Stanisław Krawczyk

The effects of hydrogen bonding and coordination interaction in visible absorption and vibrational spectra of chlorophyll a

The environmental effects in electronic and vibrational spectra of chlorophyll a are investigated. The red-shifts of the absorption maximum of chlorophyll a are analyzed using an approach based on separate evaluation of nonspecific (solvatochromic) and specific effects of the solvent. They are correlated with the frequency of the keto C = O group vibration determined by resonance Raman spectroscopy. This correlation shows that hydrogen bonding may quantitative account for spectral shifts in protic solvents. Noticeable extra red-shifts related to coordination appear only in chlorophyll six-coordinated with nitrogen ligands. Resonance Raman spectra at 77 K indicate that the formation of six-coordinated chlorophyll depends in a conspicuous way on the structure of the ligands lone-pair orbitals. In nonprotic solvents, the free keto group frequency (at least 1686 cm −1 is a function of both the refractive index and dielectric permittivity of the solvent and is remarkably dependent on temperature. The comparison of these data with absorption wavelengths and with resonance Raman frequencies of the keto C = O group of chlorophyll in vivo confirms a significant contribution of hydrogen bonding and nonspecific interactions to spectral shifts in vivo.

Electrochromism of chlorophyll a monomer and special pair dimer

The quadratic Stark effect was measured for chlorophyll monomer and dimer (Chl·ethanol) 2 in isotropic glassy solutions at low temperatures. The change in the permanent dipole moment, δμ, of chlorophyll monomer on excitation in the red absorption maximum was found to be (1.02±0.09) D for both mono- and disolvate. It forms an angle δ=20° with the electronic transition moment Q y . The electronic excitation of the dimer within its main absorption band at 703 nm is accompanied by the change in dipole moment Δμ=(5.17 ± 0.2) D, for which δ=23°. A remarkable contribution from polarizability change, Δα, to Stark spectra of both monomer and dimer was found within the Q y band. In the monomer, Δα is several times higher in the region of vibrational side bands, where it is related to the second electronic transition ( Q x ). The large increase in Δμ for the dimer as compared to the monomer is ascribed to charge-transfer contribution to the dimer excited state, due to significant overlap of electronic densities in the constituent monomers.

Spectral broadening and its effect in Stark spectra of carotenoids

Abstract Electrooptical parameters of eight carotenoids with different molecular structures were determined from the absorption and electroabsorption (Stark) spectra in glassy solvents and in poly(methyl methacrylate) films at temperatures 108–115 K. The results point to a limited significance of solvent-induced dipole moments in carotenoids resulting from local electric fields generated by solvent molecules, and indicate that the features in Stark spectra related to apparent dipole moments display a correlation with the bandwidth of individual vibronic transitions in absorption spectra. Model calculations show that it is possible to describe the Stark spectra by assuming a close link between the molecular polarizability and the inhomogeneous spectral shift. It is postulated that the dipole-related features in Stark spectra and the inhomogeneous broadening have a common origin in the distortions of molecular geometry in the form of torsional deformations about the single C–C bonds.

A study of protein-carotenoid interactions in the astaxanthin-protein crustacyanin by absorption and Stark spectroscopy; evidence for the presence of three spectrally distinct species.

Molecular mechanisms underlying the peculiar spectral properties of the carotenoid astaxanthin in alpha-crustacyanin, the blue carotenoprotein isolated from the exoskeleton of the lobster Homarus gammarus, were investigated by comparing the basic electrooptical parameters of astaxanthin free in vitro with those of astaxanthin in the complex. Absorption and electroabsorption (Stark effect) spectra were obtained for alpha-crustacyanin in low-temperature glasses to provide information about the molecular interactions that lead to the large bathochromic shift of the spectra resulting from this complexation. The low-temperature spectra reveal the presence of at least three spectral forms of alpha-crustacyanin, with vibronic (0-0) transitions at 14000 cm(-1), 13500 cm(-1) and 11600 cm(-1) (corresponding to approximately 630, 660 and 780 nm, respectively, at room temperature) and with relative aboundance 85%, 10% and 5%. The longer wavelength absorbing species have not previously been detected. The changes in polarizability and in permanent dipole moments associated with the S0-->S2 electronic transition for all these forms are about 1.5 times larger than for isolated astaxanthin. The results are discussed with reference to the symmetric polarization model for astaxanthin in alpha-crustacyanin.

Solvent effects and vibrational dependence in electrochromic spectra of carotenoids

Abstract Electrochromic (Stark effect) spectra of three carotenoids, β-carotene, lutein and violaxanthin, were obtained in glassy matrices at low temperature. When analyzed in the framework of the theory of electrochromism they were found to contain a remarkable contribution from the second derivative of the absorption spectrum, equivalent to a substantial change in dipole moment (3–5 D) on electronic excitation, in addition to the usual polarizability term. These dipole moments only weakly depend on solvent polarity; this puts in doubt the induced dipole model. In the case of violaxanthin, a variability of the electro-optical parameters along the electrochromic spectrum was found, which is related to the type of vibration involved in the electronic transition. An analogous effect was also noted for tetradecaheptaene chromophore in amphotericin B. These observations strongly indicate an essential role of vibronic coupling in determining the electro-optical parameters of carotenoids.

