The constituent tryptophans and bisANS as fluorescent probes of the active site and of a secondary binding site of stromelysin-1 (MMP-3)

The active site of the catalytic domain of stromelysin-1 (matrix metalloproteinase-3, MMP-3) was probed by fluorescence quenching, lifetime, and polarization of its three intrinsic tryptophans and by the environmentally sensitive fluorescent reporter molecule bisANS. Wavelength-dependent acrylamide quenching identified three distinct emitting tryptophan species, only one of which changes its emission and fluorescence lifetime upon binding of the competitive inhibitor Batimastat. Significant changes in the tryptophan fluorescence polarization occur upon binding by any of the three hydroxamate inhibitors Batimastat, CAS108383-58-0, and Celltech CT1418, all of which bind in the P2′-P3′ region of the active site. In contrast, the inhibitor CGS27023A, which is t hought to bind in the P1-P1′ region, does not induce any change in tryptophan fluorescence polarization. The use of the fluorescent probe bisANS revealed the existence of an auxiliary binding site extrinsic to the catalytic cleft. BisANS acts as a competitive inhibitor of stromelysin with a dissociation constant ofK _i=22 μM. In addition to this binding to the active site, it also binds to the auxiliary site with a dissociation constant of 3.40±0.17 μM. The auxiliary site is open, hydrophobic, and near the fluorescing tryptophans. The binding of bisANS to the auxiliary site is greatly enhanced by Batimastat, but not by the other competitive inhibitors tested.     

Resonance energy transfer nanobiosensors based on affinity binding between apo-enzyme and its substrate

Systems for glucose monitoring based on resonance energy transfer (RET) and competitive binding using Concanavalin A (Con A) are problematic as a result of problems of toxicity, aggregation, and irreversible binding. This paper presents an improved RET assay wherein Con A was replaced by apo-glucose oxidase (apo-GOx). The basic principle for transduction is identical to that used in assays based on Con A-dextran: a reduction in RET from fluorescein isothiocyanate (FITC) to tetramethyl rhodamine isothiocyanate (TRITC) occurs when FITC-dextran (donor) is displaced from TRITC-apo-GOx (acceptor) as a result of the competition of glucose. Fluorescence measurements confirm that the apo-GOx/dextran complexes are highly sensitive to glucose, measured as an increase in the donor peak relative to acceptor due to stepwise addition of glucose. The solution-phase assay showed strong signals and excellent repeatability, with a sensitivity of 0.0163 (ratio units)/mM over the range of 0-90 mM glucose. If properly encapsulated, these sensors can potentially be used for in vivo sensing without the concern of toxicity associated with Con A.     

Direct observation of resonance tryptophan-to-chromophore energy transfer in visible fluorescent proteins

Visible fluorescent proteins from Aequorea victoria contain next to the fluorophoric group a single tryptophan residue. Both molecules form a single donor-acceptor pair for resonance energy transfer (RET) within the protein. Time-resolved fluorescence experiments using tryptophan excitation have shown that RET is manifested by a distinct growing in of acceptor fluorescence at a rate characteristic for this process. In addition, time-resolved fluorescence anisotropy measurements under the same excitation-emission conditions showed a correlation time that is similar to the time constant of the same RET process with the additional benefit of gaining information on the relative orientation of the corresponding transition dipoles.     

The constituent tryptophans and bisANS as fluorescent probes of the active site and of a secondary binding site of stromelysin-1 (MMP-3)

