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PubMed2005

Interplay between intra- and interligand charge transfer with variation of the axial N-heterocyclic ligand in osmium(II) pyridylpyrazolate complexes: extensive color tuning by phosphorescent solvatochromism.

The rational design and syntheses of a new series of Os(II) complexes with formula [Os(fppz)(2)(CO)(L)] (1: L=4-dimethylaminopyridine; 2: L = pyridine; 3: L = 4,4'-bipyridine; 4: L = pyridazine; 5: L = 4-cyanopyridine), bearing two (2-pyridyl)pyrazolate ligands (fppz) together with one carbonyl and one N-heterocyclic ligand at the axial positions are reported. Single-crystal X-ray diffraction studies of, for example, 2 reveal a distorted octahedral geometry in which both fppz ligands reside in the equatorial plane with a trans configuration and adopt a bent arrangement at the metal center with a dihedral angle of approximately 23 degrees , while the carbonyl and pyridine ligands are located at the axial positions. Variation of the axial N-heterocyclic ligand leads to remarkable changes in the photophysical properties as the energy gap and hence the phosphorescence peak wavelength can be tuned. For complexes 1 and 2 the solvent-polarity-independent phosphorescence originates from a combination of intraligand (3)pi-pi* ((3)ILCT) and metal-to-ligand charge transfer transitions ((3)MLCT). In sharp contrast, as supported by cyclic voltammetry measurements and theoretical calculations, complexes 3--5 exhibit mainly ligand-to-ligand charge transfer (LLCT) transitions, resulting in a large dipolar change. The phosphorescence of complexes 3--5 thus exhibits a strong dependence on the polarity of the solvent, being shifted for example, from 560 (in C(6)H(12)) to 665 nm (in CH(3)CN) and from 603 (in C(6)H(12)) to 710 nm (in CH(3)CN) for complexes 3 and 5, respectively. The results clearly demonstrate that a simple, straightforward derivatization of the axial N-heterocyclic ligand drastically alters the excitation properties per se from intraligand charge transfer (ILCT) to LLCT transitions. The latter exhibit remarkable LLCT phosphorescence solvatochromism so that a broad range of color tunability can be achieved.

Journal Article↗

PubMed2007

Iridium(I) pyridyl azolate complexes with saturated red metal-to-ligand charge transfer phosphorescence; fundamental and potential applications in organic light-emitting diodes.

Preparation of a new series of neutral metal complexes [(cod)Ir(fppz)] (1), [(cod)Ir(bppz)] (2), [(cod)Ir(fptz)] (3) and [(cod)Ir(bptz)] (4), bearing one cod ligand and a pyridyl azolate chelate are reported. A single-crystal X-ray diffraction study of 3 reveals the expected distorted square-planar geometry. The lowest absorption band consists of IrI atom increased triplet dpi-->pi* transitions (3MLCT), the assignment of which is firmly supported by the theoretical approaches. Complexes 1-4 exhibit weak phosphorescence in degassed solution at room temperature, whereas much more intense, solid-state phosphorescence appears in the range 622-649 nm. The pure MLCT emission was used as a prototypical model to address its remarkable spectral differences from the IrIII isoquinoline pyrrolide complex (5), which has mainly 3pipi phosphorescence. Complex 3 was used as a dopant to fabricate red-emitting phosphorescent organic light-emitting diodes (OLEDs). For the 7 % doped device, a maximum brightness of 3010 cd m-2 was achieved at an applied voltage of 15 V and with CIE coordinates of (0.56, 0.33), demonstrating for the first time the potential of neutral IrI complexes in OLED applications.

Journal Article↗

PubMed1996

Long time-scale probing of the protein globular core using hydrogen-exchange and room temperature phosphorescence.

