PROTONATION AND DEPROTONATION EQUILIBRIA OF LUMICHROME

Abstract— In order to investigate the possibility of the tautomerization of alloxazine to isolloxazine in its ground state, the parameters affecting the redistribution of charges in the lumichrome molecule were studied. The absorption and emission spectra of lumichrome as a function of pH in the range H₀ = - 6 to pH = 12 were recorded. At extreme pH conditions the spectra of lumichrome are similar to those of the isoalloxazine system. At high acid concentration ( H ₀ < - 3.0) the absorption spectrum of lumichrome protonated at N₁₀, is practically identical to that of lumiflavin. The fluorescence quantum yield of the two cations is negligible at room temperature.
At pH = 10.5 lumichrome is deprotonated at positions N₁ or N₃, The two monoanions have different excitation spectra. Except for slight differences in the extinction coefficients, the absorption of the anion deprotonated at N₃ is very similar to the lumichrome spectrum. The absorption spectra of the N₁ monoanion and of the di-anion are similar to the spectrum of lumiflavin except for a blue shift of about 20 nm. Furthermore, the emission spectrum of the N₁ -monoanion is identical to that of the isoalloxazine system. These results indicate that the charge distribution in the lumichrome molecule depends on the protonation and deprotonation of the nitrogen atoms at positions 10 and l. Both processes cause a redistribution of charges so that an isoalloxazine ring system is formed.     

PHOTOCHEMISTRY AND PHOTOPHYSICS OF GUANINE-CONTAINING DINUCLEOSIDES

Abstract— Yields and action spectra are reported for photochemistry. fluorescence. and total lumincscence at 405 nm due to UV excitation (240–300 nm) or dilutc (-0.1 mM ) solutions or guanosine 5–monophosphate (GMP) and the dinucleosides linking guanine with adenine (ApG and GpA). cytosine (CpG) and uracil (GpU) in neutral ethylene glycol-water (7:3) glasses at 140–165 K. Phosphorescence lifetimes were determined at 140 K. Less complete data are presented for GpC, UpG and dpGpT. Quantum yields for all three processes were usually found to increase as the excitation wavelength increases. Although intramolecular exciplex formation was not dominant under these conditions interactions were strong enough to frustrate attempts at interpretation of results within the Forster very weak coupling framework. There is evidence that the GMP photochemistry proceeds from the triplet state. Surprisingly, this photochemistry is not quenched in ApG, GpA, and dpGpT at 163 K although the adenine (A) and thymine (T) moieties are known to have lower triplet states. At 140 K the phosphorescence from ApG and GpA was entirely characteristic of A but both G and T components were observed from dpGpT.     

CHEMILUMINESCENCE AND FLUORESCENCE OF THE EUROPIUM IONS-ADENINE NUCLEOTIDES SYSTEM AND ITS POSSIBLE BIOLOGICAL SIGNIFICANCE

Abstract— Chemiluminescence of the Eu(II)/Eu(III)-adenine nucleotide-H₂O₂ system and fluorescence of the Eu(III)-adenosine triphosphate system have been investigated. The spectral distribution of the chemiluminescence emission has shown an occurrence of three main bands (Λ=470–480,590–620 and ca. 700 nm). The energy transfer process from the adenosine triphosphate molecules to the Eu(III) ions has been observed in the fluorescence spectrum. The examined chemiluminescence and fluorescence spectra show that these both kinds of emission originate from the ⁵ D _***τ⁷F_*** ( n =1–4) transitions in the Eu(III) ions.     

