The importance of energy transfer in biochemistry is that the efficiency of transfer can be used to evaluate the molecular distance r between the tryptophan residues in a protein (donor) and a drug (acceptor) at the binding site  and . According to the Föster non-radiative energy-transfer theory (FRET), energy will be transferred under the following conditions: (a) the donor can produce fluorescence light, (b) there is significant overlap between the fluorescence-emission spectrum of the donor and UV-absorption spectrum of the acceptor, and (c) the distance between the donor and the acceptor is less than 8 nm. Fig. 11 shows the overlap of the fluorescence-emission spectrum of BSA and the UV Embelin spectrum of DMA at 298 K. It can be seen that energy transfer would occur. The efficiency of energy transfer (E), under the conditions of 1:1 situation of donor to acceptor concentrations, can be expressed by :equation(8)E=1−II0=R06R06+r6where I and I0 are the fluorescence intensity of BSA in the presence and absence of DMA and R0 is a characteristic distance, called the Föster distance or critical distance, at which the transfer efficiency is 50%. R0 is determined from the following equation:equation(9)R06=8.79×10−25K2n−4?Jwhere, K2 is the spatial orientation factor describing the relative orientation in space of the transition dipoles of the donor and acceptor, n is the refraction index of the medium, Φ is the fluorescence quantum yield of the donor in the absence of the acceptor, and J is the overlap integral between the donor fluorescence-emission spectrum and the acceptor absorption spectrum when both spectra are recorded at the same concentration. J can be calculated byequation(10)J=∫I(λ)ελλ4dλ∫I(λ)dλ.