Development of long-lifetime metal-ligand probes for biophysics and cellular imaging

Characterization of Single-Spheroid Oxygen Consumption Using a Microfluidic Platform and Fluorescence Lifetime Imaging Microscopy

Oxygen consumption has been used to evaluate various cellular activities. In addition, three-dimensional (3D) spheroids have been broadly exploited as advanced in vitro cell models for various biomedical studies due to their capability of mimicking 3D in vivo microenvironments and cell arrangements. However, monitoring the oxygen consumption of live 3D spheroids poses challenges because existing invasive methods cause structural and cell damage. In contrast, optical methods using fluorescence labeling and microscopy are non-invasive, but they suffer from technical limitations like high cost, tedious procedures, and poor signal-to-noise ratios. To address these challenges, we developed a microfluidic platform for uniform-sized spheroid formation, handling, and culture. The platform is further integrated with widefield frequency domain fluorescence lifetime imaging microscopy (FD-FLIM) to efficiently characterize the lifetime of an oxygen-sensitive dye filling the platform for oxygen consumption characterization. In the experiments, osteosarcoma (MG-63) cells are exploited as the spheroid model and for the oxygen consumption analysis. The results demonstrate the functionality of the developed approach and show the accurate characterization of the oxygen consumption of the spheroids in response to drug treatments. The developed approach possesses great potential to advance spheroid metabolism studies with single-spheroid resolution and high sensitivity.

Optical probes and techniques for O2 measurement in live cells and tissue

In recent years, significant progress has been achieved in the sensing and imaging of molecular oxygen (O₂) in biological samples containing live cells and tissue. We review recent developments in the measurement of O₂ in such samples by optical means, particularly using the phosphorescence quenching technique. The main types of soluble O₂ sensors are assessed, including small molecule, supramolecular and particle-based structures used as extracellular or intracellular probes in conjunction with different detection modalities and measurement formats. For the different O₂ sensing systems, particular attention is paid to their merits and limitations, analytical performance, general convenience and applicability in specific biological applications. The latter include measurement of O₂ consumption rate, sample oxygenation, sensing of intracellular O₂, metabolic assessment of cells, and O₂ imaging of tissue, vasculature and individual cells. Altogether, this gives the potential user a comprehensive guide for the proper selection of the appropriate optical probe(s) and detection platform to suit their particular biological applications and measurement requirements.

Ruthenium polypyridyl peptide conjugates: membrane permeable probes for cellular imaging

Two novel polyarginine labelled ruthenium polypyridyl dyes are reported, one conjugated to five, (Ru-Ahx-R5), and one to eight arginine residues, (Ru-Ahx-R8); both complexes exhibit long-lived, intense, and oxygen-sensitive luminescence; (Ru-R8) is passively, efficiently and very rapidly transported across the cell membrane into the cytoplasm without requirement for its permeablisation.

Real-time background suppression during frequency domain lifetime measurements

We describe real time background suppression of autofluorescence from biological samples during frequency domain or phase modulation measurements of intensity decays. For these measurements the samples were excited with a train of light pulses with widths below 1 ps. The detector was gated off for a short time period of 10 to 40 ns during and shortly after the excitation pulse. The reference signal needed for the frequency domain measurement was provided by a long-lifetime reference fluorophore which continues to emit following the off-gating pulse. Both the sample and the reference were measured under identical optical and electronic conditions avoiding the need for correction of the photomultiplier tube signal for the gating sequence. We demonstrate frequency domain background suppression using a mixture of short- and long-lifetime probes and for a long-lifetime probe in human plasma with significant autofluorescence.

Interaction of oxygen-sensitive luminescent probes Ru(phen)(3)(2+) and Ru(bipy)(3)(2+) with animal and plant cells in vitro. Mechanism of phototoxicity and conditions for non-invasive oxygen measurements

Understanding the role of oxygen in the physiology, pathophysiology and radio- and chemosensitivity of animal cells requires accurate and non-invasive measurements of oxygen concentrations in the range of 0-2x10(-4) M, in cells in vitro or in vivo. High resolution 3D imaging techniques could be particularly useful in investigating tissue oxygenation in vivo and in model tissues (multicellular spheroids) in vitro. The goals of this work were to develop microscopy techniques and (i) to define conditions under which two oxygen-sensitive luminescent dyes, Ru(bipy)(3)(2+) (tris(2,2'-bipyridyl)ruthenium(II) chloride hexahydrate) and Ru(phen)(3)(2+) (tris(1,10-phenanthroline)ruthenium(II) chloride hydrate) can be used to probe oxygen concentrations within viable cells in vitro, when no phototoxic effects are evident, and (ii) to investigate the mechanism of phototoxicity once cell damage occurs. This report demonstrates that Ru(bipy)(3)(2+) and Ru(phen)(3)(2+) do not pass through intact biological membranes, do not cause measurable photodamage to plasma membranes at a concentration of 0.2 mM and, when loaded into endosomes, yield a strong luminescent signal. However, at an extracellular concentration of 1 mM, in the presence of 457-nm light, detectable amounts of both complexes accumulate at the plasma membrane and cause a loss of membrane integrity via a mechanism which may involve the generation of singlet oxygen.

