Fluorescence Lifetime Correlation Spectroscopy Research Articles

Pulsed-Interleaved-Excitation Two-Dimensional Fluorescence Lifetime Correlation Spectroscopy.

We report on pulsed-interleaved-excitation two-dimensional fluorescence lifetime correlation spectroscopy (PIE 2D FLCS) to study biomolecular structural dynamics with high sensitivity and high time resolution using Förster resonance energy transfer (FRET). PIE 2D FLCS is an extension of 2D FLCS, which is a unique single-molecule fluorescence method that uses fluorescence lifetime information to distinguish different fluorescence species in equilibrium and resolves their interconversion dynamics with a submicrosecond time resolution. Because 2D FLCS has used only a single-color excitation so far, it was difficult to distinguish a very low-FRET (or zero-FRET) species from only donor-labeled species. We overcome this difficulty by implementing the PIE scheme (i.e., alternate excitation of the donor and acceptor dyes using two temporally interleaved excitations with different colors) to 2D FLCS, realizing two-color excitation and two-color fluorescence detection in 2D FLCS. After proof-of-principle PIE 2D FLCS analysis on the photon data synthesized with Monte Carlo simulation, we apply PIE 2D FLCS to a DNA-hairpin sample and show that this method readily distinguishes four fluorescent species, i.e., high-FRET, low-FRET, and two single-dye-labeled species. In addition, we show that PIE 2D FLCS can also quantitatively evaluate the contributions of the donor-acceptor spectral crosstalk, which often appears as artifacts in FRET studies and degrades the information obtained.

Three species multiplexing of fluorescent dyes and gold nanoclusters recovered with fluorescence lifetime correlation spectroscopy

Biosensor applications often require the simultaneous detection of multiple analytes, with a clear need to go beyond the traditional multiplexing relying on distinct fluorescent dyes across the visible spectrum. Fluorescence lifetime correlation spectroscopy (FLCS) is a powerful approach taking advantage of the fluorescence lifetime information to separate the contributions of different fluorescent species with overlapping emission spectra. However, so far FLCS detection has been demonstrated only on binary mixtures of two fluorescent dyes, limiting its multiplexing capabilities. Here, we report the first quantitative FLCS measurements within a ternary mixture composed of three different fluorescent emitters with near-identical emission spectra. Two organic fluorescent dyes, Alexa Fluor 647 and CF640R, are combined with water-soluble Au18(SG)14 gold nanoclusters. Our experimental data establish that FLCS allows to accurately determine individual concentrations within intricate ternary mixtures. Another major aspect of interest concerns the assessment of the suitability of gold nanoclusters for FLCS multiplexing applications. With their microsecond lifetime and stable emission characteristics, gold nanoclusters add a valuable new aspect to the array of FLCS probes. Extending FLCS multiplexing beyond binary mixtures paves the way for further progress in the simultaneous highly parallel biosensing of multiple species.

Evaluation of the passive permeability of antidepressants through pore-suspended lipid bilayer

HypothesisThe antidepressant drug imipramine, and its metabolite desipramine show different extents of interaction with, and passive permeation through, cellular membrane models, with the effects depending on the membrane composition. Through multimodal interrogation, we can observe that the drugs have a direct impact on the physicochemical properties of the membrane, that may play a role in their pharmacokinetics. ExperimentsMicrocavity pore-suspended lipid bilayers (MSLBs) of four different compositions, each with a different headgroup charge namely; zwitterionic dioleoylphosphatidylcholine (DOPC), mixed DOPC and negatively charged dioleoylphosphatidylglycerol (DOPG) (3:1), mixed DOPC and positively charged dioleoyltrimethylammoniumpropane (DOTAP) (3:1), and with increasing complex composition mimicking blood-brain-barrier (BBB) were prepared on gold and polydimethylsiloxane (PDMS) substrates using a Langmuir-Blodgett-vesicle fusion method. The molecular interaction and permeation of antidepressants, imipramine, and its metabolite desipramine with the lipid bilayers were evaluated using highly sensitive label-free electrochemical impedance spectroscopy (EIS) and surface-enhanced Raman spectroscopy (SERS). Drug-induced membrane packing/fluidity alterations were assessed using fluorescence lifetime imaging (FLIM) and fluorescence lifetime correlation spectroscopy (FLCS) of MSLB over microfluidic PDMS array. FindingsUsing EIS to evaluate in real-time membrane admittance changes, we found that imipramine greatly increases the ion permeability of negatively charged DOPC:DOPG (3:1) membranes. The effect was observed also at neutral (DOPC) and to a lesser extent at positively charged DOPC:DOTAP(3:1) membranes. In contrast, desipramine had a much weaker impact on ion permeability across all bilayer compositions. Temporal capacitance data show that desipramine intercalates at negatively charged membrane thereby increasing the thickness of the membrane. The overall kinetics of the imipramine permeation is higher than that of desipramine. This was confirmed using SERS, which also provides an evaluation of drug passive permeation based on arrival time across the membrane. Using FLCS, we found that imipramine increases the lipid membrane fluidity, whereas desipramine lowers it, with the exception of the negatively charged membrane. A translocation rate pharmacokinetics model was established for the first time at the MSLB platform by real-time monitoring of the variation in membrane resistance of pristine DOPC and blood-brain-barrier (BBB) membrane.

