Robert R. Kuntz

OBSERVATIONS OF C‐S AND S‐S BOND CLEAVAGE IN THE PHOTOLYSIS OF DISULFIDES IN SOLUTION

Abstract— A comparison of the transient absorption spectra from the photolysis of disulfides in solution suggests that C‐S bond breakage is a common primary photolytic process. This process becomes more important as the resulting carbon centered radical is stabilized by increasing alkyl substitution or resonance interaction with an aromatic system. The perthiyl radical product is characterized by λmax?380 nm,ε380?1700 M−1 cm−1 and decays by second order kinetics with k2?3.7×108M−1 s−1 in water.

FLASH PHOTOLYSIS OF TRYPTOPHAN AND N‐ACETYL‐L‐TRYPTOPHANAMIDE; THE EFFECT OF BROMIDE ON TRANSIENT YIELDS

Abstract— The initial yields of the cation and neutral radicals and the triplet are greatly enhanced when high concentrations of Br‐ are present during the flash photolysis of aqueous solutions containing either tryptophan or N‐acetyl‐L‐trytophanamide. The present study is an attempt to elucidate the mechanism by which Br‐ induces these effects. The results obtained indicate that the initial event involves an interaction between the fluorescent state and Br‐ to promote the formation of a long‐lived radical precursor that may be the triplet. It is shown that all of the Br‐‐induced neutral and cation radical formation originate from this long‐lived state. Furthermore, it was found that the mechanism of radical production from the Br‐‐induced long‐lived precursor does not involve hydrated electron formation.

Flash photolysis of N-acetyl-L-tryptophanamide; acid-base equilibrium of the radical transients

Abstract The transient with a λ max at 570 m observed in the flash photolysis of N-acetyl-L-tryptophanamide is confirmed to be the positive radical. This species is in rapid equilibrium with its deprotonated form (λ max =510 mm) with a pK of 4.3.

Spectral shifts of the p-aminophenylthyl radical absorption and emission in solution

Abstract The absorption and fluorescence spectra of the reactive p-aminophenylthiyl radical were observed in several solvents. Analysis of the spectral shifts yielded dipole moments for the ground and excited states of 4.3 D and 7.3 D, respectively. These values suggest a significant contribution of ionic resonance forms to both states.

FLASH PHOTOLYSIS OF N-ACETYL-L-TRYPTOPHANAMIDE: ANALYSIS OF TRANSIENTS EMPLOYING VIDICON TUBE DETECTION*

Abstract— An improved method for recording transient absorption spectra obtained by flash photolysis is described. The detection system, comprising a vidicon tube coupled to a multichannel analyzer, is shown to permit the rapid acquisition of accurate and precise spectra spanning any 360 nm interval within the 200 to 1100 nm wavelength range. The successful dissection of the complex spectrum obtained when N‐acetyl‐L‐tryptophanamide is flashed under nitrogen‐saturated conditions is utilized to highlight the capabilities of the detection system. This analysis suggests that the observed radicals must originate, in part, from an oxygen sensitive long‐lived precursor which is not the triplet state.

Chemistry of bifunctional photoprobes. 3 -- Correlation between the efficiency of CH insertion by photolabile chelating agents and lifetimes of singlet nitrenes by flash photolysis: First example of photochemical attachment of {sup 99m}Tc-complex with human serum albumin

Systematic functionalization of perfluoroaryl azides with chelating agents capable of complexing transition metals produces a new class of bifunctional photolabile chelating agents (BFPCAs). The strategy to shield the azide functionality from the electronic and steric influence of the electron-rich metal Pd through ester and amide bridges raised CH insertion efficiency to unprecedented levels (>92%) in a model solvent (cyclohexane). In contrast, perfluoroaryl azides attached to chelating agents via hydrazones show no significant CH insertion in cyclohexane upon photolysis. Measurements of the lifetimes of the singlet nitrenes derived from these agents by flash photolysis techniques correlate well with the efficiency of CH insertion by demonstrating longer lifetimes (10−50 times) for singlet nitrenes derived from azidotetrafluorinated esters and amides compared with the related hydrazones, which failed to yield significant CH insertion. A representative BFPCA 12 is chelated to diagnostic radionuclide 99mTc...

High yields of nitrene insertion into unactivated C–H bonds. First example of X-ray crystallographic and 19F NMR analysis of the photochemically produced C–H inserted adduct

Photolysis of 4-azido-tetrafluorobenzonitrile results in the highest yield reported to date, (75–80% as estimated from 19F NMR spectroscopy) for nitrene insertion into the unactivated C–H bond of cyclohexane; the photochemical adduct is characterized by 1H and 13C NMR spectroscopy and the structure is confirmed by X-ray crystallography for the first time.

FLASH PHOTOLYSIS OF N‐ACETYL‐L‐TRYPTOPHANAMIDE: THE RELATIONSHIP BETWEEN RADICAL YIELDS AND FLUORESCENCE QUENCHING

Abstract— The flash photolysis of N‐acetyl‐L‐tryptophanamide (NATA) in the presence of the fluorescence quenchers, imidazole, acrylamide and trichloroethanol, has been investigated. Imadazole and acrylamide induce a decrease in the NATA radical yield which correlates with their NATA fluorescence quenching action. These observations suggest that the fluorescent state is primarily responsible for the monophotonic photoionization processes. The acrylamide data also suggest that 40–65% of the NATA radicals arise from a long‐lived state (τ˜μs) which must originate from the fluorescent state. Unlike imidazole and acrylamide, trichloroethanol enhances the radical yield by reaction with excited state precursors. Mechanisms for the quenching of fluorescence and the long‐lived states are discussed.

A FLASH PHOTOLYSIS STUDY OF 5-METHOXYINDOLE

Abstract— The microsecond flash photolysis of 5‐methoxyindole in aqueous solutions has been studied at γexc≥ 290 nm. Transients identified in this time realm in neutral solutions are: eaq‐, the 5‐methoxyindole radical cation (γmax≅ 440 nm), the neutral transient with γmax≅ 530 nm) and an unidentified oxygen sensitive transient with γmax≅ 435 nm. Radical cations and e‐aq are shown to be produced in equal amounts consistent with a photoionization process as the only source of both transients. H+ quenching of fluorescence and radical cation production gives equivalent Stern‐Volmer constants indicating that photoionization occurs from the fluorescent state. The unidentified oxygen sensitive transient exhibits a pKa of2–2.5 and is quenched at lower pH values indicating that it also has a fluorescent state precursor.

The extinction coefficient for the p-aminophenylthiyl radical as determined by reaction with galvinoxyl

The reaction of the p-aminophenylthiyl radical (RS) with galvinoxyl has been used to determine the extinction coefficient of RS in several solvents. Values of ϵRS range from 11 000 M−1 cm−1 in cyclohexane to 19 000 M−1 cm−1 in acetonitrile. The addition of RS to galvinoxyl and the second-order recombination of RS both proceed at nearly diffusion-controlled rates with rate constants of (1.1 − 2) × 109 M−1 s−1. Rates of addition of RS to styrene have been determined to elucidate ground state radical solvation effects. In cyclohexane RS adds to styrene with a rate constant of 1.7 × 105 M−1 s−1. Increasing the polarity of the solvent lowers this rate constant to a limiting value of 4.8 X 103 M−1 s−1 in acetone and more polar solvents.