SEARCH · PubMed Health

Results for “PHOSPHORESCENCE”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 55 records · Page 3Linked to original sources

PubMed1998

Tyrosine quenching of tryptophan phosphorescence in glyceraldehyde-3-phosphate dehydrogenase from Bacillus stearothermophilus.

Tyrosine is known to quench the phosphorescence of free tryptophan derivatives in solution, but the interaction between tryptophan residues in proteins and neighboring tyrosine side chains has not yet been demonstrated. This report examines the potential role of Y283 in quenching the phosphorescence emission of W310 of glyceraldehyde-3-phosphate dehydrogenase from Bacillus stearothermophilus by comparing the phosphorescence characteristics of the wild-type enzyme to that of appositely designed mutants in which either the second tryptophan residue, W84, is replaced with phenylalanine or Y283 is replaced by valine. Phosphorescence spectra and lifetimes in polyol/buffer low-temperature glasses demonstrate that W310, in both wild-type and W84F (Trp84-->Phe) mutant proteins, is already quenched in viscous low-temperature solutions, before the onset of major structural fluctuations in the macromolecule, an anomalous quenching that is abolished with the mutation Y283V (Tyr283-->Val). In buffer at ambient temperature, the effect of replacing Y283 with valine on the phosphorescence of W310 is to lengthen its lifetime from 50 micros to 2.5 ms, a 50-fold enhancement that again emphasizes how W310 emission is dominated by the local interaction with Y283. Tyr quenching of W310 exhibits a strong temperature dependence, with a rate constant kq = 0.1 s(-1) at 140 K and 2 x 10(4) s(-1) at 293 K. Comparison between thermal quenching profiles of the W84F mutant in solution and in the dry state, where protein flexibility is drastically reduced, shows that the activation energy of the quenching reaction is rather small, Ea < or = 0.17 kcal mol(-1), and that, on the contrary, structural fluctuations play an important role on the effectiveness of Tyr quenching. Various putative quenching mechanisms are examined, and the conclusion, based on the present results as well as on the phosphorescence characteristics of other protein systems, is that Tyr quenching occurs through the formation of an excited-state triplet exciplex.

Geobacillus stearothermophilus↗

PubMed1992

Characterization of tryptophan phosphorescence of aspartate aminotransferase from Escherichia coli.

The Trp phosphorescence spectrum, intensity and decay kinetics of apo-aspartate aminotransferase, pyridoxamine-5P-aspartate-aminotransferase and pyridoxal-5P-aspartate aminotransferase were measured over a temperature range 160-273 K. The fine structure of the phosphorescence spectra in low-temperature glasses, with 0-0 vibrational bands centered at 408, 415 and 417 nm, for both apoenzyme and pyridoxamine-5P-enzyme reveals a marked heterogeneity of the chromophore environments. Only for the pyridoxal-5P form of the enzyme is the triplet emission strongly quenched and, in this case, the spectrum displays a unique 0-0 vibrational band centered at 415 nm. Concomitant to quenching, there is Trp-sensitized delayed fluorescence of the Schiff base, an indication that quenching of the excited triplet state is due, at least in part, to a process of triplet singlet energy transfer to the ketoenamine tautomer. All three forms of the enzyme are phosphorescent for temperatures up to 273 K. However, across the glass transition temperature the pyridoxal-5P enzyme shows a decrease in lifetime-normalized phosphorescence intensity, a thermal quenching that reduces even further the number of phosphorescing residues at ambient temperature. In fluid solution, the triplet decay is nonexponential and multiple lifetimes stress the heterogeneity in dynamical structure of the chromophores' sites. For the pyridoxal-5P enzyme, where only one or at most two residues are phosphorescent at 273 K, the nonexponential nature of the decay implies the presence of different conformers of the protein not interconverting in the millisecond time scale.

Apoenzymes↗

PubMed1989

Phosphorescence maxima and triplet state lifetimes of NAD+ and epsilon-NAD+ in ternary complexes with horse liver alcohol dehydrogenase.