Photoluminescence Blinking of Quantum Confined CsPbBr3 Perovskite Nanocrystals: Influence of Size

Quantum confined CsPbBr3 nanocrystals (NCs) with adjustable photoluminescence (PL) in the range of 470–500 nm are particularly desirable for applications in light-emitting diodes and many quantum technologies. Exploration of the full potential of these perovskite NCs requires an understanding of the random fluctuation of their PL at the single-particle level, commonly termed as blinking. In this work, we study the PL blinking of quantum confined single NCs of CsPbBr3 of three different sizes between 3.80 and 5.90 nm in immobilized and fluid conditions to understand the recombination pathways and dynamics of the photogenerated charge carriers. In the immobilized state, the PL intensity trajectories and PL lifetime-intensity distributions of these single NCs reveal the contributions of both Auger recombination and trapping of hot carriers to the PL fluctuation. The results suggest a higher carrier trapping rate constant for smaller NCs. The fluorescence correlation spectroscopy and fluorescence lifetime correlation spectroscopy measurements on freely diffusing NCs show a higher off-state fraction and lower per-particle brightness of the smaller NCs. It is concluded that a larger trap depth and higher probability density of the carriers at the surface in smaller NCs make the trapping process more feasible and detrapping more difficult. The results provide first information on the effect of size on PL blinking of the quantum confined CsPbBr3 NCs, and this knowledge is expected to be useful in better designing photoluminescent samples of this class for optoelectronic applications.

Single-Particle Dynamics of ZnS Shelling Induced Replenishment of Carrier Diffusion for Individual Emission Centers in CuInS2 Quantum Dots.

Insights into blinking and photoactivation of aqueous copper indium sulfide (CIS) quantum dots have been obtained using fluorescence correlation spectroscopy (FCS) and fluorescence lifetime correlation spectroscopy (FLCS). An unusual excitation wavelength-dependence of photoactivation/photocorrosion is manifested in an increase in the initial correlation amplitude G(0) for λex = 532 nm, but a decrease for λex = 405 nm. This has been rationalized in terms of different contributions from surface-assisted recombination in the two cases. Blinking times obtained from the autocorrelation functions (ACFs) of the 100-200 ns lifetime component (core Cu-mediated recombination) are almost unaffected by shelling, but those from the ACF for the 10-30 ns lifetime (surface states) increase significantly. Absence of cross-correlation between the two recombinative states of bare CIS QDs and the emergence of an anticorrelation with the introduction of the ZnS shell are observed, indicating the diffusive nature of the two states for CIS-ZnS. The diffusion is inhibited in bare CIS QDs due to the preponderance of surface states.

Galectin-3 Binding to α5β1 Integrin in Pore Suspended Biomembranes.

