OK, so I mostly did FRAP and FLIP, but did do some FRET. Can't really remember a lot, but my incomplete garbling may help fill some gaps somewhere!

R

_{0} is the distance between the donor and acceptor probes where teh energy transfer is 50%. The equation is generally written with the '0's being subscript and the '6' being superscript. I guess this is something to do with the fact that 10 Angstrom = 1 nm, so the 60 Angstrom distance between the probes is 6 nm (but don't quote me - I'm working on it!).

E = R

_{0}^{6}/(R

_{0}^{6}+R

^{6})

This graph is quite good:

So, I think essentially, in this case:

R

_{0}^{6}= 60

E = 0.75

You can re-arrange the equation as follows: (I'm not doing sub and superscript anymore, its getting annoying!)

E(R06+R6) = R06 (multiply both sides by (R06+R6))

(E x R06) + (E x R6) = R06 (multiply out)

R06 + R6 = R06/E (divide both sides by E)

R6 = (R06/E) - R06 (minus R06)

Substitute the numbers:

R6 = (60/0.75)-60 = 20 Angstrom.

Does this make sense? The two probes must be closer than 60 A to get a transfer efficiency above 50%.