FLUORESCENCE OF THYMINE IN AQUEOUS SOLUTION AT 300° K

Abstract— –Fluorescence of thymine in neutral aqueous solution at room temperature has been detected using the multiscaling operation of a multichannel analyzer. The emission maximum (2.96 μm^-1) and 0-0 transition energy (3.37-3.45 μm^-1) are close to those determined at liquid nitrogen temperature in mixed solvents. The quantum efficiency of fluorescence excited at 3.77 μm^-1 is calculated to be 1.04 × 10^-4.
The corrected relative excitation spectrum shows significant differences from the absorption spectrum when both are determined under identical conditions of concentration and spectral bandwidth on the same instrument. The quantum yield of fluorescence decreases about 2-fold as the energy of excitation is increased beyond the 0-0' transition and follows the relation 1/φ°α E _excit..
This behavior is discussed in terms of (a) n π* and ππ* states, (b) emission from a minor tautomer and (c) kinetics of competing deactivation processes.     

SPECTROSCOPY OF POLYENES–III. ABSORPTION AND EMISSION SPECTRAL INVESTIGATION OF POLYENE SCHIFF BASES AND PROTONATED SCHIFF BASES RELATED TO VISUAL PIGMENTS

Abstract— A number of n -butylamine Schiff bases of polyenals related to retinals as homologues and analogues, and their protonated forms, have been studied for absorption and emission spectral properties. The polyene Schiff bases exhibit the same general features in their absorption spectra as those of the parallel polyenals except that the ^lB_u←¹A_g and π*← n singlet transitions are at substantially higher energy in the Schiff bases (the shift being larger for the π *← n transition). The Schiff bases with short polyene chainlength ( n = 2, 3 where n is the number of double bonds including C=N) do not fluoresce or phosphoresce in 3-methylpentane in the temperature range 298–77 K. The Schiff bases with intermediate chainlength ( n = 4, 5) show fluorescence at 77 K with intensity strongly dependent on the nature of solvent. The Schiff bases with relatively long chainlength ( n = 5–7) show strong or moderately strong fluorescence at 77 K and very weak fluorescence at 298 K ( n = 7) with intrinsic radiative lifetimes much longer than those estimated from the oscillator strength of the low-energy, strong absorption band (¹B_u←¹ A_g ). A discussion on the possible state order and nature of the fluorescing state of the various polyene Schiff base systems is presented.     

THE EFFECT OF ALIPHATIC ALCOHOLS ON FLUORESCENCE QUENCHING AND FLUORESCENCE POLARIZATION OF LUMINESCENCE PROBES IN HEXADECYLTRIMETHYLAMMONIUM BROMIDE MICELLES

Abstract— The fluorescence quenching of the indole chromophore by NO⁻₂ and the fluorescence depolarization of several luminescence probes in aqueous solutions containing hexadecyltrimethylammonium bromide (HDTBr) were measured as a function of added C₂–C₄ aliphatic alcohol concentration. The fluorescence decay profiles of pyrene in the micellar solutions were also measured to estimate the aggregation number of the micelles. The addition of n -butyl alcohol significantly reduces the fluorescence quenching rate and the aggregation number and increases the extent of fluorescence depolarization in HDTBr micellar systems. The addition of ethyl alcohol shows a similar but smaller effect.     

PHOTOPHYSICAL PROPERTIES OF NUCLEIC ACID COMPONENTS—1. THE PYRIMIDINES: THYMINE, URACIL, N, N-DIMETHYL DERIVATIVES AND THYMIDINE

Abstract— Low-temperature (and some room temperature) absorption and emission, fluorescence and phosphorescence, data including quantum yields and lifetimes have been obtained from the title pyrimidine bases as a function of the nature of the solvent environment. Modest vibrational resolution has been observed for the first time in the absorption spectra, particularly for thymine and uracil. The excitation spectra also show structure. The quantum yields of fluorescence (φ_F) and phosphorescence are independent of the excitation wavelength. Thymine, thymidine and uracil have profoundly different photophysical properties in polar-aprotic vs polar-protic solvents. The N, N-dimethyl substitution of thymine and uracil produces photophysical changes comparable to the solvent change for the unsubsti-tuted bases. The species involved in the emission processes is the keto (lactam) form. It is probable that ^1,3(n,π*) state(s) has(have) changed order relative to a lowest ¹(π,π*) state as a consequence of both the solvent change and N, N-dimethyl substitution. The lowest triplet state is assigned as ³(n π*). We propose that an important factor contributing to the previously reported excitation wavelength dependence of φ_F and φ_T1 (φ_isc) for nucleic-acid components is the equilibrium coexistence of H-bonded and non-H-bonded forms each having different photophysical properties. Consideration is given of the impact of the significantly different photophysical properties of nucleic-acid bases as a function of the nature of the solvent upon the photochemical properties.     

