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

A Resonance Energy Transfer Immunoassay Based on a Thiol-Reactive Ruthenium Donor Dye and a Longwave-Emitting Acceptor

A novel immunoassay is described that applies a thiol-reactive ruthenium metal-ligand complex as the donor dye in a luminescence energy transfer (LET) detection scheme. Unlike amine-reactive labels, the LET with a thiol label allows improved specificity and better reproducibility of labelling positions on proteins, because the number of reactive thiol groups of proteins is distinctly smaller. This helps to reduce the risk of over-labelling and self-quenching of the fluorophore. The synthesis of the thiol label was significantly improved, resulting in almost quantitative yields of pure product. The absorption and emission maxima of the ruthenium donor dye are at 460 nm and 600 nm, respectively, and a Stokes' shift of 140 nm warrants distinct separation of excitation and emission wavelengths even in turbid samples. A cyanine dye with an absorption maximum at 642 nm was chosen as the acceptor label because it has good overlap with the emission spectrum of the donor label. The emission of the acceptor peaks at 660 nm, thus further increasing the Stokes' shift (to an overall 200 nm). The quantification of anti-HSA with the LET immunoassay is possible with this new approach at concentrations as low as 220 pmol L(-1).

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.

Precise boundary element computation of protein transport properties: Diffusion tensors, specific volume, and hydration

A precise boundary element method for the computation of hydrodynamic properties has been applied to the study of a large suite of 41 soluble proteins ranging from 6.5 to 377 kDa in molecular mass. A hydrodynamic model consisting of a rigid protein excluded volume, obtained from crystallographic coordinates, surrounded by a uniform hydration thickness has been found to yield properties in excellent agreement with experiment. The hydration thickness was determined to be delta = 1.1 +/- 0.1 A. Using this value, standard deviations from experimental measurements are: 2% for the specific volume; 2% for the translational diffusion coefficient, and 6% for the rotational diffusion coefficient. These deviations are comparable to experimental errors in these properties. The precision of the boundary element method allows the unified description of all of these properties with a single hydration parameter, thus far not achieved with other methods. An approximate method for computing transport properties with a statistical precision of 1% or better (compared to 0.1-0.2% for the full computation) is also presented. We have also estimated the total amount of hydration water with a typical -9% deviation from experiment in the case of monomeric proteins. Both the water of hydration and the more precise translational diffusion data hint that some multimeric proteins may not have the same solution structure as that in the crystal because the deviations are systematic and larger than in the monomeric case. On the other hand, the data for monomeric proteins conclusively show that there is no difference in the protein structure going from the crystal into solution.

Temperature dependence of solvation dynamics and anisotropy decay in a protein: ANS in bovine serum albumin

Temperature dependence of solvation dynamics and fluorescence anisotropy decay of 8-anilino-1-naphthalenesulfonate (ANS) bound to a protein, bovine serum albumin (BSA), are studied. Solvation dynamics of ANS bound to BSA displays a component (300 ps) which is independent of temperature in the range of 278-318 K and a long component which decreases from 5800 ps at 278 K to 3600 ps at 318 K. The temperature independent part is ascribed to a dynamic exchange of bound to free water with a low barrier. The temperature variation of the long component of solvation dynamics corresponds to an activation energy of 2.1 kcal mol(-1). The activation energy is ascribed to local segmental motion of the protein along with the associated water molecules and polar residues. The time scale of solvation dynamics is found to be very different from the time scale of anisotropy decay. The anisotropy decays are analyzed in terms of the wobbling motion of the probe (ANS) and the overall tumbling of the protein.

