Ronald L. Christensen

The origin of the chemiluminescence of phagocytosing granulocytes.

Granulocytes engaged in the phagocytosis of opsonized zymosan emit light by a process that is inhibited by superoxide dismutase and catalase. In the present report is is shown that light emission is the result of reactions between certain unspecified constituents of the ingested particles and some or all of the oxidizing agents (H2O2, O2),and possibly the hydroxyl radical and singlet oxygen) produced by the activated cells. This conclusion is based on a study of light emission by both activated cells ans artificial O2 generating system containing xanthine oxidase and purine. With these two systems light production required the presence of both zymosan and oxidizing agent, suggesting that the oxidation of particle components is necessary for luminescence to occur. The characteristics of the emission spectrum as well as the finding that granulocytes activated by a nonparticulate agent (F-) fail to liminesce show that light emission by the relaxation of singlet oxygen to the ground state does not contribute in a major way to the chemiluminescence of phagocytosing granulocytes; whether singlet oxygen contributes to chemiluminescence in other ways cannot be decided from the data available. Inasmuch as the oxidation of constituents of ingested particles is an important bacterial killing mechanism in the granulocyte, chemiluminescence may be viewed as a manifestation of the microbicidal activity of the cell.

LOW RESOLUTION OPTICAL SPECTROSCOPY OF RETINYL POLYENES: LOW LYING ELECTRONIC LEVELS AND SPECTRAL BROADNESS*

Abstract— Optical absorption and emission spectra of retinal, axerophtene, anhydrovitamin A, and 2,10‐dimethylundecapentaene in rigid glass matrixes at 77°K are presented. An interpretation of the observed spectra which provides a rationalization on the basis of electronic structure for the diffuse nature of the retinal spectrum is discussed. For the undecapentaene a weakly absorbing 1Ag state was found 3250 cm‐1 lower in energy than the level responsible for the strong visible absorption.

Vibronic coupling in polyenes: High resolution optical spectroscopy of 2,10‐dimethylundecapentaene

High resolution fluorescence and absorption spectra of 2,10‐dimethylundecapentaene in 4.2 °K n‐nonane matrices are presented. Analysis of these spectra indicates that the transitions between the ground and first excited state are induced by low frequency, nontotally symmetric vibrations. This is in agreement with Herzberg‐Teller expectations for forbidden transitions and supports the 1Ag assignment for the lowest lying excited singlet state. Vibrational analysis of the absorption indicates increases in the carbon–carbon double bond stretching frequencies in the 1Ag state. This is unexpected given the decreases in carbon–carbon double bond orders predicted by molecular orbital calculations. Rationalization of this apparent discrepancy may be found in substantial mixings of excited state vibrations. The photochemical implications of these findings also are discussed.

Low-lying electronic states of carotenoids

Four all-trans carotenoids, spheroidene, 3,4-dihydrospheroidene, 3,4,5,6-tetrahydrospheroidene, and 3,4,7,8-tetrahydrospheroidene, have been purified using HPLC techniques and analyzed using absorption, fluorescence and fluorescence excitation spectroscopy of room temperature solutions. This series of molecules, for which the extent of pi-electron conjugation decreases from 10 to seven carbon-carbon double bonds, exhibits a systematic crossover from S2----S0 (1(1)Bu----1(1)Ag) to S1----S0 (2(1)Ag----1(1)Ag) emission with decreasing chain length. Extrapolation of the S1----S0 transition energies indicates that the 2(1)Ag states of longer carotenoids have considerably lower energies than previously thought. The energies of the S1 states of spheroidenes and other long carotenoids are correlated with the S1 energies of their chlorophyll partners in antenna complexes of photosynthetic systems. Implications for energy transfer in photosynthetic antenna are discussed.

Absorption and emission of 2,12‐dimethyltridecahexaene

We have obtained well resolved optical spectra of a simple, methyl substituted hexaene at 4.2 K. These spectra provide detailed information on the vibronic levels associated with the ground (1 1Ag) and first excited (2 1Ag) singlet electronic states. The strongest vibronic feature, due to a totally symmetric CC double bond stretch, shifts from 1576 cm−1 in the 1 1Ag state to 1779 cm−1 in the 2 1Ag state. This shift, not accounted for by current theoretical descriptions of polyene electronic states at the crude adiabatic level, may be rationalized as a manifestation of vibronic coupling between the ground and first excited singlet states.