Galectin-3 (Gal3) is a β-galactoside binding lectin that mediates many physiological functions, including the binding of cells to the extracellular matrix for which the glycoprotein α5β1 integrin is of critical importance. The mechanisms by which Gal3 interacts with membranes have not been widely explored to date due to the complexity of cell membranes and the difficulty of integrin reconstitution within model membranes. Herein, to study their interaction, Gal3 and α5β1 were purified, and the latter reconstituted into pore-suspended lipid bilayers comprised eggPC:eggPA. Using electrochemical impedance and fluorescence lifetime correlation spectroscopy, we found that on incubation with low nanomolar concentrations of wild-type Gal3, the membrane's admittance and fluidity, as well as integrin's lateral diffusivity, were enhanced. These effects were diminished in the following conditions: (i) absence of integrin, (ii) presence of lactose as a competitive inhibitor of glycan-Gal3 interaction, and (iii) use of a Gal3 mutant that lacked the N-terminal oligomerization domain (Gal3ΔNter). These findings indicated that WTGal3 oligomerized on α5β1 integrin in a glycan-dependent manner and that the N-terminal domain interacted directly with membranes in a way that is yet to be fully understood. At concentrations above 10 nM of WTGal3, membrane capacitance started to decrease and very slowly diffusing molecular species appeared, which indicated the formation of protein clusters made from WTGal3-α5β1 integrin assemblies. Overall, our study demonstrates the capacity of WTGal3 to oligomerize in a cargo protein-dependent manner at low nanomolar concentrations. Of note, these WTGal3 oligomers appeared to have membrane active properties that could only be revealed using our sensitive methods. At slightly higher WTGal3 concentrations, the capacity to generate lateral assemblies between cargo proteins was observed. In cells, this could lead to the construction of tubular endocytic pits according to the glycolipid-lectin (GL-Lect) hypothesis or to the formation of galectin lattices, depending on cargo glycoprotein stability at the membrane, the local Gal3 concentration, or plasma membrane intrinsic parameters. The study also demonstrates the utility of microcavity array-suspended lipid bilayers to address the biophysics of transmembrane proteins.

A cell free biomembrane platform for multimodal study of influenza virus hemagglutinin and for evaluation of entry-inhibitors against hemagglutinin.

Seasonal periodic pandemics and epidemics caused by Influenza A viruses (IAVs) are associated with high morbidity and mortality worldwide. They are frequent and unpredictable in severity so there is a need for biophysical platforms that can be used to provide both mechanistic insights into influenza virulence and its potential treatment by anti-IAV agents. Host membrane viral association through the glycoprotein hemagglutinin (HA) of IAVs is one of the primary steps in infection. HA is thus a potential target for drug discovery and development against influenza. Deconvolution of the multivalent interactions of HA at the interfaces of the host cell membrane can help unravel therapeutic targets. In this contribution, we reported the effect of a multivalent HA glycoprotein association on various glycosphingolipid receptors (GD1a, GM3, GM1) doped asymmetrically into an artificial host membrane spanned across an aqueous filled microcavity array. The extent of HA association and its impact on membrane resistance, capacitance, and diffusivity was measured using highly sensitive electrochemical impedance spectroscopy (EIS) and fluorescence lifetime correlation spectroscopy (FLCS). Furthermore, we investigated the inhibition of the influenza HA glycoprotein association with the host mimetic surface by natural and synthetic sialic acid-based inhibitors (sialic acid, Siaα2,3-GalOMe, FB127, 3-sialyl lactose) using electrochemical impedance spectroscopy and observe that while all inhibit, they do not prevent host binding. Overall, the work demonstrates the platform provides a label-free screening platform for the biophysical evaluation of new inhibitors in the development of potential therapeutics for IAV infection prevention and treatment.

Reaction Cycle of Operating Pump Protein Studied with Single-Molecule Spectroscopy.

Biological machinery relies on nonequilibrium dynamics to maintain stable directional fluxes through complex reaction cycles. For such reaction cycles, the presence of microscopically irreversible conformational transitions of the protein, and the accompanying entropy production, is of central interest. In this work, we use multidimensional single-molecule fluorescence lifetime correlation spectroscopy to measure the forward and reverse conformational transitions of bacteriorhodopsin during trans-membrane H+ pumping. We quantify the flux, affinity, enthalpy and entropy production through portions of the reaction cycle as a function of temperature. We find that affinity of irreversible conformational transitions decreases with increasing temperature, resulting in diminishing flux and entropy production. We show that the temperature dependence of the transition affinity is well fit by the Gibbs-Helmholtz relation, allowing the ΔHtrans to be experimentally extracted.

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.

Protein-Bath Coupling of an Internal Reaction Coordinate at Intermediate Time Scales.

Thermally activated barrier-crossing processes are central to protein reaction kinetics. A determining factor for such kinetics is the extent to which the protein's motions are coupled to the surrounding bath. It is understood that slow large-scale conformational motions are strongly coupled to the environment, while fast librational motions are uncoupled. However, less is known about protein-bath coupling of reaction coordinates located on the interior of a protein and with dynamics on intermediate time scales. In this work, we use single molecule 2D fluorescence lifetime correlation spectroscopy to study the microsecond chemical reaction occurring in the chromophore pocket of eGFP. The equilibrium reaction involves a dihedral rotation of a glutamic acid residue and a rearrangement of the local hydrogen-bonding network surrounding the endogenous chromophore, with no accompanying large-scale conformational changes. We observe that the internal chemical reaction is coupled to the solvent viscosity, though the scaling deviates from Kramers' behavior. We attribute this deviation to the internal friction of the protein, which weakens the protein-solvent coupling at high viscosity and intermediate time scales.

