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Phosphorescence probes can have significantly long lifetimes, on the order of micro- to milli-seconds or longer. In addition, environmental changes can affect the lifetimes of these phosphorescence probes. Thus, Phosphorescence Lifetime Imaging Microscopy (PLIM) is a very useful tool to localize the phosphorescence probes based on their lifetimes to study the variance in the lifetimes due to the micro environmental changes. Since the probes respond to the biologically relevant parameters like oxygen concentration, they can be used to study various biologically relevant processes like cellular metabolism, protein interaction etc. In this case, we study the effects of oxygen on Oxyphor G4 with PLIM. Since The Oxyphor G4 can be quenched by O2, it is a good example of such a probe and has a lifetime around 250us. Here we present the digital frequency domain PLIM technique and study the lifetime of the Oxyphor G4 as a function of the O2 concentration. The lifetime data are successfully presented in a phasor plot for various O2 concentrations and are consistent with the time domain data. Overall, we can analyze the oxygen consumption of varying cells using this technique.

PIE FastFLIM microscopy allows the quantitative multi-parameter measurement of single molecule protein folding and dynamics. Using donor-acceptor FRET pair-labeled proteins, we detect changes in protein conformation and dynamics by monitoring FRET efficiency, stoichiometry and lifetime. Together with anisotropy decay information, we acquire rotational relaxation times for single molecules. By applying antibunching, FLCS and burst analysis, multi-parameters (such as copy numbers in protein complexes), diffusion coefficient and molecular brightness can be fitted for deeper understanding of the conformational dynamic behavior of single protein molecules. In this paper, we’ll focus on the multiparameters of FRET efficiency, stoichiometry and lifetime.

Changes in energy metabolism, mitochondrial functions and of reactive oxygen species are often supposed to induce alterations in cellular activity. The major intracellular endogenous fluorophores are reduced nicotinamide adenine dinucleotide (NADH) and dinucleotide phosphate (NADPH), riboflavin's, and tryptophan present inside biological tissue and they can be used to image tissue architecture without any exogenous probe. Their fluorescence can be excited by multi-photon microscopy using NIR laser wavelengths [1,2,5,6]. Using FLIM imaging the lifetime of tryptophan and NADH were monitored in cells with and without addition of glucose in the medium. The lifetime data were collected and further using the ANOVA (refer table 2) of the lifetime of free NADH, bound NADH and Tryptophan, we found on applying the null hypothesis for the P-value > 0.05, there is a significant difference between the lifetimes of bound NADH and Tryptophan from control to glucose treated cells, however, free NADH, shown to be not significant change between the control and glucose treated cells. The tryptophan puzzle comes closer and closer to a solution. The ultimate evidence supporting the existence of FRET between Tryptophan and NADH in live cells slowly could come from lifetime measurements of tryptophan in proteins and bound NADH within live cells.

Phosphatase inhibitor-2 (I2) was discovered as a regulator of protein Ser/Thr phosphatase-1 and is conserved from yeast to human. Binding between purified recombinant I2 from different species and the prolyl isomerase Pin1 has been demonstrated with pull-down assays, size exclusion chromatography and nuclear magnetic resonance spectroscopy. Despite this, questions persist as to whether these proteins associate together in living cells. In this study, we prepared fluorescent protein (FP) fusions of I2 and Pin1 and employed both Forster Resonance Energy Transfer (FRET) and Fluorescence Correlation Spectroscopy (FCS) imaging techniques to characterize their interactions in living cells. In both intensity-based and time-resolved FRET studies, we observed FRET uniformly across whole cells co-expressing I2-Cerulean and Pin1-Venus that was significantly higher than in negative controls expressing Cerulean FP (without fusing to I2) as the FRET donor and Pin1-Venus, showing a specific interaction between I2-Cerulean and Pin1-Venus in living cells. We also observed the co-diffusion of I2-Cerulean and Pin1-mCherry in Fluorescence Cross Correlation Spectroscopy (FCCS) measurements. We further showed that I2 itself as well as I2-Pin1 formed complexes in living cells (predicted from in vitro studies) via a quantitative FRET assay, and demonstrated from FCS measurements that both I2 and Pin1 (fused to Cerulean) are highly mobile in living cells.

The genetically encoded fluorescent proteins (FP), used in combination with Forster resonance energy transfer (FRET) microscopy, provide the tools necessary for the direct visualization of protein interactions inside living cells. Currently, the FPs most commonly used for live-cell FRET studies are the Cerulean and Venus variants of the cyan and yellow FPs. However, there are problems associated with this donor-acceptor pair, and these might be overcome by exploiting the characteristics of some of the newer FPs. For example, earlier we showed that the monomeric teal FP (mTFP) has advantages over Cerulean as a FRET donor for Venus. Here, using mTFP as the common donor fluorophore, we characterize a variety of different yellow, orange and red FPs as potential acceptors of FRET. We employed a "FRET standard" genetic construct to minimize variability in the separation distance and positioning of the fused donor and acceptor FPs. Using spectral FRET imaging and fluorescence lifetime measurements from living cells expressing the fused proteins, we characterized both sensitized acceptor emission and the shortening of the donor lifetime resulting from quenching for each of the fused FP pairs. Surprisingly, we found disagreements between the spectral FRET and lifetime measurements for some of the different FP pairs. Our results appear to indicate that some of the orange and red FPs can quench the mTFP donor while yielding little sensitized emission. We are characterizing the basis for this observation.

Forster resonance energy transfer (FRET) methodology has been used for over 30 years to localize protein-protein interactions in living specimens. The cloning and modification of various visible fluorescent proteins (FPs) has generated a variety of new probes that can be used as FRET pairs to investigate the protein associations in living cells. However, the spectral cross-talk between FRET donor and acceptor channels has been a major limitation to FRET microscopy. Many investigators have developed different ways to eliminate the bleedthrough signals in the FRET channel for one donor and one acceptor. We developed a novel FRET microscopy method for studying interactions among three chromophores: three-color FRET microscopy. We generated a genetic construct that directly links the three FPs - monomeric teal FP (mTFP), Venus and tandem dimer Tomato (tdTomato), and demonstrated the occurrence of mutually dependent energy transfers among the three FPs. When expressed in cells and excited with the 458 nm laser line, the mTFP-Venus-tdTomato fusion proteins yielded parallel (mTFP to Venus and mTFP to tdTomato) and sequential (mTFP to Venus and then to tdTomato) energy transfer signals. To quantify the FRET signals in the three-FP system in a single living cell, we developed an algorithm to remove all the spectral cross-talk components and also to separate different FRET signals at a same emission channel using the laser scanning spectral imaging and linear unmixing techniques on the Zeiss510 META system. Our results were confirmed with fluorescence lifetime measurements and using acceptor photobleaching FRET microscopy.