Hi everyone,

I am trying to understand if there is a mathematical relationship between brightness and fluorescence intensity (measured at a given wavelength). While fluorescence intensity is dependent on measurement conditions, I am talking in relative terms, so to give a very specific example:

I have a fluorescent compound which binds to a biomolecule (which is non-absorbing, non-fluorescent). Binding to the biomolecule results in an 80-fold absorption turn-on, and a 100-fold fluorescence intensity turn-on. After binding, the fluorophore has an extinction coefficient of 100 000 M-1.cm-1 and a quantum yield of 0.90 (brightness = 90 000 M-1.cm-1).

From this data, can I determine the extinction coefficient, quantum yield and therefore brightness of the fluorophore before binding?

Absorption seems simple, I can deduce that the extinction coefficient of the unbound fluorophore is 1250 M-1.cm-1. But what of quantum yield? Does the 100-fold fluorescence turn-on means the the brightness of the unbound fluorophore is 900 M-1.cm-1? And therefore the quantum yield is 0.72?

Thank you (preparing a big summary table for a review paper, and trying to fill in the blank on data not reported in the original research papers).