RESONANCE RAMAN SPECTRA AND NORMAL MODE DESCRIPTIONS OF A BACTERIOCHLOROPHYLL a MODEL COMPLEX

Abstract— Resonance Raman (RR) spectra are reported for the bacteriochlorophyll a (BChl a) model complex, copper methylbacteriopheophorbide a (CuBPheo a). The spectra are acquired at a number of excitation wavelengths in the Q_y+ Q_x and B regions of the absorption spectrum. The RR data obtained for CuBPheo a are correlated to those previously reported for BChl a. The normal modes of the BChl structure are calculated by using the semiempirical quantum chemistry force field (QCFF/ PI) method of Warshel and Karplus [J. Am. Chem. Soc. 94 ,5612–5625 (1972)]. The vibrational data are used in conjunction with the calculations to obtain a cohesive assignment for the normal modes of BChl a with frequencies between 1100 and 1700 cm^-1. The forms of the normal modes of BChl a are predicted to be different both from those of chlorophyll a and from those of porphyrins.     

TWENTY-TWO CARBON HOMOLOGUE OF 11-CIS RETINAL. PHOTOPHYSICAL AND PHOTOCHEMICAL PROPERTIES

Abstract— Results concerning absorption-emission spectra and fluorescence quantum yields at 77 and 298K. triplet absorption spectra, and quantum yields of intersystem crossing and photoisomerization at 298 K, are presented for 11- cis β-apo-14'-carotenal (C₂₂-Ald), the immediate higher homologue of 11- cis retinal. The absorption spectra are characterized by two band-systems with maxima at 390–400 and 270–280 nm, respectively. Upon cooling from 298 to 77 K, the intensities of these two band-systems undergo changes in opposite directions indicating 12-s- cis: 12- s-trans conformational changes. No intermediate band-system analogous to the one located at 270–310 nm in cis retinals is observed for 11- cis C₂₂-Ald. In nonpolar hydrocarbon solvents (e. g. cyclohexane) at room temperature, quantum yields of fluorescence (0.01), intersystem crossing (0.6) and photoisomerization (0.4) are all quite pronounced. The photophysical and photochemical properties of 11- cis C₂₂-Ald are discussed in the light of similarities and dissimilarities with those of all- trans C₂₂-Ald and 11- cis retinal under comparable conditions.     

PHOTOINDUCED LUMINESCENCE AND EPR SIGNALS OF POLYPHENOL AND QUINONE POLYMERS

Abstract— Aqueous basic solutions, pH 9.0 of humic acids and melanin-like, synthetic polymers, obtained with adrenochrome, hydroquinone and purpurogallin, were illuminated with visible light under N₂ or O₂ atmospheres. It has been found that light enhances a singlet electron-paramagnetic-resonance (EPR) signal of polymers both under N₂ and O₂, and induces ultra-weak luminescence in the presence of O₂. Degradative oxidation of polymers, accelerated by light, leads to a decrease of EPR signal intensity and generates weak chemiluminescence.     

