Patent ID: 7445902

Claim:
In a method for a homogeneous fluorescence resonance energy transfer (FRET) assay for measuring binding of a ligand to a GPCR polypeptide, which method comprises the steps of: i) contacting the GPCR polypeptide, which contains a Gα subunit comprising a covalently bound tag and a GTP binding site, with a fluorescent GTPase resistant GTP analogue, wherein said analogue comprises a first fluorescence resonance energy transfer (FRET) label; ii) contacting the GPCR with a detection molecule that comprises a binding moiety that specifically binds to the covalently bound tag and a second FRET label; and measuring the fluorescence intensity resulting from the intramolecular interaction between the first and second FRET labels in the presence and absence of said ligand; wherein an increase in fluorescence intensity is a measure of the potency of ligand binding to the GPCR polypeptide; the improvement comprises: using a compound having the formula hereinbelow as a donor or an acceptor of the FRET assay: wherein: X and Y are the same or different and are selected from >C(CH 3 ) 2 , —O— and —S—; one of groups R 1 , R 2 , R 3 and R 4 is the group W where W is: wherein: BASE is a purine base; R′ and R″ are independently H or OH; Z is —O—, —S—, or —NR 5 —, where R 5 is H or C 1 -C 4 alkyl; LiNK is a linking group containing from 1-20 linked carbon atoms which may optionally include one or more groups selected from —NR 5 —, —O—, —C(O)— and —CO—NR 5 —, where R 5 is hereinbefore defined; and when any of groups R 1 and R 2 is not said group W, said remaining groups R 1 and R 2 are independently selected from H and C 1 -C 4 alkyl, which may be optionally substituted with sulphonic acid or sulphonate; when any of groups R 3 and R 4 is not said group W, said remaining groups R 3 and R 4 are independently selected from H, sulphonic acid and sulphonate; and n is an integer from 1 to 3; and biologically compatible salts thereof.