The active site of the catalytic domain of stromelysin-1 (matrix metalloproteinase-3, MMP-3) was probed by fluorescence quenching, lifetime, and polarization of its three intrinsic tryptophans and by the environmentally sensitive fluorescent reporter molecule bisANS. Wavelength-dependent acrylamide quenching identified three distinct emitting tryptophan species, only one of which changes its emission and fluorescence lifetime upon binding of the competitive inhibitor Batimastat. Significant changes in the tryptophan fluorescence polarization occur upon binding by any of the three hydroxamate inhibitors Batimastat, CAS108383-58-0, and Celltech CT1418, all of which bind in the P2′-P3′ region of the active site. In contrast, the inhibitor CGS27023A, which is t hought to bind in the P1-P1′ region, does not induce any change in tryptophan fluorescence polarization. The use of the fluorescent probe bisANS revealed the existence of an auxiliary binding site extrinsic to the catalytic cleft. BisANS acts as a competitive inhibitor of stromelysin with a dissociation constant ofK _i=22 μM. In addition to this binding to the active site, it also binds to the auxiliary site with a dissociation constant of 3.40±0.17 μM. The auxiliary site is open, hydrophobic, and near the fluorescing tryptophans. The binding of bisANS to the auxiliary site is greatly enhanced by Batimastat, but not by the other competitive inhibitors tested.     

A rare protein fluorescence behavior where the emission is dominated by tyrosine: case of the 33-kDa protein from spinach photosystem II

An abnormal fluorescence emission of protein was observed in the 33-kDa protein which is one component of the three extrinsic proteins in spinach photosystem II particle (PS II). This protein contains one tryptophan and eight tyrosine residues, belonging to a "B type protein". It was found that the 33-kDa protein fluorescence is very different from most B type proteins containing both tryptophan and tyrosine residues. For most B type proteins studied so far, the fluorescence emission is dominated by the tryptophan emission, with the tyrosine emission hardly being detected when excited at 280 nm. However, for the present 33-kDa protein, both tyrosine and tryptophan fluorescence emissions were observed, the fluorescence emission being dominated by the tyrosine residue emission upon a 280 nm excitation. The maximum emission wavelength of the 33-kDa protein tryptophan fluorescence was at 317 nm, indicating that the single tryptophan residue is buried in a very strong hydrophobic region. Such a strong hydrophobic environment is rarely observed in proteins when using tryptophan fluorescence experiments. All parameters of the protein tryptophan fluorescence such as quantum yield, fluorescence decay, and absorption spectrum including the fourth derivative spectrum were explored both in the native and pressure-denatured forms.     

Porcine odorant-binding protein: structural stability and ligand affinities measured by fourier-transform infrared spectroscopy and fluorescence spectroscopy

Infrared spectra show that the binding of the odorants 2-isobuthyl-3-methoxypyrazine (PYR) and 3,7-dimethyl-1-octanol (DMO) stabilises the tertiary structure of porcine OBP-I against thermal denaturation. The fluorescence emission spectrum of the single tryptophan shows a lambdamax at 337 nm, indicating that the residue is not directly exposed to the solvent. Tryptophan does not appear to be involved in the odorant binding process and it is not accessible to the fluorescence quenchers NaI, CsCl and acrylamide. The binding of the fluorescent dye 1-aminoanthracene (1-AMA), a strong ligand, does not modify the tryptophan fluorescence spectrum. In contrast, the lambdamax of 1-AMA bound to OBP-I is shifted from 537 to 481 nm, with a lambdamax intensity increase by a factor of 80. Bound 1-AMA is displaced by odorant molecules in competitive binding assays and can be employed in simple and rapid binding assay, avoiding the use of radioactive ligands. The Scatchard plot shows that 1-AMA binds to OBP-I with a dissociation constant of 1.3 microM and an equimolar stoichiometry.     

Decay-associated fluorescence spectra and the heterogeneous emission of alcohol dehydrogenase

A procedure is described for using nanosecond time resolved fluorescence decay data to obtain decay-associated fluorescence spectra. It is demonstrated that the individual fluorescence spectra of two or more components in a mixture can be extracted without prior knowledge of their spectral shapes or degree of overlap. The procedure is also of value for eliminating scattered light artifacts in the fluorescence spectra of turbid samples. The method was used to separate the overlapping emission spectra of the two tryptophan residues in horse liver alcohol dehydrogenase. Formation of a ternary complex between the enzyme, NAD+, and pyrazole leads to a decrease in the total tryptophan fluorescence. It is shown that the emission of both tryptophan residues decreases. The buried tryptophan (residue 314) undergoes dynamic quenching with no change in the spectral distribution. Under the same conditions, the fluorescence intensity of tryptophan (residue 15) decreases without a change in decay time but with a red shift of the emission spectrum. There is also a decrease in tryptophan fluorescence intensity when the free enzyme is acid denatured (succinate buffer, pH 4.1). The denatured enzyme retains sufficient structure to provide different microenvironments for different tryptophan residues as reflected by biexponential decay and spectrally shifted emission spectra (revealed by decay association). The value of this technique for studies of microheterogeneity in biological macromolecules is discussed.     