INFLUENCE OF pH AND OXYGEN ON 315 mμ INACTIVATION AND THYMINE DIMERIZATION IN MUTANTS OF BACTERIOPHAGE T4

Abstract— 1. Irradiation with 315 mμ light inactivates phage T4v-x C, and T4v^-x- , and forms thymine dimers in their DNA.
2. Both the rates of inactivation and of thymine dimerization depend upon pH and gaseous environment during irradiation. The U.V. sensitivities are: 1 (pH 7, N₂, 03, 2.2 (pH 3.5, Oz), 3.3 (pH 3–5, N₂; and the corresponding rates of thymine dimerization 1: 2.5: 5.2. The number of thymine dimers per lethal hit observed withT4v-x + are: 5.7 (pH 7, N₂, O₂, 5.4 (pH 3.5, O₂, 10.9 (pH 3.5, N₂); and forT4v-x-: 4.6, 3.4, and 7.1 with the same sequence of conditions.
3. Also the photoreactivable sectors depend upon the environmental conditions at 315 mp inactivation. In T4v-x f this sector amounts to about 50 per cent at pH 7, 18 per cent at pH 3.5, O., and 29 per cent at pH 3.5, N, respectively.
4. The molecular basis of these findings is discussed. It is concluded that, besides thymine dimer, at least one other lethal photoproduct (probably a photoproduct of cytosine) is involved in photoreactivation.     

THE PHOTOCHEMISTRY OF ADENOSINE: INTERMEDIATES CONTRIBUTING TO ITS PHOTODEGRADATION MECHANISM IN AQUEOUS SOLUTION AT 298 K AND CHARACTERIZATION OF THE MAJOR PRODUCT

Abstract— The steady-state (254 nm) photolysis of 9–(β-d-erythropentofuranosyl)adenine (adenosine) in aqueous solution was studied. Photodestruction yields on the order of 1.3 × 10⁻³⁾ were determined at room temperature by measuring the initial decrease in the absorption maximum as a function of irradiation time. The use of high performance liquid chromatography (HPLC) permitted a more exact determination of the yield (2.5 × 10³). The formation of photoproducts was also studied using HPLC. In the photolysis of 50 μ M aqueous solutions of adenosine under anaerobic conditions at least 11 stable photoproducts are formed that absorb at 260 nm, the wavelength of maximum absorption of adenosine. The major photoproduct was also isolated and characterized as adenine; its formation yield was determined to be 4.5 × 10⁴. This yield is affected by the presence of oxygen and by the initial concentration of adenosine employed. Fluorescence emission and excitation spectra were used to monitor the formation of highly fluorescent photoproducts that emit with maxima at 365, 398 , and 430 nm and absorb in the wavelength region of 240–380 nm.
The reactive species in the photodestruction mechanism were established using substrates that react selectively with the respective short-lived species. Photoionization is a primary photoprocess implied by these studies. The triplet state of adenosine also contributes to the photodestruction mechanism.     

RESONANCE FLUORESCENCE IN CHLOROPHYLL a SOLUTIONS

Abstract— Chlorophyll a in aqueous hexane solutions has two fluorescence peaks, two fluorescence excitation peaks in the blue, and two fluorescence excitation peaks in the red. One of these set of peaks (excitation 2.34_μ^-1 and 1.52_μ-1, fluorescence 1–50_μ^-1) is identical with those found in methanol solutions of chlorophyll a and is assigned to an unaggregated species, while the other set of peaks (excitation 2.44_μ-1 and 1.475_μ^-l, fluorescence 1.48_μ^-l) is assigned to a species aggregated in the presence of water. The spectral shifts occurring in the aggregate follow the selection rules obtained by McRae and Kasha for a linear stack of parallel dye molecules. The sharply increased overlap of absorption and fluorescence leads to resonance fluorescence in the aggregate. The resonance fluorescence is demonstrated experimentally.     