This paper describes the phosphorescence emission and decay times of NAD+ and its fluorescent etheno derivative, epsilon-NAD+, in the pyrazole ternary complex with horse liver alcohol dehydrogenase (ADH). We show that the epsilon-NAD+ triplet state, as well as the tryptophan triplet state, can be utilized to monitor the coenzyme-enzyme interaction. The decays of NAD+ and AMP are single exponential, and the lifetimes are the same within experimental error. The phosphorescence lifetimes, evaluated as single exponentials, are slightly shorter in epsilon-NAD+ than they are in epsilon-AMP. Whereas the decay of epsilon-AMP was adequately fit by a single exponential with a time constant of very close to 0.5 s, it was necessary to fit the decay of epsilon-NAD+ to a double exponential. Ternary complexes with NAD+ excited at 297 nm exhibit decay kinetics nearly identical to those of ADH by itself. On the other hand, when excitation of the epsilon-NAD+ ternary complex is provided at 313 nm, where there is very little absorption by either tryptophan residue, the decay law of the ternary complex is similar to that of epsilon-NAD+ in solution. Our results demonstrate that NAD+ and epsilon-NAD+ quench tryptophan phosphorescence in ADH. Normalizing the phosphorescence intensity to the 0-0 vibronic band assigned to Trp-15 (blue-edge), we calculate a 21% decrease in the phosphorescence associated with Trp-314 at stoichiometric saturation of the coenzyme binding sites with NAD+ in the ternary complex. When the active sites are saturated with epsilon-NAD+, the relative phosphorescence due to Trp-314 decreases by 63%.(ABSTRACT TRUNCATED AT 250 WORDS)

Alcohol Dehydrogenase↗

PubMed1989

Tryptophan phosphorescence at room temperature as a tool to study protein structure and dynamics.

Fluorescence and phosphorescence resemble each other and in many ways can give the same type of information. Both originate from a dipolar interaction between light and the molecule. In this regard, both are polarized and subject to the same type of quenching phenomena. In other respects the information which they divulge are complementary. The fluorescence quantum yield is higher for exposed tryptophans and this is expressed in longer lifetime (Grinvald and Steinberg, 1976); in contrast long lifetime of phosphorescence appears to correlate with burial. Phosphorescence, spin-disallowed, is much longer lived than fluorescence. This allows the structural/dynamic characterization of proteins to be studied on a new time regime. A really remarkable finding of studies of protein phosphorescence is that there is such variability both in phosphorescence lifetime and quenchability. We would interpret this to indicate that the tryptophan environment can range from essentially a crystal, almost comparable in rigidity as found at 77 K, to tryptophans in a flexible environment, almost as flexible as free in solution. An interesting task will be to examine the relationship between the yield and lifetime of phosphorescence and details of the tryptophan environment in terms of rigidity and adjacent amino acids among the proteins with known three dimensional structure.

Luminescence↗

PubMed1991

Demonstration that phosphorescent 6-bromo-2-naphthyl sulfate can be used to probe heme accessibility in heme proteins.

The phosphorescence properties of 6-bromo-2-naphthyl sulfate (BNS) in aqueous solution were studied. The phosphorescence lifetime is several hundred microseconds and is self-quenched. Although a fluorescent photoproduct is formed from BNS, it does not interfere with the decay properties of triplet-state BNS and its utility as a probe of the accessibility of the heme group in heme proteins. Quenching of BNS phosphorescence does not occur for the non-heme protein lysozyme and apomyoglobin but occurs by a dynamic mechanism with a quenching constant of 1-2 x 10(9) M-1 s-1 for cytochrome c and myoglobin and with a quenching constant of 6.2 x 10(9) M-1 s-1 for protoporphyrin IX. The phosphorescence of an inclusion complex of 1-bromonaphthalene and beta-cyclodextrin is not quenched by heme-containing proteins. The temperature and viscosity dependencies of the rate with which BNS phosphorescence is quenched by microperoxidase-11 are consistent with unit quenching efficiency. These results indicate that quenching of BNS phosphorescence occurs only upon contact with the quencher, and the quenching constant can be used to assess the degree of accessibility of the heme group.

Fluorescent Dyes↗

PubMed1987

On the prevalence of room-temperature protein phosphorescence.

