The triplet state as a probe of dynamics and structure in biological macromolecules

Imaging and detection of long-lived fluorescence probes in presence of highly emissive and scattering background

Optical biomedical imaging and diagnostics is a rapidly growing field that provides both structural and functional information with uses ranging from fundamental to practical clinical applications. Nevertheless, imaging/visualizing fluorescence objects with high spatial resolution in a highly scattering and emissive biological medium continues to be a significant challenge. A fundamental limiting factor for imaging technologies is the signal-to-background ratio (SBR). For a long time to improve the SBR, we tried to improve the brightness of fluorescence probes. Many novel fluorophores with improved brightness (almost reaching the theoretical limit), redshifted emission, highly improved photostability, and biocompatibility greatly helped advance fluorescence detection and imaging. However, autofluorescence, scattering of excitation light, and Raman scattering remain fundamental limiting problems that drastically limit detection sensitivity. Similarly, significant efforts were focused on reducing the background. High-quality sample purification eliminates the majority of autofluorescence background and in a limited confocal volume allows detection to reach the ultimate sensitivity to a single molecule. However, detection and imaging in physiological conditions does not allow for any sample (cells or tissue) purification, forcing us to face a fundamental limitation. A significant improvement in limiting background can be achieved when fluorophores with a long fluorescence lifetime are used, and time-gated detection is applied. However, all long-lived fluorophores present low brightness, limiting the potential improvement. We recently proposed to utilize multipulse excitation (burst of pulses) to enhance the relative signal of long-lived fluorophores and significantly improve the SBR. Herein, we present results obtained with multipulse excitation and compare them with standard single-pulse excitation. Subtraction of images obtained with a single pulse from those obtained with pulse burst (differential image) highly limits background and instrumental noise resulting in more specific/sensitive detection and allows to achieve greater imaging depth in highly scattering media, including skin and tissue.

On the possibility of direct triplet state excitation of indole

We studied the luminescence properties of indole in poly (vinyl alcohol) (PVA) film. The indole molecules are effectively immobilized in this polymer film and display both fluorescence and phosphorescence emission at room temperature. We noticed that the phosphorescence of indole in PVA film can be effectively excited at a longer wavelength than its typical singlet to triplet population route involving intersystem crossing. The maximum of the phosphorescence excitation is about 410 nm which corresponds to the energy of indole's triplet state. Interestingly, the phosphorescence anisotropy excited with the longer wavelength (405 nm) is positive and reaches a value of about 0.25 in contrast to the phosphorescence anisotropy excited within the indole singlet absorption spectrum (290 nm), which is negative. Very different temperature dependences have been observed for fluorescence and phosphorescence of indole in PVA film. While fluorescence depends minimally, the phosphorescence decreases with temperature dramatically. The fluorescence lifetime was measured to be a single component 4.78 ns while the intensity weighted average phosphorescence lifetime with 290 nm and 405 nm excitations were 6.57 and 5.62 ms, respectively. We believe that the possibility of the excitation of indole phosphorescence in the blue region of visible light and its high anisotropy opens a new avenue for future protein studies.

Probing the C₆₀ triplet state coupling to nuclear spins inside and out

The photoexcitation of functionalized fullerenes to their paramagnetic triplet electronic state can be studied by pulsed electron paramagnetic resonance (EPR) spectroscopy, whereas the interactions of this state with the surrounding nuclear spins can be observed by a related technique: electron nuclear double resonance (ENDOR). In this study, we present EPR and ENDOR studies on a functionalized exohedral fullerene system, dimethyl[9-hydro (C60-Ih)[5,6]fulleren-1(9H)-yl]phosphonate (DMHFP), where the triplet electron spin has been used to hyperpolarize, couple and measure two nuclear spins. We go on to discuss the extension of these methods to study a new class of endohedral fullerenes filled with small molecules, such as H₂@C₆₀, and we relate the results to density functional calculations.

Ground- and excited-state interactions of 2,7,12,17-tetraphenylporphycene with model target biomolecules for type-I photodynamic therapy

Biosubstrate-sensitizer binding is one of the factors that enhances the type-I mechanism over the type-II in the whole photodynamic process. 2,7,12,17-Tetraphenylporphycene (TPPo), a second-generation photosensitizer, is a hydrophobic compound with good photophysical properties for photodynamic therapy applications that has proved its ability for the photoinactivation of different cell lines. Nevertheless, little is known about its mechanism of action. This paper focuses on the study of the interaction/binding of TPPo with different model biomolecules that may favor the type-I mechanism in the overall photodynamic process, including nucleosides, proteins, and phospholipids. Compared with more hydrophilic photosensitizers, it is concluded that TPPo is more likely to undergo type-II (singlet oxygen) than type-I (electron transfer) photodynamic processes in biological environments.