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

We used resonance energy transfer and frequency-domain fluorometry to measure slow donor to acceptor diffusion in viscous media. The frequency-domain RET data were analyzed using a new numerical algorithm for predicting the donor intensity decay in the presence of diffusion occurring within the donor decay time. By the use of a rhenium metal-ligand complex as a microsecond decay time donor we were able to measure mutual donor-to-acceptor diffusion coefficients as low as 2 × 10-8 cm2/s. The availability of microsecond decay time luminophores and appropriate theory suggests the use of diffusion-enhanced energy transfer for measurement of diffusive processes and structural dynamics in biological systems.

Synthesis and characterization of a sulfhydryl-reactive rhenium metal-ligand complex

We describe the synthesis and spectral characterization of a rhenium metal-ligand complex. This complex reacts with sulfhydryl groups via an iodoacetamide side chain on the phenanthroline ligand and displays a high limiting anisotropy near 0.35 when excited at 442 nm. When covalently linked to human serum albumin, this complex displayed a mean decay time of about 1 micros. This decay time is appropriate for measuring rotational correlation times on the microsecond time scale as may occur for large macromolecular complexes.

Measurement of the Intensity of Long-Lifetime Luminophores in the Presence of Background Signals Using Phase-Modulation Fluorometry

We describe a method to correct for background fluorescence and to measure the intensity of long-lifetime probes using phase-modulation fluorometry. The theoretically predicted and simulated data were supported by two experiments. The fractional contribution of background fluorescence in the sample was determined by measurement of phase angle and/or modulation at single modulation frequency. In certain experimental situations, where the mean decay times of the background and the long lifetime probe are widely separated, determination of background signals in the sample does not require a blank sample or information about the nature of intensity decay of the background. Hence, phase-modulation fluorometry can be used to directly determine the intensity of the long-lifetime probe in the presence of an unknown short-lifetime background. We also discuss the effects of ambient light (indefinitely long lifetime) and scattered excitation light (zero lifetime) on phase-modulation measurements.

Resonance energy transfer study using a rhenium metal-ligand lipid conjugate as the donor in a model membrane

We measured steady state and time-resolved resonance energy transfer between donors and acceptors in model membranes. The donor was a long lifetime rhenium-lipid complex, which displayed a mean lifetime of 1 microsecond and lifetime components as long as 3 microseconds in the labeled DOPC membranes. The transfer efficiencies were found to be substantially larger than those predicted without consideration of lateral diffusion. The larger transfer efficiencies are consistent with a mutual lateral diffusion coefficient in the membrane near 2 x 10(-8) cm2/s. These results demonstrate that lateral diffusion in membranes can be detected with microsecond lipid probes.

Long-lifetime lipid rhenium metal-ligand complex for probing membrane dynamics on the microsecond timescale

We report the luminescence and spectral properties of a phospholipid analogue containing a long-lifetime luminescent rhenium metal-ligand complex (MLC) covalently linked to the amino group of phosphatidyl ethanolamine. When incorporated into synthetic membranes, this lipid probe displays intensity decay times near 3 microseconds. Importantly, the probe displays highly polarized emission with a maximal fundamental anisotropy of 0.33. This probe is expected to have numerous applications for studies of microsecond diffusion and dynamics of membranes.

Glucose sensor for low-cost lifetime-based sensing using a genetically engineered protein

We describe a glucose sensor based on a mutant glucose/galactose binding protein (GGBP) and phase-modulation fluorometry. The GGBP from Escherichia coli was mutated to contain a single cysteine residue at position 26. When labeled with a sulfhydryl-reactive probe 2-(4'-iodoacetamidoanilino)naphthalene-6-sulfonic acid, the labeled protein displayed a twofold decrease in intensity in response to glucose, with a dissociation constant near 1 microM glucose. The ANS-labeled protein displayed only a modest change in lifetime, precluding lifetime-based sensing of glucose. A modulation sensor was created by combining ANS26-GGBP with a long-lifetime ruthenium (Ru) metal-ligand complex on the surface of the cuvette. Binding of glucose changed the relative intensity of ANS26-GGBP and the Ru complex, resulting in a dramatic change in modulation at a low frequency of 2.1 MHz. Modulation measurements at 2.1 MHz were shown to accurately determine the glucose concentration. These results suggest an approach to glucose sensing with simple devices.