A large number of proteins were tested for the property of intrinsic phosphorescence in deoxygenated aqueous solution at room temperature. The majority of proteins exhibit phosphorescence under normal solution conditions. Phosphorescence lifetimes from 0.5 millisecond to 2 seconds were observed in three-fourths of the proteins tested. The lifetime appears to correlate with relative isolation of the tryptophan indole side chain from solvent. With few exceptions, proteins in general can be expected to display a phosphorescence lifetime greater than 30 microseconds. This widespread characteristic of proteins has been largely overlooked because long-lived phosphorescence is highly sensitive to quenching by low levels of dissolved oxygen in solution. Protein phosphorescence offers a new time domain and a far wider dynamic range than has been used before for photoluminescence experimentation.

Fluorescence↗

PubMed2006

Tomographic imaging of oxygen by phosphorescence lifetime.

Imaging of oxygen in tissue in three dimensions can be accomplished by using the phosphorescence quenching method in combination with diffuse optical tomography. We experimentally demonstrate the feasibility of tomographic imaging of oxygen by phosphorescence lifetime. Hypoxic phantoms were immersed in a cylinder with scattering solution equilibrated with air. The phantoms and the medium inside the cylinder contained near-infrared phosphorescent probe(s). Phosphorescence at multiple boundary sites was registered in the time domain at different delays (t(d)) following the excitation pulse. The duration of the excitation pulse (t(p)) was regulated to optimize the contrast in the images. The reconstructed integral intensity images, corresponding to delays t(d), were fitted exponentially to give the phosphorescence lifetime image, which was converted into the three-dimensional image of oxygen concentrations in the volume. The time-independent diffusion equation and the finite element method were used to model the light transport in the medium. The inverse problem was solved by the recursive maximum entropy method. We provide what we believe to be the first example of oxygen imaging in three dimensions using long-lived phosphorescent probes and establish the potential of these probes for diffuse optical tomography.

Computer Simulation↗

PubMed2005

Simple determination of the herbicide napropamide in water and soil samples by room temperature phosphorescence.

A new, simple, rapid and selective phosphorimetric method for determining napropamide is proposed which demonstrates the applicability of heavy-atom-induced room-temperature phosphorescence for analyzing pesticides in real samples. The phosphorescence signals are a consequence of intermolecular protection and are found exclusively with analytes in the presence of heavy atom salts. Sodium sulfite was used as an oxygen scavenger to minimize room-temperature phosphorescence quenching. The determination was performed in 1 M potassium iodide and 6 mM sodium sulfite at 20 degrees C. The phosphorescence intensity was measured at 520 nm with excitation at 290 nm. Phosphorescence was easily developed, with a linear relation to concentration between 3.2 and 600.0 ng ml(-1) and a detection limit of 3.2 ng ml(-1). The method has been successfully applied to the analysis of napropamide in water and soil samples and an exhaustive interference study was also carried out to display the selectivity of the proposed method.

Herbicides↗

PubMed1995

A study of the mitochondrial F1-ATPase tryptophan phosphorescence at 273 K.

The bovine heart mitochondrial F1-ATPase complex exhibits an intrinsic tryptophan phosphorescence that can be used to monitor structural changes of the epsilon-subunit. The phosphorescence decay rate of F1 containing the tightly bound nucleotides increases upon addition of adenine nucleoside triphosphate in the presence of magnesium. The average phosphorescence lifetime of this enzyme preparation decreases from 10.2 to 7.8 ms upon Mg-ATP addition. Since increasing phosphorescence decay rate is related to increasing flexibility of proteins, Mg-ATP added to the F1-ATPase complex can enhance the flexibility of the protein structure surrounding the chromophore. Experiments carried out on F1 prepared with the three noncatalytic sites filled and the three catalytic sites vacant show a significant increase of the phosphorescence lifetime from 6.4 ms to 7.6 ms upon Mg-ATP addition. These results suggest that the mitochondrial F1-ATPase epsilon-subunit conformation senses differently the nucleoside triphosphate binding to catalytic or noncatalytic sites.

Adenosine Triphosphate↗

PubMed2004

Theoretical study of the external heavy atom effect on phosphorescence of free-base porphin molecule.