Enantioselective Quenching of Room-Temperature Phosphorescence Lifetimes of Proteins: Bovine and Human Serum Albumins

Enantioselective quenching of the room-temperature phosphorescence (RTP) lifetime of proteins was demonstrated due to the effects of various external chiral quenching agents. In the absence of quenchers, the RTP lifetimes for bovine serum albumin (BSA) and human serum albumin (HSA) were found to be 5.0 +/- 0.2 and 4.0 +/- 0.1 ms, respectively. The addition of various chiral quenchers (three pairs of binaphthols and two pairs of beta-blockers) into the deoxygenated sample solutions containing BSA and HSA reduced their RTP lifetimes significantly, i.e., from ca. 4-5 ms (in the absence) to an average lifetime of ca. 1-2 ms (in the presence) of the chiral quenchers. For the R and S enantiomers examined, marked differences in RTP lifetimes were observed, i.e., ranging from ca. 20-29% for the binaphthols to ca.14-16% for the beta-blockers. Such findings could lead to a better understanding of the relationship between chirality, dynamics/conformational changes, and biological functions of proteins.

The Triplet-state Lifetime of Indole Derivatives in Aqueous Solution¶

An important feature of tryptophan phosphorescence, crucial for probing protein structure and dynamics, is the drastic reduction of the lifetime (tau) in fluid solutions. Initial reports of indole and derivatives showed that tau decreases from 6 s in rigid glasses to about 1 ms in aqueous solutions at ambient temperature. Recently a report by Fischer et al. questioned the validity of the millisecond lifetime, claiming that in millimolar electrolyte solutions tau is about 40 micros, similar to the 12-30 micros of earlier determinations based on flash photolysis. Longer lived phosphorescence was detected in pure water but because it exhibited an initial growing phase and an anomalously large triplet yield, the emission was attributed to an artifact arising from the slow, first-order, geminate recombination of the radical cation and electron generated by photochemistry. In this study, we reexamine both the phosphorescence lifetime and the triplet quantum yield of indole, N-acetyl tryptophanamide (NATA), N-methyl tryptophan and the tryptophan-glycine-glycine tripeptide under the same conditions adopted by Fischer et al. as well as over a wider range of electrolyte and buffering salts concentrations, pH, solvent and temperature. Throughout, the results show that the phosphorescence decay is slow and uniform down to the 12 micros resolution of the instrument, with no evidence of short-lived, 40 micros-like components. Most compelling was the similarity between the fluorescence-normalized triplet yield of indole derivatives in water and that of W59 in the protein ribonuclease T1 or of NATA in rigid glasses. Its invariance over experimental conditions that varied the production of photoproducts several fold and the characteristic susceptibility of the triplet lifetime to O2, proton and ground state quenching demonstrated that the triplet state was formed predominantly through normal intersystem crossing and that its unquenched lifetime was at least 9 ms.

Exploiting luminescence spectroscopy to elucidate the interaction between sugar and a tryptophan residue in the lactose permease of Escherichia coli

The crystal structure of the Escherichia coli lactose permease at 3.5 A with a bound substrate has been reported recently. The structure reveals the sugar-protein contacts, which include hydrophobic stacking between the galactopyranosyl ring of substrate and the indole side chain of Trp-151, as proposed previously. The nature of this interaction is studied here by exploiting the luminescence properties of Trp-151 in a mutant devoid of other tryptophan residues. The following phenomena are observed. (i) The fluorescence emission spectrum of Trp-151 and fluorescence-quenching experiments with water-soluble quenchers demonstrate that Trp-151 is in a hydrophilic environment. (ii) Substrate binding leads to a blue shift in the emission spectrum and reduction in accessibility to polar quenchers, indicating that Trp-151 becomes less exposed to aqueous solvent. (iii) The phosphorescence spectrum of Trp-151 is red-shifted in the presence of substrate, indicating charge separation of the triplet state due to a direct stacking interaction between the galactopyranosyl and indole rings. The spectroscopic data fully complement the x-ray structure and demonstrate the feasibility of fluorescence spectroscopy for studying sugar-protein interactions.

