Detection of analytes using fluorescent energy transfer

Abstract

The invention provides labeled proteins suitable for determining the presence of cAMP, other second messengers and organic molecules. The proteins are separately labeled with fluorochromes which, when in close spatial proximity, preferably, less than about 6 nm, interact through the transfer of energy from one fluorochrome to the other. A composition of matter, (S.sub.1.A).sub.n1 (S.sub.2.D).sub.n2 is provided wherein S.sub.1 and S.sub.2 are two proteins which are associated in one state and substantially disassociated in another, the equilibrium between which is controlled by the free concentration of an analyte, and A and D are fluorochromes, the emission wavelength of fluorochrome D overlapping the excitation wavelength of fluorochrome A and the distance between A and D being in sufficiently close proximity to allow the radiationless transfer of energy between the fluorochromes. A and D can be selected from a variety of acceptor, donor pairs such as fluorescein and tetramethylrhodamine and derivatives thereof. The concentration of analytes such as cAMP in a sample can be determined by contacting the sample with (S.sub.1.A).sub.n1 (S.sub.2.D).sub.n2 providing energy near the excitation wavelength of D and measuring the fluorescence of A or D, the concentration of cAMP and other said analytes being determined by the ratio of emission of D to the emission of A, previously calibrated with reference solutions of known analyte concentration.