Förster energy transfer

Introduction
Lots of molecules can be brought in an electronic excited state by absorbing a photon. If the emission of that photon than occurs in a singlet state, we can describe it as fluorescence. Under certain circumstances the energy of an excited molecule can be transfered to another molecule without radiation, known as förster resonance energy transfer.

The förster resonance energy transfer happens between two molecules that we can define as aceptor (A) chromophore and donator (D) chromophore. Lets assume the following: an excited (*) D chromophore molecule in the first excited singlet state (S1,0) is in a distance (r) of an A molecule (S0,0) that is in an electronic ground state. Both can not move from their position, they are embedded in a fixed matrix. The energy between the two molecular systems (D → A) is being transfered in an nonradiation process through dipole-dipole coupling:

D*(S1,0) + A(S0,0) → D(S0,0) + A*(1,0)

If r=R0 the transfer efficiency is 50% (definition of the Förster radius).

Discussion
The strong distance-dependence of the förster resonance energy transfer efficiency has been widely utilized in studying the structure and dynamics of proteins and nucleic acids, in the detection and visualization of intermolecular association and in the development of intermolecular binding assays. Förster resonance energy transfer is a particularly useful tool in molecular biology as the fraction, or efficiency, of energy that is transferred can be measured, and depends on the distance between the two fluorophores.