The triplet-state lifetime of indole in aqueous and viscous environments: significance to the interpretation of room temperature phosphorescence in proteins

The interpretation of room temperature phosphorescence studies of proteins requires an understanding of the mechanisms governing the tryptophan triplet-state lifetimes of residues fully exposed to solvent and those deeply buried in the hydrophobic core of proteins. Since solvents exposed tryptophans are expected to behave similarly to indole free in solution, it is important to have an accurate measure of the triplet state lifetime of indole in aqueous solution. Using photon counting techniques and low optical fluence (J/cm(2)), we observed the triplet-state lifetime of aqueous, deoxygenated indole and several indole derivatives to be approximately 40 micros, closely matching the previous reports by Bent and Hayon based on flash photolysis (12 micros; Bent, D. V.; Hayon, E. J. Am. Chem. Soc. 1975, 97, 2612-2619) but much shorter than the 1.2 ms lifetime observed more recently (Strambini, G. B.; Gonnelli, M. J. Am. Chem. Soc. 1995, 117, 7646-7651). However, we have now been able to reproduce the long lifetime reported by the latter workers for aqueous indole solutions and show that it likely arises from geminate recombination of the indole radical cation and solvated electron, a conclusion based on studies of the indole radical cation in water (Bent and Hayon, 1975). The evidence for this comes from a fast rise in the phosphorescence emission and measurements of a corresponding enhanced quantum yield in unbuffered solutions. This species can be readily quenched, and the corresponding fast rise disappears, leaving a monoexponential 40 micros decay, which we argue is the true indole triplet lifetime. The work is put in the context of room temperature phosphorescence studies of proteins.

Dynamic and thermodynamic effects of glycerol on bovine serum albumin in aqueous solution: a tryptophan phosphorescence study

The phosphorescence decay of bovine serum albumin in water-glycerol solutions at room temperature is analysed by the maximum entropy method. While in pure water the decay was found to be quasi-monoexponential, in water-glycerol mixtures it is associated with more complex lifetime distributions. This is a direct proof for the existence of multiple protein conformers. It was also found that the increase of the glycerol concentration induces a continuous shift of the lifetime pattern to longer lifetimes. This effect is analysed in connection with the coupling between the solvent viscosity and the internal protein mobility.

Excited triplet state photophysics of the sulphonated aluminium phthalocyanines bound to human serum albumin

The binding of the sulphonated aluminum phthalocyanines to human serum albumin (HSA) in aqueous phosphate buffer solution at 25 degrees C has been studied by measuring the properties of the triplet excited states of these dyes. The triplet lifetimes were measured by triplet-triplet absorption flash photolysis. The triplet lifetime of the disulphonated AlS2Pc (2.5 microM) varies from 500 +/- 30 microseconds in the absence of protein to 1.100 microseconds and longer with HSA concentrations above 100 microM. Under identical conditions, the maximum triplet lifetimes of the mono-, tri- and tetrasulphonated compounds bound to HSA are shorter than those for the disulphonated species. The increase in the triplet state lifetimes is attributed to the ability of the bulk aqueous phase to interact with the sensitizer at the site of binding; the site of binding being dependent on the degree of sulphonation. For AlS2Pc and AlS3Pc at all HSA concentrations, and regardless of the degree of sulphonation, all the triplet state decay profiles follow simple pseudo-first-order kinetics. The exponential decay of the triplet phthalocyanine at all HSA concentrations is ascribed to the rapid association and dissociation of the phthalocyanine-HSA complex on the time-scales of the triplet state lifetimes. A simplified one-step binding model is utilized to describe the results. The association of AlS1Pc with HSA results in substantial quenching of the triplet state quantum yield, and a more complex model is required to analyze the results. The tetrasulphonated compound (AlS4Pc) binds to the protein at a site where it experiences some protection from the aqueous phase.

Time-resolved room temperature tryptophan phosphorescence in proteins

The application of luminescence, primarily fluorescence, to the study of protein structure and dynamics has been extensively exploited to facilitate the understanding of complex biological problems. The interest in the application of phosphorescence, however, shows that new and complementary information can be had by careful optical studies of the phosphorescence lifetime. As in the early days of fluorescence spectroscopy in proteins, a complete and rigorous interpretation of the room temperature phosphorescence remains to be developed; nevertheless, it is clear that time-resolved phosphorescence yields new information on proteins in solution, for example, the detection of subtle conformational changes during protein folding, which is outside the sensitivity of earlier techniques. In addition, the great sensitivity of the phosphorescence lifetime to structural changes associated with rigidity and of nearby quenchers suggests that detailed structural information can be obtained when this approach is combined with the power of site-directed mutagenesis or other more biophysical techniques such as energy transfer to attached acceptors. We have presented basic aspects of time-resolved room temperature phosphorescence spectroscopy and demonstrated some useful features of the spectroscopic signals as well as the general approach to data analysis. However, it should be understood that extensions of this approach will easily allow faster and improved time resolution with greater sensitivity to highly quenched phosphorescing states. In addition, many extensions of this approach that are common to fluorescence spectroscopy have yet to be developed. For example, combining time-resolved phosphorescence with anaerobic stopped-flow techniques and more rapid data acquisition electronics will enable studies of conformational dynamics with considerably shortened dead times. Other possibilities include extending the preliminary studies of in vivo-based spectroscopy, such as to microscopy. In conclusion, time-resolved phosphorescence presents a new dimension to biophysical methodologies for the study of proteins, and it is likely that this area will continue to grow in capability as the fundamental understanding improves.

