This invention relates to attachment chemistries for binding macromolecules to a substrate surface or to other conjugation targets. More particularly, this invention relates to attachment chemistries involving branched or linear structures having one or more hydrazide attachment moieties for binding the macromolecules to a substrate surface, or for other conjugation reactions. Novel modifying reagents are provided for the introduction of protected hydrazide attachment moieties or precursor forms of such hydrazides to the macromolecule, either as a single hydrazide or as multiple hydrazides.