Preliminary room temperature phosphorescence measurements of the highly buried Trp109 in E. coli alkaline phosphatase have been used to report on the kinetics of protein hydrogen-deuterium exchange. Upon dilution in D2O the phosphorescence lifetime increases (at 20 degrees C) in a biphasic manner with an immediate change (< 30 seconds) followed by a slow change occurring on an extremely long timescale (days). The immediate D2O-induced lifetime increase is similar to that observed upon dilution into glycerol, a known protein hydrating agent. On the other hand, the slow D2O-induced first order growth in Trp109 lifetime is due to exchange at highly protected protein groups. As the phosphorescence lifetime of Trp109 is dependent on local rigidity, this increase in lifetime reflects changes in alkaline phosphatase structure. This first use of room temperature phosphorescence to monitor proton exchange shows promise as a sensitive and selective probe of protein core dynamics.

Alkaline Phosphatase↗

PubMed1993

Optical oxygen sensor based on phosphorescence lifetime quenching and employing a polymer immobilised metalloporphyrin probe. Part 1. Theory and instrumentation.

A review of the theory of phosphorescence quenching is given, and its particular application to the sensing of oxygen is outlined. The advantages of measuring phosphorescence lifetime as opposed to phosphorescence intensity are reviewed. The advantages of using the metalloporphyrins as such sensors are identified and in particular the characteristics of palladium coproporphyrin are discussed. The exceptionally long room temperature lifetime of this material makes it possible to use relatively simple PC-based instrumentation to measure lifetimes, with a xenon flashlamp light source. The design of such a system is given and its performance in measuring phosphorescence lifetime in aqueous solutions is demonstrated.

Fiber Optic Technology↗

PubMed1985

Conformational changes in the (Ca2+ + Mg2+)-ATPase of sarcoplasmic reticulum detected using phosphorescence polarization.

The technique of time-averaged phosphorescence has been used to study the interaction of calcium ions and ATP with the (Ca2+ + Mg2+)-ATPase in sarcoplasmic reticulum vesicles. The presence of excess calcium ions was found to cause a 20% decrease in the phosphorescence emission anisotropy. This is interpreted as being due to a conformational change in the protein and is supported by data from time-resolved phosphorescence measurements which also show a lowering of the anisotropy. This change in the decay of the emission anisotropy is associated with only minor changes in the rotational relaxation time of the protein and is again suggestive of a conformational change in the protein. In some cases ATP was also observed to lower the time-averaged phosphorescence anisotropy possibly via an interaction with the low-affinity regulatory site of the protein.

Adenosine Triphosphate↗

PubMed1981

Phosphorescence and optical detection of magnetic resonance of cowpea chlorotic mottle virus.

Phosphorescence spectra of the tryptophan residues in cowpea chlorotic mottle virus were recorded at 77 K and the influence of the quaternary structure on the emission characteristics was investigated. The position of the phosphorescence maxima appeared to be invariant under changes in the aggregation state of the virus particle. In contrast to the results of fluorescence experiments, the phosphorescence probably originates from tryptophan residues, buried in the hydrophobic interior of the virus. Optical detection of magnetic resonance on the triplet state of the tryptophan residues at 1.2 K shows a slight shift in the zero-field transitions, when the interaction between the protein and the RNA is abolished. This shift is discussed in relation with changes in polarity and in polarizability of the environment of the phosphorescing tryptophan residues when the interaction between RNA and the protein subunits decreases. The zero-field transitions in the virus are further characterized by a large linewidth, when comparisons are made with similar transitions observed in other proteins. This shows great heterogeneity in the environment of tryptophan residues, and makes the recognition and interpretation of changes in the transitions very complicated.

Luminescent Measurements↗

PubMed1994

Lack of correspondence between the room-temperature phosphorescence decay-components and Trp residues in a series of Trp-->Cys or Trp-->Phe mutants of human carbonic anhydrase II.

The room temperature phosphorescence of native human carbonic anhydrase (CA), and several mutants of this enzyme has been investigated. In these mutants the seven tryptophan residues in the native protein have sequentially been replaced by cysteine or phenylalanine. All of the mutants as well as native CA show room-temperature tryptophan phosphorescence (RTP) spectra. Surprisingly, only small differences in RTP life-times are noticeable among these mutants, indicating that there is more than one tryptophan residue with similar phosphorescence decay kinetics in the protein. The present results illustrate the danger in attributing the room temperature phosphorescence of a multi-tryptophan protein to a particular residue based solely on an analysis of the protein structure.

