4-nitrophenyl (4-carboxybutyl)phosphonate is an organophosphonate oxoanion that is a nitrophenyl phosphonate hapten and transition-state analogue derived from pentanoic acid, with affinity for the germline precursor to catalytic antibody 48G7. It has a role as an epitope. It is a C-nitro compound, a monocarboxylic acid and an organophosphonate oxoanion.