Long-lifetime Ru(II) complexes for the measurement of high molecular weight protein hydrodynamics

We describe the synthesis and characterization of two asymmetrical ruthenium(II) complexes, [Ru(dpp)2(dcbpy)]2+ and [Ru(dpp)2(mcbpy)]2+, as well as the water soluble sulfonated derivatives [Ru(dpp(SO3Na)2)2(dcbpy)]2+ and [Ru(dpp(SO3Na)2)2(mcbpy)]2+ (dpp is 4,7-diphenyl-1,10-phenanthroline, dcbpy is 4,4'-dicarboxylic acid-2,2'-bipyridine, mcbpy is 4-methyl,4'-carboxylic acid-2,2'-bipyridine, and dpp(SO3Na)2 is the disulfonated derivative of dpp) as probes for the measurement of the rotational motions of proteins. The spectral (absorption, emission, and anisotropy) and photophysical (time-resolved intensity and anisotropy decays) properties of these metal-ligand complexes were determined in solution, in both the presence and absence of human serum albumin (HSA). These complexes display lifetimes ranging from 345 ns to 3.8 microseconds in deoxygenated aqueous solutions under a variety of conditions. The carboxylic acid groups on these complexes were activated to form N-hydroxysuccinimide (NHS) esters which were used to covalently lable HSA, and were characterized spectroscopically in the same manner as above. Time-resolved anisotropy measurements were performed to demonstrate the utility of these complexes in measuring long rotational correlation times of bioconjugates between HSA and antibody to HSA. The potential usefulness of these probes in fluorescence polarization immunoassays was demonstrated by an association assay of the Ru(II)-labeled HSA with polyclonal antibody.

Use of a long-lifetime Re(I) complex in fluorescence polarization immunoassays of high-molecular-weight analytes

We describe a new class of fluorescence polarization immunoassays based on the luminescence from a Re(I) metal-ligand complex. Re(I) complexes are extremely photostable and possess useful photophysical properties including long lifetimes, high quantum yields, and high emission polarization in the absence of rotational diffusion. In the present study, a conjugatable, highly luminescent Re(I) metal-ligand complex, [Re(bcp)(CO)3(4-COOHPy)](ClO4), where bcp is 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline and 4-COOHPy is isonicotinic acid, has been evaluated for use in fluorescence polarization immunoassays (FPIs) with high-molecular-weight antigens. This Re(I) complex (Re) displays highly polarized emission (with a maximum anisotropy near 0.3) in the absence of rotational diffusion and a long average lifetime (2.7 microseconds) when bound to human serum albumin (HSA) in oxygenated aqueous solution. The emission polarization of the Re-HSA conjugate is sensitive to the binding of anti-HSA, resulting in a significant increase in anisotropy. The labeled HSA was also used in a competition immunoassay where unlabeled HSA was also used as an antigen. These experimental results, combined with theoretical predictions, demonstrate the potential of this Re(I) metal-ligand complex as a luminescence probe in FPIs of high-molecular-weight analytes (10(5)-10(8) Da).

Long-lifetime Ru(II) complexes as labeling reagents for sulfhydryl groups

We report the synthesis and spectral properties of two long-lifetime highly luminescent Ru(II) complexes containing either a sulfhydryl reactive iodoacetamido group or a less reactive choloroacetamido group, [Ru(bpy)2(5-iodoacetamido-1,10-phenanthroline)] (PF6)2 and [Ru(bpy)2(5-chloroacetamido-1,10-phenanthroline)](PF6) 2, respectively, where bpy is 2,2'-bipyridine. Ru(bpy)2(phen-IA)](PF6)2 was covalently linked to human serum albumin (HSA) and human immunoglobulin G (IgG). The photoluminescence lifetime of protein-bound probes approaches 1 microsecond under ambient conditions. In the absence of rotational motions, this probe displayed an anisotropy of 0.18 for excitation at 472 nm. Anisotropy decay data were used to determine the overall rotational correlation times of HSA and IgG. These long-lifetime sulfhydryl-reactive probes can be used to recover microsecond rotational motions and/or domain motions of proteins and/or macromolecular complexes.

Synthesis and spectral characterization of a thiol-reactive long-lifetime Ru(II) complex

We report the synthesis and spectral properties of a long-lifetime luminescent Ru complex containing a sulfhydryl-reactive maleimide group, [Ru (2,2'-bipyridine)2(1, 10-phenanthroline-5-maleimide)](PF6)2. [Ru(bpy)2(phen-mi)]2+ was covalently linked to human serum albumin, immunoglobulin G, and beta-galactosidase. The lifetimes for probe bound to proteins were near 1.1 micros. In the absence of rotational motions, the probe displayed an anisotropy near 0.17 for excitation near 475 nm. Anisotropy decay data were used to determine rotational correlation times of the proteins, which showed local probe motions in addition to overall rotational diffusion. This long-lifetime sulfhydryl-reactive probe can be used to recover microsecond rotational motions and/or domain motions of proteins and/or macromolecular complexes.