Fast and robust two-dimensional inverse Laplace transformation of single-molecule fluorescence lifetime data

Fluorescence spectroscopy at the single-molecule scale has been indispensable for studying conformational dynamics and rare states of biological macromolecules. Single-molecule two-dimensional (2D) fluorescence lifetime correlation spectroscopy is an emerging technique that holds promise for the study of protein and nucleic acid dynamics, as the technique is 1) capable of resolving conformational dynamics using a single chromophore, 2) resolves forward and reverse transitions independently, and 3) has a dynamic window ranging from microseconds to seconds. However, the calculation of a 2D fluorescence relaxation spectrum requires an inverse Laplace transform (ILT), which is an ill-conditioned inversion that must be estimated numerically through a regularized minimization. Current methods for performing ILTs of fluorescence relaxation can be computationally inefficient, sensitive to noise corruption, and difficult to implement. Here, we adopt an approach developed for NMR spectroscopy (T1-T2 relaxometry) to perform one-dimensional (1D) and 2D-ILTs on single-molecule fluorescence spectroscopy data using singular-valued decomposition and Tikhonov regularization. This approach provides fast, robust, and easy to implement Laplace inversions of single-molecule fluorescence data. We compare this approach to the widely used maximal entropy method.

Robust Photoelectric Biomolecular Switch at a Microcavity-Supported Lipid Bilayer.

Biomolecular devices based on photo-responsive proteins have been widely proposed for medical, electrical, and energy storage and production applications. Also, bacteriorhodopsin (bR) has been extensively applied in such prospective devices as a robust photo addressable proton pump. As it is a membrane protein, in principle, it should function most efficiently when reconstituted into a fully fluid lipid bilayer, but in many model membranes, lateral fluidity of the membrane and protein is sacrificed for electrochemical addressability because of the need for an electroactive surface. Here, we reported a biomolecular photoactive device based on light-activated proton pump, bR, reconstituted into highly fluidic microcavity-supported lipid bilayers (MSLBs) on functionalized gold and polydimethylsiloxane cavity array substrates. The integrity of reconstituted bR at the MSLBs along with the lipid bilayer formation was evaluated by fluorescence lifetime correlation spectroscopy, yielding a protein lateral diffusion coefficient that was dependent on the bR concentration and consistent with the Saffman–Delbrück model. The photoelectrical properties of bR-MSLBs were evaluated from the photocurrent signal generated by bR under continuous and transient light illumination. The optimal conditions for a self-sustaining photoelectrical switch were determined in terms of protein concentration, pH, and light switch frequency of activation. Overall, a significant increase in the transient current was observed for lipid bilayers containing approximately 0.3 mol % bR with a measured photo-current of 250 nA/cm2. These results demonstrate that the platforms provide an appropriate lipid environment to support the proton pump, enabling its efficient operation. The bR-reconstituted MSLB model serves both as a platform to study the protein in a highly addressable biomimetic environment and as a demonstration of reconstitution of seven-helix receptors into MSLBs, opening the prospect of reconstitution of related membrane proteins including G-protein-coupled receptors on these versatile biomimetic substrates.

Revealing Explicit Microsecond Carrier Diffusion from One Emission Center to Another in an All-Inorganic Perovskite Nanocrystal.

Blinking of freely diffusing CsPbBr3 nanocrystals (NCs) is studied using fluorescence lifetime correlation spectroscopy (FLCS). Emitted photons from each NCs are assigned to an emission state (exciton or trap) based on their lifetime. Subsequently, an intrastate autocorrelation function (ACF) and an interstate cross-correlation function (CCF) are constructed. Fitting of the AFCs with an analytical model shows that, at low excitation power, the microsecond blinking timescale of the exciton state matches well with that of the trap state. Most interestingly, both of those timescales further correlate with the microsecond growth timescale of the CCF. The strong anti-correlation of the CCF along with the stretched exponential nature of the blinking kinetics confirms the involvement of carrier diffusion and dispersive trap states in NC blinking. At high excitation power, enhanced sample heterogeneity causes a more dispersive blinking. To the best of our knowledge, this is the first report of a NC blinking study using a single-molecule-based FLCS technique.

Microsecond Folding of preQ1 Riboswitch and Its Biological Significance Revealed by Two-Dimensional Fluorescence Lifetime Correlation Spectroscopy.