A spectroscopic analysis of thermal stability of the Chromobacterium viscosum lipase

The thermal stability of the lipase from Chromobacterium viscosum was assessed by deactivation (loss of activity), fluorescence, circular dichroism (CD) and static light scattering (SLS) measurements. Lipase fluorescence emission is dominated by the tryptophyl contribution. An increase in the tyrosyl contribution from 2 to 16% was only observed upon prolonged incubation at 60 degrees C. The effect of temperature on the tryptophyl quantum yield was studied and two activation energies were calculated. Tryptophan residues in the native structure have an activation energy of 1.9 kcal mol(-1) for temperature-dependent non-radiative deactivation of the excited state. A structural change occurs at approximately 66.7 degrees C and the activation energy increases to 10.2 kcal mol(-1). This structural change is not characterized by tryptophan exposure on the surface of the protein. The deactivation and the evolution of structural changes with time after lipase incubation at 60 degrees C were assessed by fluorescence, CD and SLS measurements. CD spectra show that both secondary and tertiary structures remain native-like after incubation at 60 degrees C in spite of the fluorescence changes observed (red-shift from 330 to 336 nm on the trytophyl emission). SLS measurements together with the CD data show that deactivation may be due to protein association between native molecules. Deactivation and the decrease on the fraction of non-associated native lipase evaluated by changes in fluorescence intensity with time, show apparent first order kinetics. According to the rate constants, fluorescence changes precede deactivation pointing to an underestimation of the deactivation. Reactivation upon dilution during the activity assay and substrate-induced reactivation due to lipase interfacial adsorption are possible causes for this underestimation.     

Fluorescence-based sensing of glucose using engineered glucose/galactose-binding protein: a comparison of fluorescence resonance energy transfer and environmentally sensitive dye labelling strategies

Fluorescence-based glucose sensors using glucose-binding protein (GBP) as the receptor have employed fluorescence resonance energy transfer (FRET) and environmentally sensitive dyes, but with widely varying sensitivity. We therefore compared signal changes in (a) a FRET system constructed by transglutaminase-mediated N-terminal attachment of Alexa Fluor 488/555 as donor and QSY 7 as acceptor at Cys 152 or 182 mutations with (b) GBP labelled with the environmentally sensitive dye badan at C152 or 182. Both FRET systems had a small maximal fluorescence change at saturating glucose (7% and 16%), badan attached at C152 was associated with a 300% maximal fluorescence increase with glucose, though with badan at C182 there was no change. We conclude that glucose sensing based on GBP and FRET does not produce a larger enough signal change for clinical use; both the nature of the environmentally sensitive dye and its site of conjugation seem important for maximum signal change; badan-GBP152C has a large glucose-induced fluorescence change, suitable for development as a glucose sensor.     

ES complexes of Aeromonas aminopeptidase: direct observation by stopped-flow fluorescence

Intermediates of Aeromonas aminopeptidase are monitored through fluorescence generated by radiationless energy transfer (RET) between enzyme tryptophans and the dansyl group of the bound substrate. Upon binding of the substrate enzyme tryptophan fluorescence is quenched and substrate dansyl fluorescence enhanced. These processes are reversed upon hydrolysis of the Leu-Ala bond and release of Ala-DED from the enzyme. Stopped-flow RET kinetic analysis yields values of kcat = 36 sec-1 and Km = 3.7 microM at pH 7.5 and 20 degrees C. These values represent the highest kcat/Km ratio, 1 X 10(7) M-1 sec-1, of any substrate for Aeromonas aminopeptidase. The excellent binding properties of the peptide permit direct visualization of ES complexes even at enzyme concentrations of 10(-7) M.     

