Chemiluminescence quenching of pteroic acid–N-sulfonyl-acridinium-9-carboxamide conjugates by folate binding protein

N10-Trifluoroacetylpteroic acid was conjugated to chemiluminescent N-sulfonylacridinium-9-carboxamide labels at the N10 or 9-position carboxamide. Upon binding to folate binding protein the light output of the N10 derivative (9) was quenched up to 62% upon triggering with basic peroxide, while the 9-position carboxamide conjugate (7) was quenched only 12%. The utility of this effect was demonstrated in a model homogeneous chemiluminescent assay for folic acid.