Neutral labeling reactants and conjugates derived thereof

A method of dissociation enhanced lanthanide fluorescence immunoassay, wherein the method comprises detecting a signal derived from a biospecific binding reactant conjugated with a chelate of formula (I)

• wherein:
• -A- is a linker comprising from one to ten moieties, wherein said from one to ten moieties are selected from the group consisting of phenylene, alkyl containing 1-12 carbon atoms, ethynediyl (—C≡C—), ethylenediyl (—C═C—);
• ether (—O—), thioether (—S—), amide (—CO—NH— and —NH—CO— and —CO—NR′ and —NR′—CO—), carbonyl (—CO—), ester (—COO— and —OOC—), disulfide (—SS—), diaza (—N═N—) and a tertiary amine (—NR′—), wherein R′ represents an alkyl containing less than 5 carbon atoms;
• R is selected from the group consisting of —CONH2, —CONHR1 and —CONR1R2, wherein R1 and R2 are the same or different and independently comprise from one to ten moieties, wherein said from one to ten moieties are selected from the group consisting of phenylene, alkyl containing 1-12 carbon atoms, ethynediyl (—C≡C—), ethylenediyl (—C═C—), ether (—O—), thioether (—S—), amide (—CO—NH— and —NH—CO— and —CO—NR′ and —NR′—CO—), carbonyl (—CO—), ester (—COO— and —OOC—), disulfide (—SS—), diaza (—N═N—) and a tertiary amine (—NR′—), wherein R′ represents an alkyl containing less than 5 carbon atoms;
• X is a reactive group for conjugation of the chelate to said biospecific binding reactant and is selected from the group consisting of amino, aminooxy, haloacetamido, isothiocyanato, 3,5-dichloro-2,4,6-triazinylamino, maleimido, and a thioester or an active ester of a carboxylic acid;
• and
• M is selected from the group consisting of europium, terbium, samarium and dysprosium.