Interaction of tyrosine 65 of RecA protein with the first and second DNA strands

We investigated the structure of the active RecA-DNA complex by analyzing the environment of tyrosine residue 65, which is on the DNA-binding surface of the protein. We prepared a modified RecA protein in which the tyrosine residue was replaced by tryptophan, a natural fluorescent reporter, and measured the change in its fluorescence upon binding of DNA and cofactor. The fluorescence of the inserted tryptophan 65 (Trp65) was centered at 345 nm, indicating a partly exposed residue. Binding cofactor, adenosine 5'-O-3-thiotriphosphate (ATPgammaS), alone at a low salt concentration did not change the fluorescence of Trp65, confirming that the residue is not close to the nucleotide. In contrast, the binding of single-stranded DNA quenched the fluorescence of Trp65 in both the presence and absence of ATPgammaS. Trp65 fluorescence was also quenched upon binding a second DNA strand. The fluorescence change depended upon the presence and absence of ATPgammaS, reflecting the difference in the DNA binding. These results indicate that residue 65 is close to both the first and second DNA strands. The degree of quenching depended upon the base composition of DNA, suggesting that the residue 65 interacts with the DNA bases. Binding of DNA with ATPgammaS as well as binding of ATPgammaS alone at high salt concentration shifted the fluorescence emission peak from 345 to 330 nm, indicating a change from a polar to a non-polar environment. Therefore, the environment change around residue 65 would also be linked to a change in conformation and thus the activation of the protein.     

Thermodynamic and kinetic studies of the interaction of vesicular dipalmitoylphosphatidylcholine with Agkistrodon piscivorus piscivorus phospholipase A2

The tryptophan fluorescence emission intensity at 340 nm of monomeric phospholipase A2 from Agkistrodon piscivorus piscivorus increased about 70% upon addition of dipalmitoylphosphatidylcholine small unilamellar vesicles (DPPC SUV) at 25 degrees C. The emission spectrum was also blue-shifted 6-8 nm, suggesting that the environment of 1 or more tryptophan residues had become less polar. This effect of SUV on the phospholipase A2 fluorescence was independent of Ca2+ at 25 degrees C, and the apparent association constant for the interaction was approximately 1.7 x 10(4) M-1. The apparent Km for hydrolysis of DPPC SUV was equal to the inverse of the estimated association constant. In the absence of Ca2+, the change in fluorescence intensity decreased with increasing temperature. Thermodynamic analysis of this reversible, temperature-dependent fluorescence change indicated that the A. p. piscivorus monomer phospholipase A2 interacts only with SUV in the true gel phase existing below the pretransition of gel to "ripple" phase lipid in the absence of Ca2+. In contrast, the fluorescence intensity change upon addition of SUV in the presence of Ca2+ was independent of temperature over the range of 25-48 degrees C. Under these conditions, hydrolysis of the lipid occurred concomitantly with the change in fluorescence which could not be reversed by the addition of EDTA. With a nonhydrolyzable analog of DPPC, however, the fluorescence changes upon mixing of SUV, Ca2+, and phospholipase A2 were reversible and temperature-dependent. Thus, the apparent irreversibility of the change in fluorescence observed with Ca2+ and DPPC SUV was correlated with hydrolysis of the vesicles. These results indicate that the magnitude of the initial interaction of enzyme with substrate is reversible, is Ca2+-independent, depends upon the lipid state, and is quantitatively correlated to the maximum rate of hydrolysis.     

