Leslie S. Forster

THE FLUORESCENCE OF TRYPTOPHYL PEPTIDES

Abstract— Tryptophan lifetimes and relative quantum yields have been determined for a group of small (1–4 residues) peptides and peptide hormones [luteinizing releasing factor. mellitin, glucagon. glucagon 22–29, glucagon 1–26. glucagon 1–27 (homoserine)]. All of the larger peptides and most of the smaller peptide anions exhibit nonexponential decay. Peptide quenching in the small peptides is more effective when the bonding is at the amino rather than at the carboxyl end of tryptophan. With the exception of tryptophylglycine. quenching by NH+3 is thought not to involve proton transfer. The results suggest that a decay component of 3–4 ns is expected whenever large peptides and proteins contain a solvent exposed tryptophan.

FLUORESCENCE CHARACTERISTICS OF INDOLES IN NON-POLAR SOLVENTS: LIFETIMES, QUANTUM YIELDS AND POLARIZATION SPECTRA

Abstract— Fluorescence lifetimes, quantum yields and polarization spectra were measured for indole, 3‐methylindole and 2,3‐dimethylindole in non‐polar solvents. The results indicate simultaneous emission from thermally equilibrated 1La and 1Lb levels, with 1La→1A dominating the 2,3‐dimethylindole emission, and 1Lb→1A dominating the indole emission. These results are consistent with previous assignments of the 0‐0 transitions in absorption for these compounds. Radiative rates are: 1La→1A, 2·0 × 108 S‐1 and 1Lb→1A. 0·62 → 108 S‐1. In addition, the temperature dependence of the excitation and emission spectra are presented, which show that aggregation occurs with these indoles in hydrocarbons below approximately ‐ 110°C. Possible applications to tryptophyl emission in the hydrophobic interiors of proteins are briefly discussed.

The luminescence of oxines and metal oxinates

Abstract The quantum yields of fluorescence (and phosphorescence) of a number of free oxines and metal oxinates have been measured. The effect of high Z closed shell atoms on the anion fluorescence is small. The chelation of a transition metal ion reduces the fluorescence yield markedly but phosphorescence is not observed. It is concluded that the quenching produced by these ions occurs in the singlet manifolds of the ligands.

Electronic transitions in chromium(III) complexes—IV radiative and non-radiative transition probabilities☆

Abstract The luminescence quantum yields and lifetimes of a group of Cr(III) complexes have been measured. The variation in the 2 E → 4 A 2 quantum yield is largely due to the change in the non-radiative transition probability ( 2 E → 4 A 2 ) with ligand. The ligand field theory is adequate for the calculation of the 2 E → 4 A 2 radiative transition probability in most cases.

Relaxation of Excited States in Cr(III) Complexes

The quantum yields of luminescence and 2E lifetimes have been determined as a function of temperature for Cr3+ in the crystalline hosts, K3Co(CN)6, NaMgAl(C2O4)3·9H2O, C(NH2)3Al(SO4)2·6H2O(GASH), C(NH2)3Al(SO4)2·6D2O(GASD), AlCl3·6H2O, and AlCl3·6D2O. The effect of structure and environment on the nonradiative transitions has been assessed. For a given complex, the probability of the nonradiative 2E↝4A2 transition may or may not depend upon environment. In all cases, only phosphorescence is observed at low temperatures, but upon warming, the aquo complexes fluoresce. Depopulation via 4T2 is the dominant thermal relaxation process of 2E.

Thermal relaxation in excited electronic states of d3 and d6 metal complexes

Abstract Photophysical kinetic results have played an important role in assessing excited state relaxation pathways in transition metal complexes. The applicability of a kinetic analysis is critically dependent on the quality of the individual decay rates, the temperature range examined, and the model used to extract the activation parameters. The extensive literature describing the temperature dependence of excited state depopulation in d 3 and d 6 complexes permits an evaluation of both the power and limitations of kinetic arguments in assessing the mechanism of excited state relaxation.

Electronic transitions in chromium (III) complexes—III: Luminescence quantum yields and lifetimes

Abstract The luminescence quantum yields and lifetimes of a group of β-diketone chelates of chromium have been measured. The changes in these quantities with ligand are ascribed to the variation in the non-radiative transition probabilities from the 4 T 2 and 2 E states to the ground state. A wave length dependence is observed in some cases which appears to be correlated with the energy of the lowest ligand triplet state.

Electronic transitions in chromium (III) complexes—I: Absorption spectra of β-diketone chelates

Abstract Absorption spectra of solutions of chromium (III) (β-diketone chelates were obtained at 85°K. The effects of substituents on the chelate rings upon the positions of the bands in the 22–40,000 cm −1 region were determined. These shifts are similar for all the bands and in accord with the predictions of molecular orbital theory for π-π ∗ transitions.

Electronic structures of lumazines and isoalloxazines

Abstract Electronic structures of uracil, two lumazine isomers, isoalloxazine (oxidized and reduced), and alloxazine have bee computed by a self consistent field (SCF) method. The calculated transition energies and intensities are significantly better than those obtained with the Huckel approximation. The SCF calculations indicate that reduced isoalloxazine is non-planar.

Thermal Quenching of Cr(III) Luminescence

The luminescences of chromium acetylacetonate and the nitro‐ and haloderivatives dissolved in three different isomorphous crystalline hosts have been studied as a function of temperature. The lifetimes and relative quantum yields were determined and evidence for two thermally activated nonradiative processes has been obtained. The activation energies and frequency factors are E1<1 kcal/mole, s1≃103 sec−1 and E2≃7 kcal/mole, s2≃1012 sec−1. This latter value is very large for a spin‐forbidden process. The lattice appears to affect nonradiative processes originating in the 4T2 more than those involving the 2E state.