Carbonic Anhydrases↗

PubMed1989

Tryptophan environments in glutathione transferase of human placenta from temperature-dependent phosphorescence studies.

An investigation of the tryptophan emission properties of glutathione transferase from human placenta was conducted in order to characterize the environments of the two aromatic residues. The low-temperature phosphorescence spectra and temperature dependence of the phosphorescence quantum yield of the tryptophan residues revealed a difference in the chemical nature and dynamical structure of the surrounding protein matrix. Thus, one tryptophan residue seems to be deeply embedded within the polypeptide in a rigid weakly polar environment, characteristic of a beta-type secondary structure. The other is located in a more polar site, probably near the surface, in a rather flexible region of the macromolecule. At high temperature, the heterogeneity in the triplet lifetime of the internal residue attests to the presence of multiple conformers which are not in rapid equilibrium in the phosphorescence time scale. The anisotropy of the phosphorescence emission of glutathione transferase indicates that no energy transfer occurs between the two residues, and measurement of the rotational correlation time yields an hydrodynamic volume which is in good agreement with the molecular weight reported in the literature for the dimer.

Female↗

PubMed1993

Tryptophan phosphorescence as a monitor of the solution structure of phosphoglycerate kinase from yeast.

The enzyme phosphoglycerate kinase from yeast possesses two tryptophan residues whose phosphorescence spectrum in low-temperature glasses is resolved into two distinct components with 0-0 vibronic bands centered at 408 and 412.5 nm. The thermal profile of the phosphorescence intensity and lifetime shows that the red (longer wavelength) component is quenched in fluid solutions so that the long-lived phosphorescence observed at ambient temperature in buffer is due entirely to the blue (shorter wavelength) component. The remarkable heterogeneity in flexibility of the two chromophores' sites inferred from the thermal behaviour, when analyzed in terms of the crystallographic structure, allows to make a straightforward assignment of the long-lived emission to internal Trp-333. Because in buffer the phosphorescence is due to only one Trp residue the biphasic nature of the decay reveals the presence of stable, slowly interconverting, conformers with profound differences in the internal fluidity of the C-domain. Further, according to the triplet lifetime, complex formation with substrates affect the protein structure in a very selective way. Thus, while 3-phosphoglycerate has practically no influence on the average lifetime, Mg ATP and Mg ADP increases tau by a factor of 1.9 and 5.3, respectively. The change in lifetime implies a remarkable stiffening of the C-domain which is partly relaxed in ternary complexes with 3-phosphoglycerate. These findings are discussed in terms of ligand-induced "closed" conformations of the protein.

Buffers↗

PubMed2005

Simplex optimization of the variables affecting the micelle-stabilized room temperature phosphorescence of 6-methoxy-2-naphthylacetic acid and its kinetic determination in human urine.

This article reports the kinetic determination of 6-methoxy-2-naphthylacetic acid (6-MNA), the major metabolite of nabumetone, from micelle-stabilized room temperature phosphorescence (MS-RTP) measurements made by using the stopped-flow mixing technique. This methodology allows one to determine analytes in complex matrices without the need for a tedious separation process. It also shortens analysis times substantially. The proposed method uses simplex methodology to optimize the chemical and instrumental variables affecting the phosphorescence. It was applied to the determination of 6-MNA in human urine. The maximum phosphorescence signal is obtained within only 10 s after the sample is prepared. The maximum slope of the kinetic curve, which corresponds to the maximum rate of the phosphorescence development, is measured at lambda(ex)=273 nm and lambda(em)=516 nm. Least-squares regression was used to fit experimental data, and the detection limit, repeatability, and standard deviation for replicate samples were determined.

Enzyme Inhibitors↗

PubMed2005

Study of the heavy atom-induced room temperature phosphorescence properties of melatonin and its analytical application.