INTERFACIAL ELECTRON TRANSFER INVOLVING RADICAL IONS OF CAROTENE AND DIPHENYLHEXATRIENE IN MICELLES AND VESICLES

Abstract— The radical cations and anions of diphenylhexatriene have been produced and characterized in homogenous and micellar solutions by pulse radiolysis and laser flash photolysis techniques. Both types of radical ions were formed in cyclohexane on pulse radiolysis. The radical cation was formed in dichloroethane on pulse radiolysis, and by two photon photoionization in ethanol, dichloroethane, and various micelles. Both radical ions have intense ( 10⁵ M ^-1 cm^-1) absorption peaks at600–650nm. The cation peak occurs at slightly shorter wavelengths than that of the anion.
In micelles and vesicles the radical anion of carotene was formed by electron transfer from e_a– on pulse radiolysis. The radical cation was formed on pulse radiolysis of micellar solutions containing Br^-₂ as counterion, presumably by electron transfer to Br₂^-. The spectra agree with those of the radical cation and anion of carotene that have previously been obtained in homogenous solutions (Dawe and Land, 1975).
Electron transfer in micelles and vesicles from the radical anion of biphenyl to carotene and diphenylhexatriene, and from the radical anions of these to inorganic acceptors has been studied.     

SOLVENT EFFECTS ON THE ELECTRONIC ABSORPTION AND FLUORESCENCE SPECTRA OF INDOLE-4-CARBOXYLIC ACID: PROTOTROPIC EQUILIBRIA IN AQUEOUS SOLUTIONS

Abstract— The absorption and fluorescence spectra of indole-4-carboxylic acid in various solvents have indicated that the -COOH group is more planar with respect to the indole ring in the first excited singlet state (S₁) than in the ground (S₀) state. Relatively large Stokes' shifts indicate that polarisability and dipole moment of the molecule are increased predominantly upon excitation. Prototropic reactions in the S₀ and S₁ states are the same. The -COO^- and -COOH+₂ groups are not coplanar in the S₀, but coplanar in the S₁ state. pH-dependent fluorescence spectra have revealed that both protonation and deprotonation of the -COOH group increase the basicity of the molecule upon excitation.     

RESONANCE RAMAN SPECTRA OF THE II-CATION RADICALS OF COPPER, COBALT, AND NICKEL METHYLOCTAETHYLCHLORINS: VIBRATIONAL CHARACTERISTICS OF CHLOROPHYLL MODELS

Abstract— By using excitation at 363.8 nm, resonance Raman (RR) spectra were obtained for Cu(II), Co(II), and Ni(II) complexes of methyloctaethylchlorin (MeOEC) and their ir-cation radicals. Additionally, the Raman spectra of the Cu(II) derivatives of (rarcs-octaethylchlorin (r-OEC) were included for comparison. The alkyl-substituted CuMeOEC exhibits a Raman spectrum that is nearly identical to that of the simpler (-CuOEC in both the neutral and oxidized states. Unlike the latter species, the cation radical of CuMeOEC is immune to oxidative dehydrogenation to porphyrin, and this has facilitated vibrational characterization of the ring-oxidized species. This study aims to compare the vibrational characteristics in the 1450 to 1700 cm^-t region of the metallochlorin ir-cation radicals to those of the corresponding oxidized metalloporphyrins. We focus on two modes in the1500–1520 cm^-1 and1620–1650 cm^-1 region that are analogous to the v₃ and v₁₀ vibrations, respectively, in the metalloporphyrin analogs. These vibrational modes are clearly defined in all species and exhibit a strong core-size dependency in the porphyrin complexes. The core-size study as well as the frequency changes upon oxidation support the conclusion that the v₃-like vibration in the chlorin species features substantial C_bC_b in addition to C_aC_m stretching character. The v,₀-like mode of the chlorin macrocycle, on the other hand, displays characteristics that closely resembles that of the porphyrin analog; consequently, these vibrations are of predominantly C_aC_m stretching character in both cases.     