Light-emitting iridium(iii) and ruthenium(ii) polypyridyl complexes containing quadruple hydrogen-bonding moieties

A novel compound containing both a 2,2'-bipyridine as well as a 2-ureido-4[1H]-ureidopyrimidinone supramolecular moiety (3) has been synthesised and fully characterized by 1H-NMR, MALDI-TOFMS, UV-vis and IR spectroscopy. Subsequent coordination to iridium and ruthenium polypyridyl precursors allowed the formation of iridium(III) and ruthenium(II) polypyridyl dimers (5 and 7) assembled via quadruple hydrogen-bonding as well as metal coordination interactions. The syntheses and complete characterization of these materials by means of two-dimensional NMR techniques (1H-1H COSY and 1H-1H DOSY) as well as IR and MALDI-TOFMS are described in detail. Comparative studies of the optical properties of the luminescent model complexes (5' and '7) and the dimer species (5 and 7) are also illustrated. In addition, good processability of the materials has been demonstrated by inkjet printing leading to thin films revealing their potential for light-emitting devices.

[Os(bpy)2(CO)(enIA)][OTf]2: A Novel Sulfhydryl−Specific Metal−Ligand Complex

The synthesis and physical-chemical characterization of the metal-ligand complex [Os(bpy)2(CO)(enIA)][OTf]2 (where enIA = ethylenediamine iodoacetamide) with a sulfhydryl-specific functional group is described. The UV and visible absorption and luminescence emission, including lifetime and steady-state anisotropy, are reported for the free probe and the probe covalently linked to two test proteins. The spectroscopic properties of the probe are unaffected by chemical modification and subsequent covalent linkage to the proteins. The luminescence lifetime in aqueous buffer is approximately 200 ns and the limiting anisotropy is greater than 0.125, suggesting a potentially useful probe for biophysical investigations.

High-molecular-weight protein hydrodynamics studied with a long-lifetime metal-ligand complex

[Ru(2,2'-bipyridine)(2)(4,4'-dicarboxy-2,2'-bipyridine)](2+) (RuBDc) is a very photostable probe that possesses favorable photophysical properties including long lifetime, high quantum yield, large Stokes' shift, and highly polarized emission. In the present study, we demonstrated the usefulness of this probe for monitoring the rotational diffusion of high-molecular-weight (MW) proteins. Using frequency-domain fluorometry with a high-intensity, blue light-emitting diode (LED) as the modulated light source, we compared the intensity and anisotropy decays of RuBDc conjugated to immunoglobulin G (IgG) and immunoglobulin M (IgM), which show a six-fold difference in MW We obtained slightly longer lifetimes for IgM (=428 ns in buffer) than IgG (=422 ns in buffer) in the absence and presence of glycerol, suggesting somewhat more efficient shielding of RuBDc from water in IgM than in IgG. The anisotropy decay data showed longer rotational correlation times for IgM (1623 and 65.7 ns in buffer) as compared to IgG (264 and 42.5 ns in buffer). Importantly, the ratio of the long rotational correlation times of IgM to IgG in buffer was 6.2, which is very close to that of MW of IgM to IgG (6.0). The shorter correlation times are most likely to be associated with domain motions within the proteins. The anisotropy decays reflect both the molecular size and shape of the immunoglobulins, as well as the viscosity. These results show that RuBDc can have numerous applications in studies of high-MW protein hydrodynamics and in fluorescence polarization immunoassays (FPI) of high-MW analytes.

On the Possibility of Glucose Sensing Using Boronic Acid and a Luminescent Ruthenium Metal-Ligand Complex

We describe a new approach to optical sensing of glucose based on the competitive interactions between a ruthenium metal ligand complex, a boronic acid derivative and glucose. The metal-ligand complex [Ru(2,2'-bipyridme)2(5,6-dihydroxy-1,10-phenanthrolme)](PF6)2 at pH 8 forms a reversible complex with 2-toluylboronic acid or 2-methoxyphenyl boronic acid. Complexation is accompanied by a several-fold increase in the luminescent intensity of the ruthenium complex. Addition of glucose results in decreased luminescent intensity, which appears to be the result of decreased binding between the metal-ligand complex and the boronic acid. Ruthenium metal-ligand complexes are convenient for optical sensing because their long luminescent decay times allow lifetime-based sensing with simple instrumentation.

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.