Vibronic coupling in polyenes: High resolution optical spectroscopy of all‐trans‐2,4,6,8,10,12,14‐hexadecaheptaene

High resolution fluorescence and fluorescence excitation spectra of all‐trans‐2,4,6,8,10,12,14‐hexadecaheptaene have been obtained in n pentadecane at 10 K. The most prominent vibronic features, like those in spectra of shorter polyene hydrocarbons, are due to combinations of symmetric carbon–carbon stretching vibrations. These in turn are dominated by a double bond mode whose frequency increases from 1555 to 1782 cm−1 upon electronic excitation (1 1Ag→2 1Ag). This 227 cm−1 increase, the largest yet observed in polyene spectroscopy, can be explained by vibronic coupling between the ground and first excited singlets. The possible role of vibrational mixing due to mutual polarizibilities between carbon–carbon bonds also is discussed.

The Electronic States of Carotenoids

The photochemistry of Carotenoids is determined by the electronic structures of their low energy, excited states. This chapter first relates the optical spectroscopy of carotenoids to current theoretical descriptions of polyene electronic states. Highly detailed spectroscopic information now available for model polyenes leads to a simple, three-levelenergy scheme: S2(I1 Bu)>S1(21Ag)>S0(I1 Ag) Optical studies of polyenes and carotenoids of intermediate length demonstrate that increases in conjugation lengths invariably result in S2→S0 emissions replacing the characteristic S1→S0 emissions of short polyenes. This cross-over to S2 fluorescence can be accounted for by the energy gap law: decreases in S1 energies lead to sharp increases in the rates of S1→S0 nonradiative decay with increasing conjugation. The dominance of S2 emissions in long polyenes means that the S1 states of the carotenoids employed by photosynthetic systems are difficult to locate using fluorescence detection. Even less information is available regarding the energies of carotenoid T1 states due, in large part, to the absence of confirmed phosphorescence in any polyene or carotenoid. This chapter critically examines the use of extrapolation procedures, including applications of the energy gap law, in estimating the S1 and T1 energies of long, non-emissive carotenoids. In spite of the inherent limitations of extrapolations, estimates of S1 energies in molecules such as spheroidene and β-carotene now are adequate for understanding the mechanisms of singlet energy transfer in photosynthetic systems. Outstanding issues still to be addressed include: the development of a more quantitative understanding of the effects of conjugation, substitution, isomeric structure, and solvent environment on the energies and dynamics of S1 and S2 states; the extension of the limited data base of T1 energies to longer polyenes and carotenoids; the evaluation of the effects of conformational disorder on carotenoid spectroscopy and photochemistry; and the consideration of additional low-lying singlet states in the energy level diagram.

Fluorescence excitation spectra of the S1 states of isolated trienes

First observation of fluorescence for simple, linear trienes is reported. S1←S0 fluorescence excitation spectra of hexatriene and octatriene indicate large differences between the S0 and S1 potential energy surfaces. Activation energy of <200 cm−1 for the S1 state nonradiative decay is tentatively ascribed to isomerization.

The 2 1Ag state of trans,trans‐1,3,5,7‐octatetraene in free jet expansions

One‐ and two‐photon fluorescence excitation and emission spectra of the S1↔S0 transition of trans,trans‐1,3,5,7‐octatetraene have been measured for the first time in free jet expansions. The one‐photon excitation spectrum is the same, with the exception of significant differences in the intensities of a few lines, as the two‐color, resonance‐enhanced, two‐photon ionization spectrum, previously assigned to the 2 1A’←1 1A’ transition of cis,trans‐1,3,5,7‐octatetraene. However, comparison of the one‐ and two‐photon fluorescence excitation spectra shows clearly that the carrier of the spectrum has inversion symmetry, as expected for trans,trans‐1,3,5,7‐octatetraene. The one‐photon spectrum is built on bu Herzberg–Teller promoting modes, which are origins of progressions in ag modes, while the two‐photon spectrum is due to a single progression in ag modes starting from the 2 1Ag←1 1Ag electronic origin. The appearance of out‐of‐plane vibrations, possibly including torsions of the polyene framework, suggests la...

EXCITATION SPECTROSCOPY OF RETINAL AND RELATED POLYENES

Abstract— Fluorescence excitation spectra of all‐trans retinal and the related polyene hydrocarbons anhydrovitamin A and diphenyloctatetraene show that only retinal has a wavelength‐dependent fluorescence quantum yield. Three possibilities for the unusual quantum yield behavior of retinal are considered. We conclude that competition between a fast radiationless process (perhaps of photochemical origin) and internal conversion between 1ππ* states and a low‐lying 1nπ* state provides the best rationalization for the data now in hand.