Two-Dimensional Fluorescence Lifetime Correlation Spectroscopy: Concepts and Applications

Diffusion of Multiple Species Resolved by Fluorescence Lifetime Recovery after Photobleaching (FLRAP)

Fluorescence recovery after photobleaching (FRAP) is now an indispensable tool to analyze the diffusion of molecules in vivo and in vitro. However, a conventional fluorescence intensity-based approach has difficulty in analyzing the diffusion of multiple species simultaneously. Here, we report fluorescence lifetime recovery after photobleaching (FLRAP) that incorporates fluorescence lifetime information into FRAP. By using FLRAP, the fluorescence intensity-recovery curves of each species can be successfully extracted from the ensemble photon data by utilizing their species-specific fluorescence decay curves, which are verified by applying FLRAP to two heterogeneous systems. Thus, FLRAP can be a powerful tool to quantitatively elucidate the molecular diffusion of multiple species in complex systems such as in living cells.

Scanning Two-Dimensional Fluorescence Lifetime Correlation Spectroscopy: Conformational Dynamics of DNA Holliday Junction from Microsecond to Subsecond

Single-molecule Förster resonance energy transfer (smFRET) is widely utilized to investigate the structural heterogeneity and dynamics of biomolecules. However, it has been difficult to simultaneously achieve a wide observation time window, a high structure resolution, and a high time resolution with the current smFRET methods. Herein, we introduce a new method utilizing two-dimensional fluorescence lifetime correlation spectroscopy (2D FLCS) and surface immobilization techniques. This method, scanning 2D FLCS, enables us to examine the structural heterogeneity and dynamics of immobilized biomolecules on a time scale from microsecond to subsecond by slowly scanning the sample stage at the rate of ∼1 μm/s. Application to the DNA Holliday junction (HJ) complex under various [Mg2+] conditions demonstrates that scanning 2D FLCS enables tracking reaction kinetics from 25 μs to 30 ms with a time resolution as high as 1 μs. Furthermore, the high structure resolution of scanning 2D FLCS allows us to unveil the ensemble nature of each isomer state and the heterogeneity of the dynamics of the HJ.

Microsecond Equilibrium Dynamics of Hairpin-Forming Oligonucleotides Quantified by Two-Color Two-Dimensional Fluorescence Lifetime Correlation Spectroscopy

RNA and DNA play distinct roles in biological systems. However, the underlying physicochemical difference has been poorly understood, in particular, that in dynamical aspects. In this paper, we report on a comparative study of the formation-dissociation dynamics of a hairpin structure of RNA and DNA with development of two-color two-dimensional fluorescence lifetime correlation spectroscopy (two-color 2D FLCS). In this extension of 2D FLCS, we newly introduce the two-color detection scheme to analyze not only donor fluorescence photons but also acceptor fluorescence photons from a doubly labeled Förster resonance energy transfer (FRET) pair. This new 2D FLCS is utilized to resolve multiple species present in an equilibrated condition with a microsecond time resolution and enhanced sensitivity, and the combined use with the filtered fluorescence correlation spectroscopy (FCS) method enables a quantitative discussion on microsecond structural dynamics occurring in the equilibrium. This integrated approach is applied to FRET-labeled RNA/DNA oligonucleotides having analogous hairpin-forming sequences, and it was revealed that the hairpin dissociation rate of RNA is an order of magnitude slower than that of DNA while their hairpin-forming rates are comparable. This marked difference is attributable to the distinct duplex structure of RNA and DNA. The present study demonstrates that the integrated approach combining two-color 2D FLCS and filtered FCS has a high potential for quantifying microsecond kinetics at the single-molecule level, which allows us to experimentally construct a free energy landscape.

Effect of electrostatic interaction on the leaflet-specific diffusion in a supported lipid bilayer revealed by fluorescence lifetime correlation analysis

Lipid–support electrostatic interaction determines the lipid dynamics in the proximal leaflet of a SLB.

Reduction of glass-surface charge density slows the lipid diffusion in the proximal leaflet of a supported lipid bilayer

Understanding the effect of a solid support on the dynamical properties of a supported lipid bilayer (SLB) is a prerequisite for the applications of SLB as a model biomembrane. Here, we applied two-dimensional fluorescence lifetime correlation spectroscopy to examine the effect of solution pH on the diffusion of lipids in the proximal/distal leaflets of a zwitterionic SLB. Leaflet-specific diffusion analyses at various pH revealed that the diffusion of lipids in the proximal leaflet facing a glass surface becomes slower by decreasing pH with the transition pH of ∼7.4. We attributed it to the reduction of the surface charge density of a glass support. Furthermore, the data clearly showed that the lipid diffusion in the distal leaflet facing a bulk solution is insensitive to the change in the diffusion property of the proximal leaflet. This reflects a weak interleaflet coupling between the proximal and distal leaflets of the SLB.