where B is a third linking group ultimately attached to a protein, polypeptide or label selected from the group consisting of; ##STR15## where Z is a fourth hydrocarbyl linking gr