End-to-end diffusion on the microsecond timescale measured with resonance energy transfer from a long-lifetime rhenium metal-ligand complex

We measured the end-to-end diffusion coefficient of an alkyl chain-linked d onor-acceptor pair using the time-resolved frequency-domain decay of the do nor. The donor was a rhenium metal-ligand complex with a mean decay time ra nging from 2.1 to 7.9 mu s in the absence of the Texas red acceptor. The de cay time was used to measure the donor-to-acceptor distance distribution an d the mutual diffusion coefficient. Using this long lifetime donor, it was easily possible to determine a diffusion coefficient near 2 x 10(-8) cm(2)/ s and diffusion coefficients as low as 1.3 x 10(-9) cm(2)/s were measurable . Such long lifetime donors should be valuable for measuring the flexing of peptides on the microsecond timescale, domain motions of proteins and late ral diffusion in membranes. The availability of microsecond decay time lumi nophores now allows luminescence spectroscopy to be useful generally for st udies of microsecond dynamics of biological macromolecules.