Single site electronic spectroscopy of magnesium isobacteriochlorin in n-octane matrixes at 7 K

The high resolution, single site emission and absorption spectra of magnesium isobacteriochlorin (MgiBC) in n-octane matrixes at 7 K are reported. Its emission and Q and Soret band absorption regions have been investigated. The vibrational frequencies of the ground and the lowest energy pipi* singlet excited states were determined from luminescence and excitation spectra, respectively. The emission from MgiBC at 7 K seems to be entirely fluorescence. The luminescence and Q(y) region spectra of the complex are similar, having intense, narrow origin bands followed by relatively weak, but orderly vibrational structure. The Q(x) region does not have a clear origin and exhibits complex vibrational structure that increases in intensity going to higher energy. In the Soret region the individual pipi* origins are clearly identifiable and some vibrational structure was also observed.     

KINETIC AND SPECTROSCOPIC FEATURES OF SOME CAROTENOID TRIPLET STATES: SENSITIZATION BY SINGLET OXYGEN

Abstract— Triplet-triplet absorption spectra of a series of carotenoid pigments in benzene solution have been determined by pulse radiolysis experiments. The natural lifetimes in deaerated solution have also been measured. They fall in the range 2–10 µ s as found for other carotenoids under similar conditions. Pulsed laser (337 nm) excitation of benzene solutions containing oxygen, carotenoid and a photosensitized molecule (anthracene) showed the generation of absorption spectra of the triplet states. These absorptions decayed by first order kinetics in such a way as to indicate that they were formed in reactions with singlet oxygen, itself generated by interaction with the anthracene triplet state. Bimolecular rate constants for energy transfer from O*₂ (¹△_g), to carotenoid have been evaluated.     

RESOLUTION ENHANCEMENT AND DECONVOLUTION INTO VIBRONIC BANDS OF THE FLUORESCENCE AND ABSORPTION SPECTRA IN THE RED REGION OF DILUTE SOLUTIONS OF CHLOROPHYLL a IN VARIOUS SOLVENTS

Abstract— The fluorescence spectra of chlorophyll a in less than 10–⁶ mol dm^-3 solutions of benzene, toluene, tetrahydrofurane, EPA and ethanol were determined at both room temperature and at 77 K. Resolution enhancement using Fourier transform methods revealed the presence of vibronic bands with essentially solvent-invariant separation between their origins; this was confirmed by the quantitative deconvolution of the spectra into Gaussian bands. It was concluded that the fluorescence is due to a single spectroscopically distinct species. The absorption spectra in the above solvents include a band absent from the fluorescence spectra which, from its position and intensity, may be identified as the longest wavelength X-polarized electronic transition of chlorophyll a. Differences observed between the shapes of the low-and high-temperature absorption spectra may be attributed primarily to the narrowing of bandwidths with decreasing temperature and to intensity redistribution within the bands; the apparent increase in the integrated intensity on cooling the solutions appears to be due largely to the increase in the volume concentration of the solute as a result of solvent contraction.     

SPECTRAL AND TIME DEPENDENCE STUDIES OF THE ULTRA WEAK BIOLUMINESCENCE EMITTED BY THE BACTERIUM Escherichia coli

Abstract— Weak luminescence was detected using photon counting equipment, from oxygenated, liquid cultures of Escherichia coli during two stages of its growth cycle. The first period of emission occurred during the exponential phase of growth and comprised a UV(210–330 nm) band and a visible region(450–620 nm) band, the total intensity being (1.65 ± 0.12) x 10³ counts s^-1. The second period of emission occurred during the stationary phase of growth and comprised only a visible region(450–620 nm) band of intensity (8.72 ± 0.15) x 10³ counts s^-1. When the growth temperature was raised from 306.15 to 310.15 K, the above emission intensities were approximately halved, but the spectra were not changed significantly. No luminescence was observed at either temperature when the E. coli was grown anaerobically. The visible region luminescence was attributed to excited carbonyl groups and excited singlet O₂ dimers formed during the decomposition of lipid peroxides. The UV component was tentatively assigned to oxidative side reactions accompanying the synthesis of proteins.