The conformation of serum albumin in solution: a combined phosphorescence depolarization-hydrodynamic modeling study

There is a striking disparity between the heart-shaped structure of human serum albumin (HSA) observed in single crystals and the elongated ellipsoid model used for decades to interpret the protein solution hydrodynamics at neutral pH. These two contrasting views could be reconciled if the protein were flexible enough to change its conformation in solution from that found in the crystal. To investigate this possibility we recorded the rotational motions in real time of an erythrosin-bovine serum albumin complex (Er-BSA) over an extended time range, using phosphorescence depolarization techniques. These measurements are consistent with the absence of independent motions of large protein segments in solution, in the time range from nanoseconds to fractions of milliseconds, and give a single rotational correlation time phi(BSA, 1 cP, 20 degrees C) = 40 +/- 2 ns. In addition, we report a detailed analysis of the protein hydrodynamics based on two bead-modeling methods. In the first, BSA was modeled as a triangular prismatic shell with optimized dimensions of 84 x 84 x 84 x 31.5 A, whereas in the second, the atomic-level structure of HSA obtained from crystallographic data was used to build a much more refined rough-shell model. In both cases, the predicted and experimental rotational diffusion rate and other hydrodynamic parameters were in good agreement. Therefore, the overall conformation in neutral solution of BSA, as of HSA, should be rigid, in the sense indicated above, and very similar to the heart-shaped structure observed in HSA crystals.

End-to-end diffusion on the microsecond timescale measured with resonance energy transfer from a long-lifetime rhenium metal-ligand complex

We measured the end-to-end diffusion coefficient of an alkyl chain-linked donor-acceptor pair using the time-resolved frequency-domain decay of the donor. The donor was a rhenium metal-ligand complex with a mean decay time ranging from 2.1 to 7.9 microseconds in the absence of the Texas red acceptor. The decay time was used to measure the donor-to-acceptor distance distribution and the mutual diffusion coefficient. Using this long lifetime donor, it was easily possible to determine a diffusion coefficient near 2 x 10(-8) cm2/s and diffusion coefficients as low as 1.3 x 10(-9) cm2/s were measurable. Such long lifetime donors should be valuable for measuring the flexing of peptides on the microsecond timescale, domain motions of proteins and lateral diffusion in membranes. The availability of microsecond decay time luminophores now allows luminescence spectroscopy to be useful generally for studies of microsecond dynamics of biological macromolecules.

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.

Anisotropy-based sensing with reference fluorophores

We describe a new approach to fluorescence sensing based on measurements of steady-state anisotropies in the presence of reference fluorophores with known anisotropies. The basic concept is that the anisotropy of a mixture reflects a weighted average of the anisotropies of the emitting species. By use of reference fluorophores the starting anisotropy can be near zero, or near 0.9 for oriented films which contain the reference fluorophore. Changing intensities of the analyte result in changes in anisotropy. A wide dynamic range of anisotropies is available because of the freedom to select high or low starting values. Anisotropy-based sensing was demonstrated for pH using 6-carboxyfluorescein and for protein affinity or immunoassay using an oriented film with high anisotropy and a protein labeled with a metal-ligand complex. The latter measurements were performed with a simple light-emitting diode excitation source without an excitation polarizer. The sensitive range of the assay can be adjusted by changing the intensity of the reference fluorophore. Anisotropy-based sensing can have numerous applications in clinical and analytical chemistry.

Low-frequency modulation sensors using nanosecond fluorophores

We describe a new approach to fluorescence sensing based on a mixture of fluorophores, one of which is sensitive to the desired analyte. If a long-lifetime analyte-insensitive fluorophore is mixed with a short-lifetime analyte-sensitive fluorophore, the modulation of the emission at conveniently low frequencies becomes equal to the fractional fluorescence intensity of the sensing fluorophore. Under these conditions, the modulation can be used to determine the analyte concentration. This can be used with any fluorophore that changes intensity in response to analyte and does not require the sensing fluorophore to display a change in lifetime. The feasibility of modulation-based sensing was demonstrated using mixtures of 6-carboxyfluorescein and [Ru 2,2'-(bipyridyl)3]2+ as a pH sensor and of the calcium probe Fluo-3 and [Ru 2,2'-(bipyridyl)3]2+ as a calcium sensor.