Effects of diffusion on energy transfer in solution using a microsecond decay time rhenium metal-ligand complex as the donor

Effect of diffusion on Förster resonance energy transfer in low-viscosity solution

The effect of translational diffusion on Förster resonance energy transfer between rhodamine 6G (the donor) and malachite green (the acceptor) was investigated by examining the donor's fluorescence decay profile. Eight straight-chain alkyl alcohols were used, and the temperature of ethanol was changed to vary the viscosity; the decay profiles were analyzed using the theory developed by Gösele. The critical transfer distance obtained from the decay profile is in good agreement with that evaluated using the spectroscopic quantities, and the diffusion coefficients are consistent with the Stokes-Einstein relation. The fluorescence decay profile was described well by Gösele's theory and the effect of diffusion is clearly verified by the temperature/viscosity dependence of the diffusion constant.

Role of diffusion in excitation energy transfer and migration

Effect of diffusion on excitation energy transfer and migration in a dye pair sodium fluorescein (donor) and Rhodamine-6G (acceptor) has been studied for different viscosities by both steady state and time domain fluorescence spectroscopic measurements. The donor-donor interaction appears to be weaker as compared to donor-acceptor interaction and thus favors direct Forster-type energy transfer. Interestingly, at low viscosity (water in this case) transfer appears to be controlled by material diffusion/energy migration. Further, acceptor dynamics reveals the fact that direct Forster transfer dominates in viscous media.

Luminescent metal-ligand complexes as probes of macromolecular interactions and biopolymer dynamics

The knowledge of microsecond dynamics is important for an understanding of the mechanism and function of biological systems. Fluorescent techniques are well established in biophysical studies, but their applicability to probe microsecond timescale processes is limited. Luminescent metal-ligand complexes (MLCs) have created interest mainly due to their unique luminescent properties, such as the exceptionally long decay times and large fundamental anisotropy values, allowing examination of microsecond dynamics by fluorescence methods. MLC properties also greatly simplify instrumentation requirements and enable the use of light emitting diode excitation for time-resolved measurements. Recent literature illustrates how MLC labels take full advantage of well developed fluorescence techniques and how those methods can be extended to timescales not easily accessible with nanosecond probes. MLCs are now commercially available as reactive labels which give researchers access to methods that previously required more complex approaches. The present paper gives an overview of the applications of MLC probes to studies of molecular dynamics and interactions of proteins, membranes and nucleic acids.

Lateral diffusion coefficients in membranes measured by resonance energy transfer and a new algorithm for diffusion in two dimensions

We describe measurements of lateral diffusion in membranes using resonance energy transfer. The donor was a rhenium (Re) metal-ligand complex lipid, which displays a donor decay time near 3 micros. The long donor lifetime resulted in an ability to measure lateral diffusion coefficient below 10(-8) cm(2)/s. The donor decay data were analyzed using a new numerical algorithm for calculation of resonance energy transfer for donors and acceptors randomly distributed in two dimensions. An analytical solution to the diffusion equation in two dimensions is not known, so the equation was solved by the relaxation method in Laplace space. This algorithm allows the donor decay in the absence of energy transfer to be multiexponential. The simulations show that mutual lateral diffusion coefficients of the donor and acceptor on the order of 10(-8) cm(2)/s are readily recovered from the frequency-domain data with donor decay times on the microsecond timescale. Importantly, the lateral diffusion coefficients and acceptor concentrations can be recovered independently despite correlation between these parameters. This algorithm was tested and verified using the donor decays of a long lifetime rhenium lipid donor and a Texas red-lipid acceptor. Lateral diffusion coefficients ranged from 4.4 x 10(-9) cm(2)/s in 1,2-dimyristoyl-sn-glycero-3-[phospho-rac-(1-glycerol)] (DMPG) at 10 degrees C to 1.7 x 10(-7) cm(2)/s in 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) at 35 degrees C. These results demonstrated the possibility of direct measurements of lateral diffusion coefficients using microsecond decay time luminophores.

Non-Arrhenius kinetics for the loop closure of a DNA hairpin

Intramolecular chain diffusion is an elementary process in the conformational fluctuations of the DNA hairpin-loop. We have studied the temperature and viscosity dependence of a model DNA hairpin-loop by FRET (fluorescence resonance energy transfer) fluctuation spectroscopy (FRETfs). Apparent thermodynamic parameters were obtained by analyzing the correlation amplitude through a two-state model and are consistent with steady-state fluorescence measurements. The kinetics of closing the loop show non-Arrhenius behavior, in agreement with theoretical prediction and other experimental measurements on peptide folding. The fluctuation rates show a fractional power dependence (beta = 0.83) on the solution viscosity. A much slower intrachain diffusion coefficient in comparison to that of polypeptides was derived based on the first passage time theory of SSS [Szabo, A., Schulten, K. & Schulten, Z. (1980) J. Chem. Phys. 72, 4350-4357], suggesting that intrachain interactions, especially stacking interaction in the loop, might increase the roughness of the free energy surface of the DNA hairpin-loop.