The radiative lifetime of phosphorescence of free-base porphin (H2P) molecule and its complexes with noble-gas atoms are calculated by time-dependent density functions theory (TD DFT) with quadratic response functions for account of spin-orbit coupling and electric dipole activity. The complexes with Ne, Ar, Kr, and Xe are used to simulate the external heavy atom (EHA) effect on phosphorescence of the H2P molecule in the corresponding noble gas matrices. The B3LYP functional and small basis set (3-21G) are used throughout the study and comparison of all complexes but other basis sets are also utilized to support the chosen approach. A slow radiative rate constant of free-base porphin phosphorescence (about 10(-3) s(-1)) is obtained with all basis sets being in the order of magnitude agreement with experimental estimations. A strong enhancement of the H2P phosphorescence rate (by 360 times) is calculated for Xe complex; while for Ne, Ar, and Kr complexes, the enhancement is equal to 1.1, 1.3, and 10.3 times, respectively. In these complexes, the noble gas atom is disposed at 3.6 A above the center of the porphin ring. In spite of shortcomings of the chosen simple model, the TD DFT calculations explain the most important features of the EHA effect on phosphorescence of free-base porphin. Calculations of the hyperfine coupling tensors for all magnetic nuclei in the lowest triplet state of H2P molecule and its complexes with noble-gas atoms indicate an appreciable penetration of the spin density to the EHA region. This can be connected with the enhancement of spin-orbit coupling in the H2P molecule.

Electrons↗

PubMed2003

Fast determination of propranolol in urine and pharmaceutical preparations by stopped-flow and micellar-stabilized room-temperature phosphorescence: validation of the method.

The stopped-flow mixing technique has been used to study the kinetic determination of propranolol by means of micellar-stabilized room-temperature phosphorescence. This mixing system diminishes the time required for the deoxygenation of micellar medium by sodium sulfite, allowing a kinetic curve that levels off within only 7s to be obtained. The phosphorescence enhancers thallium (I) nitrate, sodium dodecyl sulfate, and sodium sulfite were optimized to obtain maximum sensitivity and selectivity. A pH value of 6.54 was selected as adequate for phosphorescence development. The kinetic curves of propranolol phosphorescence were scanned at lambda(ex)=290 nm and lambda(em)=524 nm. The calibration graphs were linear for the concentration range from 25 to 400 ng mL(-1). The phosphorescence lifetime of propranolol is approximately 1210 micros. The detection limit calculated as proposed clayton was 13.53 ng mL(-1) and by applying the error propagation theory, the detection limit was 8.37 ng mL(-1). The repeatability was studied using 10 solutions of 200 ng mL(-1) of propranolol; if error propagation theory is assumed, the relative error is 1.94%. The standard deviation for a replicate sample was 4.0 ng mL(-1). This method was successfully applied to the determination of propranolol in commercial formulations and in urine. Suitable recovery values were obtained.

Health↗

PubMed2001

Determination of photophysical rate constants for the non-protected fluid room temperature phosphorescence of several naphthalene derivatives.

The determination of kinetic parameters for luminescence processes is very important in understanding the phosphorescence process and the mechanisms of the heavy atom effect (HAE). In our previous work, we reported that room temperature phosphorescence (RTP) emission of many naphthalene derivatives can be induced directly from their aqueous solution without using any kind of protective medium, and the name Non-Protected Fluid Room Temperature Phosphorescence (NP-RTP) is suggested for this new type of RTP emission. In order to further understand this kind of luminescence phenomenon, the influence of heavy atom perturber (HAP) concentration on RTP lifetime of several naphthalene derivatives was studied in detail in this paper. The possibility of determination of photophysical parameters for emission of NP-RTP was explored based on the definition on the phosphorescence lifetime and the relation with the concentration of HAP in this paper. A static Stern-Volmer equation for phosphorescence was derived and the luminescence kinetic parameters were calculated. The results obtained by two different ways proved that photophysical parameters for RTP emission can be determined based on the changes of the RTP lifetime.

Kinetics↗

PubMed1987

Optical detection of triplet-state magnetic resonance studies on individual tryptophan residues of serum albumin: correlation between phosphorescence and zero-field splittings.