Riboswitches are regulatory elements of bacterial mRNA which function with conformational switching upon binding of specific cellular metabolites. In particular, transcriptional riboswitches regulate gene expression kinetically through the conformational change of the aptamer domain. In this study, we investigate the conformational dynamics and ligand binding mechanisms of the aptamer domain of a transcriptional prequeuosine (preQ1) riboswitch from Bacillus subtilis using two-dimensional fluorescence lifetime correlation spectroscopy (2D FLCS) with microsecond time resolution. The obtained time-resolved single-molecule data indicate that the aptamer domain undergoes folding/unfolding including three forms, which are attributed to hairpin (O), pseudoknot-like (pF), and H-type pseudoknot (fF) structures. It is found that a cofactor, Mg2+, binds only to the fF form with the conformational selection mechanism. In contrast, it is indicated that the ligand, preQ1, binds to the O form with the induced-fit mechanism and significantly accelerates the microsecond O → pF folding process. It is also shown that the binding with preQ1 substantially stabilizes the fF form that is generated from the pF form with a long time constant (>10 ms). Combining these results with the results of a former smFRET study on the slower time scale, we obtain an overall picture of the folding/unfolding dynamics of the aptamer domain as well as its energy landscape. On the basis of the picture obtained, we discuss the significance of the microsecond folding/unfolding of the aptamer domain for biological function of the riboswitch and propose the molecular mechanism of the gene expression controlled by the structural dynamics of the aptamer domain.

A Nanoplasmonic Assay of Oligonucleotide-Cargo Delivery from Cationic Lipoplexes.

A powerful new biophysical model is reported to assay nanocarrier lipid membrane permeability. The approach employs a nanophotonic biophysical membrane model as an assay to study oligonucleotide escape from delivery vector following fusion with endosomal membrane that relies on plasmonic hotspots within the receptor well, below the membrane to follow cargo arrival. Through the combined use of surface enhanced Raman spectroscopy and fluorescence lifetime correlation spectroscopy (FLCS), the model enables identification of a lipoplex-mediated endosomal-escape mechanism facilitated by DOTAP-oligonucleotide interaction that dictates the rate of oligonucleotide release. This work reveals a hitherto unreported release mechanism as a complex multistep interplay between the oligonucleotide cargo and the target membrane, rather than a process based solely on lipid mixing at the fusing site as previously proposed. This substantiates the observations that lipid mixing is not necessarily followed by cargo release. The approach presents a new paradigm for assessment of vector delivery at model membranes that promises to have wide application within the drug delivery design application space. Overall, this plasmonic membrane model offers a potential solution to address persistent challenges in engineering the release mechanism of large therapeutic molecules from their nanocarrier, which is a major bottleneck in intracellular delivery.

Spectral Detection Enables Multi-Color Fluorescence Fluctuation Spectroscopy Studies in Living Cells

Fluorescence fluctuation spectroscopy techniques provide a powerful toolbox to quantify transport dynamics and interactions of biomolecules in living cells. Cross-correlation analysis of spectrally well-separated fluctuations allows to investigate hetero-interactions between different biomolecules. This analysis is conventionally limited to two fluorescent species that are excited with one or two laser lines and detected in two non-overlapping spectral channels. However, signaling pathways in biological systems often involve interactions between multiple biomolecules, e.g. formation of ternary or quaternary protein complexes. Here, we implement a methodology to investigate such interactions at the plasma membrane (PM) of living cells, in the context of e.g. viral assembly or receptor-ligand interactions. To this aim, we introduce scanning spectral fluorescence correlation spectroscopy (SSFCS), a combination of scanning FCS with spectrally resolved detection and decomposition, inspired by fluorescence lifetime correlation spectroscopy. We first demonstrate that SSFCS allows cross-talk free cross-correlation analysis on cells expressing spectrally highly overlapping fluorescent proteins (FPs) such as mEGFP and mEYFP at the PM, with a single excitation line. We then verify the performance of SSFCS for the quantification of diffusion dynamics and protein oligomerization (based on molecular brightness analysis) of two protein species tagged with spectrally overlapping FPs. Adding a second laser line, we show the possibility of three- and four-species (cross-) correlation analysis using mApple and mCherry2 as overlapping FP tags in the red spectral region. Using this set of fluorophores, we furthermore extend the recently presented raster spectral image correlation spectroscopy (RSICS) approach to four species, and successfully demonstrate multiplexed RSICS measurements of protein interactions in cellular cytoplasm. Finally, we apply SSFCS and RSICS to investigate the assembly of the Influenza A virus matrix proteins and ternary polymerase complex at the PM and in the nucleus of living cells, respectively.