Mechanisms of quenching of the fluorescence of a benzo[a]pyrene tetraol metabolite model compound by 2'-deoxynucleosides

The hydrophobic interactions of bulky polycyclic aromatic hydrocarbons with nucleic acid bases and the formation of noncovalent complexes with DNA are important in the expressions of the mutagenic and carcinogenic potentials of this class of compounds. The fluorescence of the polycyclic aromatic residues can be employed as a probe of these interactions. In this work, the interactions of the (+)-trans stereoisomer of the tetraol 7,8,9,10-tetrahydroxytetrahydrobenzo[a]pyrene (BPT), a hydrolysis product of a highly mutagenic and carcinogenic diol epoxide derivative of benzo[a]pyrene, were studied with 2'-deoxynucleosides in aqueous solution by fluorescence and UV spectroscopic techniques. Ground-state complexes between BPT and the purine derivatives 2'-deoxyguanosine (dG), 2'-deoxyadenosine (dA), and 2'-deoxyinosine (dI) are formed with association constants in the range of approximately 40-130 M(-1). Complex formation with the pyrimidine derivatives 2'-deoxythymidine (dT), 2'-deoxycytidine (dC), and 2'-deoxyuridine (dU) is significantly weaker. Whereas dG is a strong quencher of the fluorescence of BPT by both static and dynamic mechanisms (dynamic quenching rate constant k(DYN) = [2.5 +/- 0.4] x 10(9) M(-1)s(-1), which is close to the estimated diffusion-controlled value of approximately 5 x 10(9) M(-1)s(-1), both dA and dI are weak quenchers and form fluorescence-emitting complexes with BPT. The pyrimidine derivatives dC, dU, and dT are efficient dynamic fluorescence quenchers (k(DYN) approximately [1.5-3.0] x 10(9) M (-1)s(-1), with a small static quenching component due to complex formation evident only in the case of dT. None of the four nucleosides dG, dA, dC and dT are dynamic quenchers of BPT in the triplet excited state; the observed lower yields of triplets are attributed to the quenching of single excited states of BPT by 2'-deoxynucleosides without passing through the triplet manifold of BPT. Possible fluorescence quenching mechanisms involving photoinduced electron transfer are discussed. The strong quenching of the fluorescence of BPT by dG, dC and dT accounts for the low fluorescence yields of BPT-native DNA and of pyrene-DNA complexes.

Triplet radical ion pair state of the Zn-porphyrin-viologen dyad as a magnetic field sensitive probe of phase transitions in small unilamellar vesicles

The magnetic field effect on the recombination kinetics of the triplet radical ion pair state (RIPS) of the Zn-porphyrin-viologen dyad (P-Ph-Vi2+) in the small unilamellar vesicles (SUV) of D,L-dipalmitoyl-alpha-phosphatidylcholine has been studied by the nanosecond laser flash photolysis technique at 5-60 degrees C. The increase in temperature from 25 to 40 degrees C enhances the rate constant (kr) of the RIPS recombination in zero magnetic field from 0.9 x 10(6) to 1.6 x 10(6) s-1, while kr is temperature insensitive at 5-25 and 40-60 degrees C. The typical break in the kr temperature dependence is observed in the temperature range of the phase transition of the SUV bilayers from the solid to the fluid state. The kr value in a strong magnetic field (B = 0.24 T) is equal to 2.7 x 10(5) s-1 and it is independent of temperature at 5-60 degrees C. The shape of the magnetic field dependence of kr is unaffected by the phase transition of the SUV bilayers and is characterized by the existence of an initial plateau of kr at B = 0 to 0.5 mT.

Measurement and analysis of triplet-state lifetimes by multifrequency cross-correlation phase and modulation phosphorimetry

In this paper we describe a novel approach to study the triplet-state lifetimes by a conventional multifrequency cross-correlation phase and modulation apparatus. The analysis of phase and modulation data of eosin-labeled band 3 erythrocyte ghosts revealed the existence of two phosphorescence lifetime values of 2700 and 750 microseconds, with a fractional contribution of 78 and 22%, respectively, which are in good agreement with those reported in the literature. Differential polarization phase analysis, which facilitates the study of the rotational properties of band 3, provided data in good agreement with those reported in the literature. The method proposed in this paper to study the radiative emission from the triplet state may represent a convenient alternative to the pulse laser flash technique.