Liquid phase room temperature phosphorescence (RTP) properties of melatonin were studied using heavy atom induced-room temperature phosphorescence (HAI-RTP) technique. 1.2 M potassium iodide was used as a heavy atom reagent together with 0.002 M sodium sulphite as deoxygenating agent to produce the RTP signal. The maximum phosphorescence emission and excitation wavelengths of melatonin were 290 and 457 nm, respectively. The effect of potassium iodide concentration on the RTP lifetime of melatonin was also investigated and based on the results, the rate constants for phosphorescence decay (k(p)) and radiationless deactivation through reaction with heavy atom (k(h)) were determined. Based on the obtained results, a simple and sensitive room temperature phosphorimetric method was developed for the determination of melatonin. The method allowed the determination of 10.0-200 ng ml(-1) melatonin in aqueous solution with the limits of detection and quantification of 3.6 and 12 ng ml(-1), respectively. The proposed method was satisfactorily applied to the determination of melatonin in commercial pharmaceutical formulations.

Hydrogen-Ion Concentration↗

PubMed1993

Temperature dependence of the phosphorescence quantum yield of various alpha-lactalbumins and of hen egg-white lysozyme.

The radiative quantum yield, phi op, of the triplet state of human alpha-lactalbumin (HLA) has been measured in the temperature range between 6 K and the softening point of the aqueous glass (approximately 150 K). phi op has little temperature dependence below approximately 30 K, but above this it decreases sharply with increasing temperature. The unusual temperature dependence is fitted by a phenomenological two-state model in which the phosphorescence originates primarily from a donor, tryptophan (Trp) 104, and an acceptor, Trp 60, the populations of which are coupled by a thermally activated triplet-triplet energy transfer process. The model assumes that the acceptor (Trp 60) triplet state undergoes radiationless deactivation by a proximal disulfide residue, while the donor (Trp 104) has no such extrinsic quencher. The decrease of phi op with increasing temperature is accounted for by the thermally activated triplet-triplet energy transfer process. The disulfide quenching rate constant itself is assumed to be temperature independent, in accord with recent measurements of simple disulfide quenching in long chain snake venom neurotoxins (Schlyer, B. D., E. Lau, and A. H. Maki. 1992. Biochemistry. 31:4375-4383; Li, Z., A. Bruce, and W. C. Galley. 1992. Biophys. J. 61:1364-1371). We find that the phosphorescence quenching in HLA occurs with an activation energy of 97 cm-1, which we associate with a barrier to the energy transfer process. The data are fit well by the model if we assume a value for the temperature-independent disulfide quenching constant of kQ > 3 s-1 that is consistent with recent measurements on indole-disulfide model systems (Li, Z., A. Bruce, and W. C. Galley. 1992. Biophys. J. 61:1364-1371). Similar results are reported for bovine alpha-lactalbumin (BLA) and for hen egg-white lysozyme (HEWL) that contains the structural equivalents of Trp 104 and Trp 60 of HLA. HLA provides the best agreement with calculations since it is the simplest, lacking Trp 26, a residue not considered in the model, that probably contributes significantly to the phosphorescence of BLA, guinea pig alpha-lactalbumin (GPLA), and HEWL. GPLA, which contains Trp 104 but lacks Trp 60, shows qualitatively less thermally induced phosphorescence quenching than HLA, BLA, and HEWL, thus supporting the postulated quenching model.

Animals↗

PubMed2002

Simple determination of propranolol in pharmaceutical preparations by heavy atom induced room temperature phosphorescence.