Stark effect spectroscopy of exciton states in the dimer of acridine orange

Abstract Low temperature absorption and Stark effect (electroabsorption) spectra of the cationic dye acridine orange (AO) were recorded in the wide spectral range comprising the lowest energy electronic transition at 20,150 cm −1 (496 nm) and the intense UV band at 36,800 cm −1 (272 nm). By changing the water, glycerol and ethanol contents in the solvent, different proportions of monomers, dimers and higher oligomers can be obtained in frozen glasses, enabling the decomposition of spectra and determination of dipole moment and polarizability changes for electronic transitions in AO monomer and dimer. These data are analysed on grounds of exciton theory for a dimer in an electric field. While the split UV transition fits the simplified theory well, the lower- and higher-energy dimer transitions in the VIS region exhibit significantly increased polarizability changes of opposite sign. Comparison with literature data for similar molecular systems points to orbital overlap as a source of the effects observed.

Stark effect in P700, the primary electron donor of Photosystem I

Quadratic Stark effect in CP1 pigment‐protein complex was examined at low temperatures in the red spectral region. The Stark spectra of samples containing P700 in reduced form exhibit a strong negative band at 704 nm, which disappears on chemical oxidation of P700. The change in permanent dipole moment, Δμ, of P700 on electronic excitation estimated from these spectra was found to be between 4.7 and 7.7 Debye units. It is suggested to reflect the charge‐transfer contribution to the excited state of P700. For antenna chlorophyll, Δμ≌ 1 D was obtained in accordance with the data for monomeric chlorophyll.

Spectral analysis of pigment photobleaching in photosynthetic antenna complex LHCIIb

Light-induced photooxidation of chlorophyll (Chl) a, b and xanthophylls was investigated in LHCIIb, the antenna pigment-protein complex of photosystem II. Absorption difference spectra at normal and low temperatures show initially (at less than 25% Chl a decay) a selective bleaching of a red-shifted Chl b with absorption bands at 487 and 655 nm, Chl b (460/650 nm) and Chl a (433/670 nm), which changes to a less selective photooxidation pattern at deeper bleaching stages. Difference absorption spectra and HPLC analyses indicate different photooxidation rates of pigments in the order neoxanthin>Chl a>lutein approximately Chl b. Despite significant pigment loss as monitored with absorption spectra, CD spectra indicate an essentially complete persistence of the protein secondary structure. Fluorescence excitation spectra suggest the conversion of a small fraction of Chl a into pheophytin a which acts as a fluorescence quencher, possibly through temporary charge separation process. The strong features in the electroabsorption (Stark effect) spectra due to chlorophyll b at 655 nm and a xanthophyll at 510 nm, and the spectral changes mentioned above are assigned to Chl molecules located at several binding sites in LHCIIb protein and are discussed in the context of spatial configuration and interactions of pigment molecules.

Absorption and electroabsorption spectra of carotenoid cation radical and dication

Radical cations and dications of two carotenoids astaxanthin and canthaxanthin were prepared by oxidation with FeCl3 in fluorinated alcohols at room temperature. Absorption and electroabsorption (Stark effect) spectra were recorded for astaxanthin cations in mixed frozen matrices at temperatures about 160 K. The D0→D2 transition in cation radical is at 835 nm. The electroabsorption spectrum for the D0→D2 transition exhibits a negative change of molecular polarizability, Δα=−1.2·10−38 C·m2/V (−105 A3), which seems to originate from the change in bond order alternation in the ground state rather than from the electric field-induced interaction of D1 and D2 excited states. Absorption spectrum of astaxanthin dication is located at 715–717 nm, between those of D0→D2 in cation radical and S0→S2 in neutral carotenoid. Its shape reflects a short vibronic progression and strong inhomogeneous broadening. The polarizability change on electronic excitation, Δα=2.89·10−38 C·m2/V (260 A3), is five times smaller than in neutral astaxanthin. This value reflects the larger energetic distance from the lowest excited state to the higher excited states than in the neutral molecule.

Stark signals associated with the reduced and oxidized states of P700 in P700-enriched particles

Abstract Quadratic Stark effect was measured in the red and near infrared spectral regions for Photosystem I particles devoid of most antenna pigments, with the chlorophyll/P700 ratio of 11. Stark spectra of reduced primary electron donor exhibit a strong band at 701 nm, corresponding to a change in permanent dipole moment Δμ = 5–5.3 D. When the primary donor is oxidized, two other bands appear at 807 nm and at 687 nm with Δμ equal to 5.15 D and ≅ 1.75 D, respectively. They are assigned to electronic transitions in chlorophyll cation and in neutral Chl molecule constituting the oxidized primary electron donor. In addition, two other features at ≅ 688 nm and around 670 nm were revealed, which are sensitive to the oxidation state of the primary donor. They are tentatively attributed to the primary electron acceptor A0 and to an accessory chlorophyll a in the reaction center of Photosystem I. Exciton coupling and induced dipole moments are indicated as the sources of the effects observed.