Nanosecond pulse fluorometry of conformational change in phenylalanine hydroxylase associated with activation

Conformational change in rat liver phenylalanine hydroxylase associated with activation by phenylalanine or N-(1-anilinonaphth-4-yl)maleimide was investigated by measuring fluorescence spectra and fluorescence lifetimes of tryptophanyl residues as well as the probe fluorophore conjugated with SH groups of the hydroxylase. The fluorescence spectrum of tryptophan exhibited its maximum at 342 nm. It shifted by 8 nm toward longer wavelength accompanied by an increase in its intensity, by preincubation with 1 mM phenylalanine. The fluorescence intensity of tryptophan increased by 36% upon the activation. On the other hand, the binding of (6R)-L-erythro-tetrahydrobiopterin, a natural cofactor of the enzyme, induced a decrease in the fluorescence intensity by 79% without a shift of the maximum wavelength. The fluorescence lifetime of tryptophan of phenylalanine hydroxylase exhibited two components with lifetimes of 1.7 and 4.1 ns. The values of the lifetimes changed to 1.4 and 5.6 ns, respectively, upon the activation. It is considered that the change in the longer lifetime is correlated with the shift of the emission peak upon the activation. The values of both the lifetimes decreased to 0.64 and 3.6 ns upon the binding of (6R)-L-erythro-tetrahydrobiopterin, which is coincident with the decrease in the fluorescence intensity. Conjugation of N-(1-anilinonaphth-4-yl)maleimide with SH of phenylalanine hydroxylase brought about a decrease in both the fluorescence intensity and the value of the shorter lifetime of the tryptophanyl residues, while the longer lifetime remained unchanged.(ABSTRACT TRUNCATED AT 250 WORDS)     

Tryptophan fluorescence in electron-transfer flavoprotein:ubiquinone oxidoreductase: fluorescence quenching by a brominated pseudosubstrate

We have studied the intrinsic fluorescence of the 12 tryptophan residues of electron-transfer flavoprotein:ubiquinone oxidoreductase (ETF:QO). The fluorescence emission spectrum (lambda ex 295 nm) showed that the fluorescence is due to the tryptophan residues and that the contribution of the 22 tyrosine residues is minor. The emission maximum (lambda m 334 nm) and the bandwidth (delta lambda 1/2 56 nm) suggest that the tryptophans lie in hydrophobic environments in the oxidized protein. Further, these tryptophans are inaccessible to a range of ionic and nonionic collisional quenching agents, indicating that they are buried in the protein. Enzymatic or chemical reduction of ETF:QO results in a 5% increase in fluorescence with no change of lambda m or delta lambda 1/2. This change is reversible upon reoxidation and is likely to reflect a conformational change in the protein. The ubiquinone analogue Q0(CH2)10Br, a pseudosubstrate of ETF:QO (Km = 2.6 microM; kcat = 210 s-1), specifically quenches the fluorescence of one tryptophan residue (Kd = 1.6-3.2 microM) in equilibrium fluorescence titrations. The ubiquinone homologue UQ-2 (Km = 2 microM; kcat = 162 s-1) and the analogue Q0(CH2)10OH (Km = 2 microM; kcat = 132 s-1) do not quench tryptophan fluorescence; thus the brominated analogue acts as a static heavy atom quencher. We also describe a rapid purification for ETF:QO based on extraction of liver submitochondrial particles with Triton X-100 and three chromatographic steps, which results in yields 3 times higher than previously published methods.     

Tryptophan interactions with glycerol/water and trehalose/sucrose cryosolvents: infrared and fluorescence spectroscopy and ab initio calculations