Cyanogen bromide cleavage of bovine serum albumin (BSA) yields two fragments, N (1-183) and C (184-582), containing 183 and 399 amino acid residues, respectively. Each in each fragment are characterized in this study by phosphorescence and optically detected magnetic resonance spectroscopy, and the results are compared with those of the intact albumin. Trp-134 in fragment N is located in a hydrophobic environment in the interior of the protein, as reflected by its red-shifted phosphorescence and characteristic zero-field splittings. The spectral properties of Trp-212 in fragment C suggest its location in a partially buried, inhomogeneous environment. They show great similarity to those of human serum albumin, which contains a single Trp at position 214. The Trp phosphorescence 0,0-bands of fragments C and N are fitted with Gaussian functions by computer, and their relative contributions to the phosphoresence 0,0-band of BSA are adjusted to fit the observed BSA 0,0-band. The wavelength dependence of the [D[-[E[ transition frequencies of fragments N and C is then weighted by their 0,0-band intensity, taking into account differences in spin alignment, and summed to predict the peak frequency of the [D[-[E[ band profile as a function of phosphorescence wavelength for the intact BSA. Good agreement between predicted and observed behavior of [D[-[E[ vs. wavelength for the intact protein provides strong evidence for the additivity of the phosphorescence and ODMR spectra of the individual Trp sites in BSA. We find that Trp-134 and Trp-212 have wavelength-independent and wavelength-dependent zero-field splittings, respectively.

Cyanogen Bromide↗

PubMed1987

Characterization of tryptophan environments in glutamate dehydrogenases from temperature-dependent phosphorescence.

Tryptophan room temperature phosphorescence in solution was detected in glutamic dehydrogenase from bovine liver and Escherichia coli with lifetimes of 1.2 and 0.65 s, respectively. Although these enzymes possess three and five tryptophanyl residues per polypeptide chain, respectively, the temperature dependence of the phosphorescence quantum yield estimates that the room temperature emission is due, in either case, to a single residue. Long triplet-state lifetimes and very small rates of O2 quenching indicate that these tryptophanyl side chains are embedded in a highly inflexible internal region of the macromolecule. Aided by sequence homology with dehydrogenases of known structure and theoretical predictions of secondary structure [Wootton, J.C. (1974) Nature (London) 252, 542-546; Brett, M., Chambers, G.K., Holder, A. A., Fincham, J.R.S., & Wootton, J.C. (1976) J. Mol. Biol. 106, 1-22], the phosphorescing tryptophans have been tentatively placed in the catalytic coenzyme binding domain of each enzyme. The particular sensitivity of the triplet-state lifetime in probing local changes in conformation provides a strong indication that within the time window of phosphorescence measurements the six subunits in the hexameric enzymes are equivalent. Furthermore, while in the bovine enzyme this parameter is markedly affected by the interaction with ligands which have a functional role, the constancy of the phosphorescence lifetime at various degrees of polymerization suggests that the association process is not accompanied by important conformational changes in the macromolecule.

Alcohol Dehydrogenase↗

PubMed1982

Phosphorescence and optically detected magnetic resonance study of the tryptophan residue in human serum albumin.

The phosphorescence and optically detected magnetic resonance (ODMR) of the lone tryptophan residue in the protein human serum albumin (HSA) were studied. This residue shows a red-shifted phosphorescence and D-E triplet zero-field splitting frequency, which suggests that it is buried in a hydrophobic region of the protein, based on trends in triplet state properties of tryptophan residues in peptides and proteins, which have recently been observed by other workers. This conclusion is in agreement with the results of previous fluorescence quenching studies of HSA with hydrophobic quenchers. Also, the ODMR line width is consistent with the line widths usually obtained from buried tryptophans, and the ODMR line shifts and broadens when the protein is heat denatured with urea, in qualitative agreement with results observed with several other proteins. Phosphorescence wavelengths, decay times, and zero-field splittings are also studied as a function of binding of halides and acetate to HSA. The tryptophan triplet lifetime is shortened when bromide and iodide bind, apparently due to the external heavy atom effect; chloride and acetate have no effect on the phosphorescence lifetime. ODMR and phosphorescence measurements suggest that the degree of solvent exposure of the tryptophan does not change appreciably upon the binding of these anions.

