Protein triplet states

Fluorescence in "Nonfluorescent" Polymers

Recently, a great deal of research has been started on generating fairly strong photoluminescence from organic molecules without having any conjugated π-system or fluorophore. Discrete chromophores or auxochromophores termed as "subfluorophores" may undergo "space conjugation" via co-operative intramolecular conformation followed by intermolecular aggregation to generate fluorescence or sometimes phosphorescence emission. Polymeric materials are important in this regard as nonconjugated polymers self-assemble/aggregate in a moderately concentrated solution and also in the solid state, producing membranes, films, and so forth with good physical and mechanical properties. Therefore, promoting fluorescence in these commodity polymers is very much useful for sensing, organic light emitting diodes (OLED), and biological applications. In this perspective, we have discussed the aggregation-induced emission from four different types of architectures, for example, (i) dendrimers or hyperbranched polymers, (ii) entrapped polymeric micellar self-assembly, (iii) cluster formation, and (iv) stretching-induced aggregation, begining with the genesis of fluorescence from aggregation of propeller-shaped small organic molecules. The mechanism of induced fluorescence of polymers with subfluorophoric groups is also discussed from the theoretical calculations of the energy bands in the aggregated state. Also, an attempt has been made to highlight some useful applications in the sensing of surfactants, bacteria, cell imaging, drug delivery, gene delivery, OLED, and so forth.

Temperature dependence of the disulfide perturbation to the triplet state of tryptophan

Variability in the temperature dependence of disulfide quenching of the tryptophan phosphorescence of globular proteins in rigid glasses is illustrated with lysozyme and alpha-bungarotoxin. A laser-pulsed phosphorescence study of this short-range interaction with a model indole-disulfide system is described. The perturbation of secondary dibutyl disulfide on the triplet state of the indole moiety in 2-(3-indolyl)ethyl phenyl ketone in rigid media is found to display a bimodal temperature dependence. The quenching rate constant at contact between chromophore and perturber is observed to be temperature independent below 30 K, but to increase with temperature between 30 and 100 K with an activation energy of approximately 200 cm(-1). The results suggest that the underlying quenching interaction involves a photo-induced one-electron transfer from the excited state of indole to the disulfide.

Time-resolved phosphorescence of tyrosine, tyrosine analogs, and tyrosyl residues in oxytocin and small peptides

We present the time-resolved phosphorescence of oxytocin, two oxytocin derivatives, vasopressin and a series of compounds that serve as models for free tyrosine. One of the oxytocin derivatives, desaminodicarbaoxytocin, has the disulfide bridge replaced by an ethylene bridge, and lacks the N-terminus. Similar to the reported fluorescence decays of tyrosine in these peptides, the phosphorescence decays generally are not single exponentials, but can be fit as biexponentials. The decay times for the oxytocin peptides are shorter than for desaminodicarbaoxytocin or the model compounds, and this we attribute to enhanced spin-orbit coupling due to the presence of sulfur. We measured the phosphorescence decay of the model cyclic pentapeptide that contains tyrosine and compared it to that observed for the same cyclic pentapeptide in which tyrosine is replaced by tryptophan. We also report the phosphorescence of 2-tryptophan-oxytocin, and deamino-2-tryptophan-oxytocin in which biexponential phosphorescence decay is also observed.

Hydrogen atoms are produced when tryptophan within a protein is irradiated with ultraviolet light

The UV photolysis of the aromatic amino acid, tryptophan (Trp), in the Ca(2+)-binding protein, cod parvalbumin, type III, was studied using electron paramagnetic resonance (EPR) spectroscopy in the temperature range 4-80 K. For the Ca(2+)-bound protein, irradiation with UV light (250-400 nm) resulted in the generation of atomic hydrogen with a hyperfine splitting of 50.9 mT, whereas in the Ca(2+)-free form, where the Trp is exposed to solvent, the trapped atomic hydrogen was not in evidence. In the same spectra, the radical signal in the g = 2.00 region could be detected. The line shape of the Ca(2+)-bound form is similar to the EPR line shape obtained for Trp in micellar systems. In contrast, the EPR line shape for the Ca(2+)-free form is essentially featureless up to 80 K. The EPR spectra of the photoproducts of Trp and the nature of the photoreactions are therefore sensitive to the environment of Trp within the protein.

Phosphorescence and ODMR study of the binding interactions of acetylcholine receptor alpha-subunit peptides with alpha-cobratoxin

Optical detection of magnetic resonance (ODMR) and phosphorescence spectroscopy have been applied to synthetic peptides derived from the alpha-subunit of the nicotinic acetylcholine receptor of Torpedo californica and their complexes with alpha-cobratoxin (CBTX). The CBTX Trp phosphorescence is strongly quenched by the proximal disulfide linkage, while the emission wavelengths and ODMR frequencies of the 18-mer alpha 181-198 indicate a more hydrophobic Trp environment than in the 12-mer alpha 185-196. Binding to CBTX produces a subtle increase in the hydrophobicity of the Trp environment for the peptides, in qualitative agreement with a recently proposed binding model, in which a receptor Trp residue interacts strongly with a hydrophobic cleft of the toxin.