In order to correlate how the solvent affects emission properties of tryptophan, the fluorescence and phosphorescence emission spectra of tryptophan and indole model compounds were compared for solid sugar glass (trehalose/sucrose) matrix and glycerol/water solution and under the same conditions, these matrices were examined by infrared spectroscopy. Temperature was varied from 290 to 12 K. In sugar glass, the fluorescence and phosphorescence emission spectra are constant over this temperature range and the fluorescence remains red shifted; these results are consistent with the static interaction of OH groups with tryptophan in the sugar glass. In sugar glass containing water, the water retains mobility over the entire temperature range as indicated by the HOH infrared bending frequency. The fluorescence of tryptophan in glycerol/water shifts to the blue as temperature decreases and the frequency change of the absorption of the HOH bend mode is larger than in the sugar glass. These results suggest rearrangement of glycerol and water molecules over the entire temperature change. Shifts in the fluorescence emission maximum of indole and tryptophan were relatively larger than shifts for the phosphorescence emission-as expected for the relatively smaller excited triplet state dipole for tryptophan. The fluorescence emission of tryptophan in glycerol/water at low temperature has maxima at 312, 313, and 316 nm at pH 1.4, 7.0, and 10.6, respectively. The spectral shifts are interpreted to be an indication of a charge, or Stark phenomena, effect on the excited state molecule, as supported by ab initio calculations. To check whether the amino acid remains charged over the temperature range, the infrared spectrum of alanine was monitored over the entire range of temperature. The ratio of infrared absorption characteristic of carboxylate/carbonyl was constant in glycerol/water and sugar glass, which indicates that the charge was retained. Tryptophan buried in proteins, namely calcium parvalbumin from cod and aldolase from rabbit, showed temperature profiles of the fluorescence spectra that were largely independent of the solvent (glycerol/water or sugar glass) and temperature whereas the fluorescence and phosphorescence yields were dependent. The results demonstrate how the rich information found in tryptophan luminescence can provide information on the dipolar nature and dynamics of the matrix.     

Engineered Tet repressor mutants with single tryptophan residues as fluorescent probes. Solvent accessibilities of DNA and inducer binding sites and interaction with tetracycline

Mutants of the Tn10-encoded Tet repressor containing single or no tryptophan residues were constructed by oligonucleotide-directed mutagenesis. The Trp-75 to Phe exchange reduces the dissociation rate of the complex with the inducer tetracycline by a factor of 2. The Trp-43 to Phe exchange has no effect on inducer binding. The fluorescence emission spectra of both tryptophan residues are quenched to a different extent by binding of tetracycline: Trp-75 is quenched to zero and Trp-43 to only 50%. It is concluded that Trp-75 is in the vicinity of the inducer binding site. The different fluorescence emission spectra of both tryptophan residues depend on the native structure of Tet repressor. Quenching studies with iodide indicate that the DNA binding motif is solvent exposed in free repressor and moves towards the interior of the protein upon inducer binding. The inducer binding site is in the interior of the protein. The fluorescence of tetracycline is enhanced upon binding to Tet repressor. The excitation at 280 nm results mainly from the change in environment and in part from energy transfer from tryptophan to the drug.     

Conformation of adenosine deaminase in complexes with inhibitors: application of selective quenching of fluorescence emission

The effect of inhibitors, 1-deazaadenosine (1-dAdo) and erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA), on the conformation of adenosine deaminase was studied using the method of selective quenching of fluorescence emission by acrylamide, I- and Cs+. Both in free adenosine deaminase and in its complexes with the inhibitors, the wavelength maxima and half-width of the emission characterize the environment of fluorescing tryptophan residues in adenosine deaminase as weak polar with limited access to solvent. The formation of complexes with the ground state inhibitors used did not quench or change the main emission characteristics of tryptophan fluorescence in adenosine deaminase. Small blue shifts of emission maxima were observed upon quenching in all three samples. The Stern-Volmer parameters of tryptophan fluorescence quenching by acrylamide were not essentially influenced by complex formation of the enzyme with the inhibitors: in general, the folding of the enzyme molecule in the complexes is not perturbed. On the contrary, the emission quenching by charged heavy ions, I- and Cs+, in the complexes was hindered in comparison with free adenosine deaminase. In the complex with 1-deazaadenosine, the parameters for quenching by both ions evidence the essential worsening of their interaction with tryptophans. In the complex with erythro-9-(2-hydroxy-3-nonyl)adenine, along with the worse quenching by I-, complete prohibition of quenching by Cs+ was observed. These data indicate that the local environments of fluorescing tryptophan residues is substantially distorted compared with free adenosine deaminase, which leads to their screening from charged heavy ions.     