The applicability of heavy atom induced room temperature phosphorescence in real samples is demonstrated in this work. In this methodology only two reagents, potassium iodide as heavy atom salt and sodium sulphite as oxygen scavenger, were used to obtain phosphorescent signal of propranolol in solution. Thus a new simple, rapid and selective phosphorimetric method is proposed for propranolol determination in pharmaceutical preparations. The phosphorescence intensity was measured at 492 nm exciting at 294 nm. Phosphorescence was fully developed instantly, obtaining a linear concentration range between 0 and 500 ng ml(-1) with a detection limit of 14.4 ng ml(-1), an analytical sensitivity of 6.7 ng ml(-1) and a standard deviation of 1.4% at a 300 ng ml(-1) concentration level. The method has been successfully applied to the analysis of propranolol in an antidepressive pharmaceutical preparation and it was validated using standard addition methodology.

Luminescent Measurements↗

PubMed2001

Detection of nonderivatized peptides in capillary electrophoresis using quenched phosphorescence.

A capillary electrophoresis detection technique for (small) peptides is presented, i.e. quenched phosphorescence, a method that is generally applicable and does not require chemical derivatization. For this purpose, a novel phosphorophore, 1-bromo-4-naphthalenesulfonic acid (BrNS), was synthesized. BrNS has sufficient water solubility and provides strong phosphorescence at room temperature over a wide pH range. The detection is based on the dynamic quenching of the BrNS phosphorescence background signal by electron transfer from the amino group of the peptides at pH 9.5-10. For the di- and tripeptides Val-Tyr-Val, Val-Gly-Gly, Ala-Ser, Gly-Asn, Gly-Ala, and Gly-Tyr, detection limits in the range of 5-20 microg/L were obtained. The novel technique is even a good alternative for the (limited) group of peptides containing tyrosine and, thus, exhibiting native fluorescence as well as strong UV absorption: using Gly-Tyr, Val-Tyr-Val, methionine enkephalin, and human angiotensin II as test compounds, quenched phosphorescence detection was found to compare favorably with absorption detection at 190- and 266-nm laser-induced fluorescence detection, as performed with a recently developed, small-size, quadrupled Nd:YAG laser.

Angiotensin II↗

PubMed1995

Characterization of the tryptophan binding site of Escherichia coli tryptophan holorepressor by phosphorescence and optical detection of magnetic resonance of a tryptophan-free mutant.

The L-tryptophan binding site of the Escherichia coli tryptophan holorepressor (trpR) is characterized by low-temperature phosphorescence and optical detection of magnetic resonance (ODMR) spectroscopy. Measurements are made on a tryptophan-free mutant of trpR, W19/99F, in which both intrinsic tryptophan residues of apo-trpR have been replaced with phenylalanine. Thus, essentially all of the phosphorescence that is observed from trpR originates from the bound L-tryptophan corepressor. The phosphorescence and ODMR results for the bound corepressor agree quite well with those obtained previously for the corepressor site in both single tryptophan-containing mutants, W19F and W99F [Burns, L.E., & Maki, A.H. (1994) J. Fluorescence 4, 217-226]. A red shift of the L-tryptophan phosphorescence origin as well as a decrease in the D-E ODMR frequency result from an increase in the local polarizability upon binding at the corepressor binding site. A large decrease in the ODMR line widths signals a reduction of local heterogeneity upon binding. Subsequent binding of trpR to a self-complementary DNA sequence that mimics the trp operator, 5'-CGTACTAGTTAACTAGTACG-3', produces a further decrease in line widths and additional changes in the ODMR frequencies, attributable to an increase in both the D and E parameters. This result demonstrates that binding of holo-trpR to the operator affects the local environment of the bound corepressor.

Bacterial Proteins↗

PubMed1995

Phosphorescence lifetime of tryptophan in proteins.