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.

Single-Color Fluorescence Lifetime Cross-Correlation Spectroscopy In Vivo

The ability to quantify protein concentrations and to measure protein interactions in vivo is key information needed for the understanding of complex processes inside cells, but the acquisition of such information from living cells is still demanding. Fluorescence-based methods like two-color fluorescence cross-correlation spectroscopy can provide this information, but measurement precision is hampered by various sources of errors caused by instrumental or optical limitations such as imperfect overlap of detection volumes or detector cross talk. Furthermore, the nature and properties of used fluorescent proteins or fluorescent dyes, such as labeling efficiency, fluorescent protein maturation, photostability, bleaching, and fluorescence brightness can have an impact. Here, we take advantage of previously published fluorescence lifetime correlation spectroscopy which relies on lifetime differences as a mean to discriminate fluorescent proteins with similar spectral properties and to use them for single-color fluorescence lifetime cross-correlation spectroscopy (sc-FLCCS). By using only one excitation and one detection wavelength, this setup avoids all sources of errors resulting from chromatic aberrations and detector cross talk. To establish sc-FLCCS, we first engineered and tested multiple green fluorescent protein (GFP)-like fluorescent proteins for their suitability. This identified a novel, to our knowledge, GFP variant termed short-lifetime monomeric GFP with the so-far shortest lifetime. Monte-Carlo simulations were employed to explore the suitability of different combinations of GFP variants. Two GFPs, Envy and short-lifetime monomeric GFP, were predicted to constitute the best performing couple for sc-FLCCS measurements. We demonstrated application of this GFP pair for measuring protein interactions between the proteasome and interacting proteins and for measuring protein interactions between three partners when combined with a red florescent protein. Together, our findings establish sc-FLCCS as a valid alternative for conventional dual-color fluorescence cross-correlation spectroscopy measurements.

Plasma membrane asymmetry of lipid organization: fluorescence lifetime microscopy and correlation spectroscopy analysis

A fundamental feature of the eukaryotic cell membrane is the asymmetric arrangement of lipids in its two leaflets. A cell invests significant energy to maintain this asymmetry and uses it to regulate important biological processes, such as apoptosis and vesiculation. The dynamic coupling of the inner or cytoplasmic and outer or exofacial leaflets is a challenging open question in membrane biology. Here, we combined fluorescence lifetime imaging microscopy (FLIM) with imaging total internal reflection fluorescence correlation spectroscopy (ITIR-FCS) to differentiate the dynamics and organization of the two leaflets of live mammalian cells. We characterized the biophysical properties of fluorescent analogs of phosphatidylcholine, sphingomyelin, and phosphatidylserine in the plasma membrane of two mammalian cell lines (CHO-K1 and RBL-2H3). Because of their specific transverse membrane distribution, these probes allowed leaflet-specific investigation of the plasma membrane. We compared the results of the two methods having different temporal and spatial resolution. Fluorescence lifetimes of fluorescent lipid analogs were in ranges characteristic for the liquid ordered phase in the outer leaflet and for the liquid disordered phase in the inner leaflet. The observation of a more fluid inner leaflet was supported by free diffusion in the inner leaflet, with high average diffusion coefficients. The liquid ordered phase in the outer leaflet was accompanied by slower diffusion and diffusion with intermittent transient trapping. Our results show that the combination of FLIM and ITIR-FCS with specific fluorescent lipid analogs is a powerful tool for investigating lateral and transbilayer characteristics of plasma membrane in live cell lines.

Multifocus Fluorescence Correlation Spectroscopy with Spatially Separated Excitation Beams

Abstract We report on a new multifocus optical system developed for fluorescence correlation spectroscopy (FCS). This system utilizes a beamsplitter array to spatially separate excitation beams. Fluorescence from each spot is individually detected with a single-photon avalanche photodiode (SPAD) through a fiber bundle. The combined use of beamsplitters and multiple SPADs is beneficial to give a higher detection uniformity among focal spots than the existing multifocus FCS systems. Applications of this setup show that the variation of the optical parameters among seven focal spots are as small as ∼5% and prove its high performance in FCS measurements. High efficiency of this system is also demonstrated in application to an advanced FCS method, two-dimensional fluorescence lifetime correlation spectroscopy. Along with the results of these applications, we describe a method for proper correction of the crosstalk effect in the multifocus setup.