Intrinsic fluorescence of elongation factor Tu in its complexes with GDP and elongation factor Ts

The intrinsic fluorescence properties of elongation factor Tu (EF-Tu) in its complexes with GDP and elongation factor Ts (EF-Ts) have been investigated. The emission spectra for both complexes are dominated by the tyrosine contribution upon excitation at 280 nm whereas excitation at 300 nm leads to exclusive emission from the single tryptophan residue (Trp-184) of EF-Tu. The fluorescence lifetime of this tryptophan residue in both complexes was investigated by using a multifrequency phase fluorometer which achieves a broad range of modulation frequencies utilizing the harmonic content of a mode-locked laser. These results indicated a heterogeneous emission with major components near 4.8 ns for both complexes. Quenching experiments on both complexes indicated limited accessibility of the tryptophan residue to acrylamide and virtually no accessibility to iodide ion. The quenching patterns exhibited by EF-Tu-GDP and EF-Tu X EF-Ts were, however, different; both quenchers were more efficient at quenching the emission from the EF-Tu x EF-Ts complex. Steady-state and dynamic polarization measurements revealed limited local mobility for the tryptophan in the EF-Tu x GDP complex whereas formation of the EF-Tu x EF-Ts complex led to a dramatic increase in this local mobility.     

Radiative decay engineering. 2. Effects of Silver Island films on fluorescence intensity, lifetimes, and resonance energy transfer

Metallic surfaces can have unusual effects on fluorophores such as increasing or decreasing the rates of radiative decay and the rates of resonance energy transfer (RET). In the present article we describe the effects of metallic silver island films on the emission spectra, lifetimes, and energy transfer for several fluorophores. The fluorophores are not covalently coupled to the silver islands so that there are a range of fluorophore-to-metal distances. We show that proximity of fluorophores to the silver islands results in increased fluorescence intensity, with the largest enhancement for the lowest-quantum-yield fluorophores. Importantly, the metal-induced increases in intensity are accompanied by decreased lifetimes and increased photostability. These effects demonstrate that the silver islands have increased the radiative decay rates of the fluorophore. For solvent-sensitive fluorophores the emission spectra shifted to shorted wavelengths in the presence of the silver islands, which is consistent with a decrease of the apparent lifetime for fluorophores near the metal islands. We also observed an increased intensity and blue spectral shift for the protein human glyoxalase, which displays a low quantum yield for its intrinsic tryptophan emission. In this case the blue shift is thought to be due to increased emission from a buried low-quantum-yield tryptophan residue. Increased intensities were also observed for the intrinsic emission of the nucleic acid bases adenine and thymine and for single-stranded 15-mers poly(T) and poly(C). And finally, we observed increased RET for donors and acceptors in solution and when bound to double-helical DNA. These results demonstrate that metallic particles can be used to modify the emission from intrinsic and extrinsic fluorophores in biochemical systems.     

The ultraviolet fluorescence spectra of DPN-linked isocitrate dehydrogenase from bovine heart.

The emission maximum of DPN-linked isocitrate dehydrogenase from bovine heart shifted from 316 nm to 324 nm as the excitation wavelength was varied from 265 nm to 300 nm. This shift was accompanied by a nonproportional change in fluorescence intensity. Comparisons of the emission spectra of model compounds in aqueous buffer at pH 7.07 and n-butanol showed that lowered solvent polarity led to a blue shift of the peak of free tryptophan without significant change of fluorescence intensity, whereas the fluorescence intensity of tyrosine amide increased markedly without change in emission maximum. The emission peak of mixtures of tryptophan and tyrosine amide shifted to shorter wavelengths as the proportion of tyrosine amide increased. The results suggest a major contribution of tyrosine to the overall fluorescence of the dehydrogenase. DPNH caused quenching and a blue shift of the protein fluorescence maximum when excited between 270 nm and 290 nm, indicating that the two tryptophan residues per subunit of enzyme are located in different microenvironments of the protein and that DPNH may interact preferentially with the residue emitting at the longer wavelength.