Anions↗

PubMed1997

Fluorescence, phosphorescence, and optically detected magnetic resonance studies of the nucleic acid association of the nucleocapsid protein of the murine leukemia virus.

Fluorescence, phosphorescence, and optical detection of triplet state magnetic resonance (ODMR) are employed to investigate the interaction of p10, the nucleocapsid protein of the Moloney murine leukemia virus, with nucleic acids. p10 is a 55-amino acid protein containing a single zinc finger motif, C26C29H34C39, that includes Y at position 28 and W at position 35. In addition, the interactions of a zinc finger peptide, p10-ZF, comprising residues 24-41 of p10, and a doubly mutated 24-41 peptide, p10-ZF' in which the positions of Y and W are interchanged, also are reported. The measurements focus on the direct involvement of the sole W residue in the nucleic acid interaction. Fluorescence quenching and salt-back titrations indicate complex formation of p10 with several octanucleotides--(dT)8, (dI)8, (dU)7dT, and (5-BrdU)7dT--and with the polynucleotides poly(dT) and poly(dI). Poly(dI) binds with the highest affinity. Apparent binding constants and salt-back midpoints are reported. Neither p10-ZF nor p10-ZF' exhibits significant fluorescence quenching by these DNA substrates. Binding of p10-ZF to fluorescent poly(ethenoadenylic acid) was detected with greatly reduced affinity relative to p10, but binding of p10-ZF' was undetectable. These results are in general agreement with phosphorescence and ODMR measurements monitoring W. Addition of poly(I) to p10 leads to a phosphorescence red shift, reduction in the zero-field splitting (ZFS) parameters D and E, and a significantly reduced phosphorescence lifetime, each consistent with aromatic stacking interactions between W and the nucleobases. These effects are smaller with p10-ZF and undetectable with p10-ZF'. Poly(U) produces no significant changes in the triplet state parameters of W; no stacking interactions are observed even for p10. (5-BrdU)7dT yields large phosphorescence red shifts in p10 and p10-ZF, and reductions of D, but no significant heavy atom effects. These effects probably are due to enhanced local polarizability caused by Br, but any stacking interactions in these complexes would exclude van der Waals contacts between W and the Br atoms.

Amino Acid Sequence↗

PubMed2006

Triplet energy back transfer in conjugated polymers with pendant phosphorescent iridium complexes.

The nature of Dexter triplet energy transfer between bonded systems of a red phosphorescent iridium complex 13 and a conjugated polymer, polyfluorene, has been investigated in electrophosphorescent organic light-emitting diodes. Red-emitting phosphorescent iridium complexes based on the [Ir(btp)(2)(acac)] fragment (where btp is 2-(2'-benzo[b]thienyl)pyridinato and acac is acetylacetonate) have been attached either directly (spacerless) or through a -(CH(2))(8)- chain (octamethylene-tethered) at the 9-position of a 9-octylfluorene host. The resulting dibromo-functionalized spacerless (8) or octamethylene-tethered (12) fluorene monomers were chain extended by Suzuki polycondensations using the bis(boronate)-terminated fluorene macromonomers 16 in the presence of end-capping chlorobenzene solvent to produce the statistical spacerless (17) and octamethylene-tethered (18) copolymers containing an even dispersion of the pendant phosphorescent fragments. The spacerless monomer 12 adopts a face-to-face conformation with a separation of only 3.6 A between the iridium complex and fluorenyl group, as shown by X-ray analysis of a single crystal, and this facilitates intramolecular triplet energy transfer in the spacerless copolymers 17. The photo- and electroluminescence efficiencies of the octamethylene-tethered copolymers 18 are double those of the spacerless copolymers 17, and this is consistent with suppression of the back transfer of triplets from the red phosphorescent iridium complex to the polyfluorene backbone in 18. The incorporation of a -(CH(2))(8)- chain between the polymer host and phosphorescent guest is thus an important design principle for achieving higher efficiencies in those electrophosphorescent organic light-emitting diodes for which the triplet energy levels of the host and guest are similar.

Journal Article↗

PubMed2002

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.

Disulfides↗