Evidence for ligand-induced conformational changes in proteins from phosphorescence spectroscopy

Phosphorescence spectroscopy on mouse myeloma IgA J539 in rigid solution at 77K revealed the type of anomalous short-lived component in the tryptophan decay originally observed with lysozyme (Churchich, J.E., 1966. Biochim. Biophys. Acta. 120:406-412) and seen in a large number of Bence Jones proteins (Longworth, J.W., C.L. McLaughlin, and A. Solomon. 1976. Biochemistry. 15:2953-2958). The decay time of the anomalous component that results from the interaction between tryptophan side chains and disulfide linkages in proteins was observed to significantly lengthen in J539 in response to binding of a galactan antigen. With hen egg-white lysozyme in which the type of fluorescence enhancement on ligand binding seen with J539 has also been observed, phosphorescence measurements revealed a similar lengthening of the decay time of the disulfide-induced anomalous component in the tryptophan decay. These perturbations are interpreted as ligand-induced changes to the tryptophan-disulfide proximities that have been shown to exist in these structures. Given the short-range nature of the disulfide perturbation (see following article) the observations suggest, in particular when combined with x-ray crystallographic data, that phosphorescence decay-time measurements of disulfide perturbations can serve as a sensitive spectroscopic indicator of subtle conformational changes in immunoglobulins and other tryptophan-disulfide containing proteins.

Triplet-singlet energy transfer in the complex of auramine O with horse liver alcohol dehydrogenase

Triplet-singlet energy transfer has been studied in the complex formed between auramine O (AO) and horse liver alcohol dehydrogenase with optically detected magnetic resonance (ODMR) spectroscopy. The results show that Trp-15 and Tyr residues transfer triplet energy mainly by a trivial process, whereas Trp-314 transfers triplet energy by a Förster process with two observed lifetimes at 77 K of 170 and 50 ms. The different Förster energy-transfer lifetimes are ascribed either to quenching of the two Trp-314 residues of the dimer by a single asymmetrically bound AO or to two distinct conformations of the enzyme-dye complex with differing separations and/or orientations of donor and acceptor. Individual spin sublevel transfer rate constants are reported for the major decay component with the 170-ms Trp triplet-state lifetime; these are found to be highly selective with kxtr much greater than kytr and kztr.

Optically detected magnetic resonance in lysozyme: evidence for three phosphorescent TRP residues

The Optically Detected Magnetic Resonance spectrum of lysozyme has been shown to consist of a multiplet of narrow components, at -1565 MHz, 1585 MHz, and 1620 MHz. The 1585 MHz component is the strongest feature of the spectrum. This is consistent with earlier reports which apparently resolved only this principal component in lysozyme. The linewidths reported here are the narrowest ever reported for tryptophan in proteins. Using Microwave-Induced Phosphorescence techniques, the dominant 1585 MHz line is seen to be coupled to a "narrow" phosphorescence emission component at about 4134A. This component has a bandwidth of about 25A compared to 42A for the normal O-O band for tryptophan in lysozyme.

Laser chemically induced dynamic nuclear polarization study of the reaction between photoexcited flavins and tryptophan derivatives at 360 MHz

Chemically induced dynamic nuclear polarization (CIDNP) is generated when tryptophan (Trp), its derivatives, or Trp-containing peptides react with photoexcited flavins in a 360-MHz NMR spectrometer. In contrast to tyrosine (Tyr), we find that the nuclear polarization of Trp originates in an electron-transfer reaction. By use of a series of Trp derivatives, the unpaired spin-density distribution of the Trp radical cation and the ground-state NMR spectrum of Trp are analyzed in detail. The signs and the relative magnitudes of the proton isotropic hyperfine coupling constants for each position around the indole ring in the radical cation deduced from these measurements are the following: position 3 greater than 2 approximately 4 approximately 6 greater than 1 greater than 5 greater than 7, with positions 1, 2, 3, 4, and 6 positive, 5 negative, and 7 essentially zero. This result is inconsistent with most available calculations of the unpaired spin-density distribution but is compatible with the pattern of electrophilic aromatic substitution. The origin of this discrepancy is discussed in detail. Possible mechanistic complications in the reaction leading to CIDNP are discussed. The laser CIDNP spectra of the Trp-rich peptides gramicidins A and B are presented as examples of the resolution enhancement obtained with this technique.

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.

Effect of stacking interactions with poly(riboadenylic acid) on the triplet state properties of tryptophan

Optically detected magnetic resonance (ODMR) signals of tryptophan (Trp) have been measured in L-lysyl-L-tryptophyl-L-lysine (Lys-Trp-Lys) and in its complex with poly(riboadenylic acid) [poly(rA)]. Measurements were made with optical narrow band detection through the Trp O-O band. Plots of [D] and [E] vs. lambda are distinctly different for Lys-Trp-Lys and its complex with poly(rA). A reduction of [D], in particular, is consistent with stacking of Trp with adenine in the complex, since this effect is expected from charge-transfer contributions in the excited triplet state. Triplet energy transfer from poly(rA) to Lys-Trp-Lys is nearly complete at 77 K, with a Trp:adenine ratio of 0.1. The energy transfer efficiency is considerably reduced at 4.2 K and below, probably resulting from reduction of the triplet mobility in the polymer. Analysis of the phosphorescence decays shows that the triplet states of poly(rA), Lys-Trp-Lys, and their complex decay nonexponentially. Binding of polylysine to poly(rA) has no effect on the phosphorescence spectrum, but the decay kinetics are changed.

Triplet state energy transfer in several proteins

Energy transfer between excited triplet states of aromatic amino acid residues was observed at 1.4 degrees K. The distance necessary for energy transfer between monomeric tyrosine and tryptophan residues was determined to be roughly 63 A. Total phosphorescence decay rate constants for several proteins were determined while emission corresponding to tyrosine and tryptophan residues was monitored. The observed decay rate constants are interpreted in terms of intramolecular interactions of the polypeptide residues.