This investigation enquires into the factors that are responsible for the wide range of room-temperature Trp phosphorescence lifetimes (tau) in proteins. By exploiting the enhanced sensitivity and time resolution of phosphorescence measurements, experiments were conducted to evaluate the triplet quenching potential of each amino acid side chain. From the magnitude of the Stern-Volmer rate constant it is concluded that, among the amino acids, quenching reactions at 20 degrees C are quite effective with His, Tyr, Trp, cysteine, and cystine, with rate enhancements of 20 and 50 times when the side chains of Tyr and His, respectively, are in the ionized form. The distance dependence of the quenching interaction, estimated from the quenching of internal Trp residues in proteins separated from the amino acid in solution by a protein spacer of various thickness, emphasizes the very short-range nature of the process. The importance of these side chains, and to some extent that of the peptide linkage, as intrinsic quenchers of Trp phosphorescence in proteins was also confirmed with short synthetic peptides prepared appositely with only one type of these residues. Finally, very short (microseconds) phosphorescence lifetimes of Trp residues in proteins were shown to be invariably associated with the presence of Tyr or Cys in the immediate neighborhood of the chromophore. From a survey of the amino acids that are nearest neighbors to Trp in proteins and the corresponding value of tau it was established that, in the absence of His, Tyr, Trp, and Cys, tau is > or = 1 ms and appears to reflect mainly the local fluidity of the protein structure. Otherwise, tau can be much shorter, and for bulky His, Tyr, and Trp side chains it seems to depend dramatically on the mutual chromophore-quencher orientation. In these cases the triplet decay kinetics is shown to be a complex function of temperature, pH, and flexibility of the protein site.

Amino Acid Sequence↗

PubMed1992

A comparative investigation of snake venom neurotoxins and their triplet-state tryptophan-disulfide interactions using phosphorescence and optically detected magnetic resonance.

We have investigated the luminescence and optically detected magnetic resonance (ODMR) of the highly homologous snake venom neurotoxins alpha-bungarotoxin (BgTX), alpha-cobratoxin (CbTX), and cobrotoxin (CoTX) in frozen aqueous glasses. The phosphorescence intensity and lifetime of the single invariant tryptophan, Trp29, are found to be diminished in BgTX and CbTx relative to CoTX both at 77 K and at 4.2 K. Selective reduction of the Cys30-Cys34 disulfide proximal to Trp29 in BgTX and CbTX, that is absent in CoTX, results in the enhancement of the phosphorescence to fluorescence intensity ratio of Trp29 and identifies this disulfide as the source of the triplet-state quenching. Variations of the phosphorescence parameters are observed for differently frozen BgTX and CbTX samples. We argue that this observation is consistent with conformational flexibility in the region of Trp29. For BgTX and CbTX, changing the wavelength of excitation from 285 to 300 nm results in a small bathochromic phosphorescence shift of 0.4 nm, an average decrease in the lifetime, and a change in the polarity of the normally positive D-E ODMR signal. From the small excitation-dependent emission shift, we infer that Trp29 is in a relatively hydrophobic environment. The excitation-dependent changes in lifetime and ODMR signal parameters arise from subtle heterogeneity in the disposition of Trp29 with respect to Cys30-Cys34. We discuss the mechanism of disulfide-induced quenching of the Trp29 triplet state in BgTX and CbTX and argue that it most probably is due to electron transfer.

Amino Acid Sequence↗

PubMed1989

Phosphorescence properties and protein structure surrounding tryptophan residues in yeast, pig, and rabbit glyceraldehyde-3-phosphate dehydrogenase.

An investigation of the phosphorescence emission properties of tryptophan (Trp) was carried out in glyceraldehyde-3-phosphate dehydrogenase from yeast and from pig and rabbit muscle. Aided by the external heavy-atom effect of iodide, the dependence on excitation wavelength, and thermal quenching profiles, it was established that the 0,0 vibronic band peaked at 406 nm in the pig and rabbit proteins is made up of overlapping contributions from two Trp residues. In contrast to a previous report [Davis, J.M., & Maki, A.H. (1984) Biochemistry 23, 6249-6256], this implies that even in the muscle enzymes all three aromatic side chains are phosphorescent. Further, when the nature of the local environment of each residue is compared to the crystallographic structure of lobster GPDH, it leads to a complete new assignment of the individual phosphorescence spectra. With each protein, a single Trp, identified as Trp-310, was found to display long-lived phosphorescence at room temperature. The decay of this emission gives evidence of conformational homogeneity among the subunits of the tetrameric molecule.

Animals↗