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  • 1.
    Bondesson, Laban
    et al.
    KTH, School of Biotechnology (BIO), Theoretical Chemistry.
    Mikkelsen, Kurt V.
    Department of Chemistry, University of Copenhagen.
    Luo, Yi
    KTH, School of Biotechnology (BIO), Theoretical Chemistry.
    Garberg, Per
    Biovitrum AB, Stockholm.
    Ågren, Hans
    KTH, School of Biotechnology (BIO), Theoretical Chemistry.
    Hydrogen bonding effects on infrared and Raman spectra of drug molecules2007In: Spectrochimica Acta Part A - Molecular and Biomolecular Spectroscopy, ISSN 1386-1425, E-ISSN 1873-3557, Vol. 66, no 2, p. 213-224Article in journal (Refereed)
    Abstract [en]

    Infrared and Raman spectra of three drug molecules, aspirin, caffeine and ibuprofen, in gas phase and in aqueous solution have been simulated using hybrid density functional theory. The long range solvent effect is modelled by the polarizable continuum model, while the short range hydrogen bonding effects are taken care of by the super-molecular approach with explicit inclusion of water molecules. The calculated spectra are found to compare well with available experimental results. The agreement obtained make grounds for proposing theoretical modeling as a tool for characterizing changes in the bonding environments of drug molecules in terms of particular variations in their IR and Raman spectra.

  • 2. Hammarstrom, L.
    et al.
    Sun, Licheng C.
    Akermark, B.
    Styring, S.
    A biomimetic approach to artificial photosynthesis: Ru(II)-polypyridine photo-sensitisers linked to tyrosine and manganese electron donors2001In: Spectrochimica Acta Part A - Molecular and Biomolecular Spectroscopy, ISSN 1386-1425, E-ISSN 1873-3557, Vol. 57, no 11, p. 2145-2160Article, review/survey (Refereed)
    Abstract [en]

    The paper describes recent advances towards the construction of functional rr mics of the oxygen evolving complex in photosystem II (PSII) that are coupled to photoinduced charge separation. Some key principles of PSII and artificial systems for light-induced charge accumulation are discussed. Systems are described where biomimetic electron donors - manganese complexes and tyrosine - have been linked to a Ru(II)-polypyridine photosensitiser. Oxidation of the donors by intramolecular electron transfer from the photo-oxidised Ru(III) complex has been studied using optical flash photolysis and EPR experiments. A step-wise electron transfer Mn-2(III,III) --> tyrosine --> Ru(III) has been demonstrated, in analogy to the reaction on the donor side of PSII Electron transfer from the tyrosine to Ru(III) was coupled to tyrosine deprotonation. This resulted in a large reorganisation energy and thus a slow reaction rate, unless the tyrosine was hydrogen bonded or already deprotonated. A comparison with analogous reactions in PSH is made. Finally, light-induced oxidation of a manganese dimer linked to a Ru(II)-photosensitiser has been observed. Preliminary results suggest the possibility of photo-oxidising manganese dimers in several steps, which is an important advancement towards water oxidation.

  • 3. Huang, P.
    et al.
    Anderlund, M.
    Weihe, H.
    Barra, A. L.
    Sun, Licheng C.
    Sign of excited spin state magnetic anisotropy parameters2006In: Spectrochimica Acta Part A - Molecular and Biomolecular Spectroscopy, ISSN 1386-1425, E-ISSN 1873-3557, Vol. 63, no 3, p. 541-543Article in journal (Refereed)
    Abstract [en]

    By using high frequency high field EPR spectroscopy we demonstrate how to extract the sign of magnetic anisotropy parameters pertinent to excited spin multiplets of antiferromagnetically coupled clusters. The method is demonstrated on a manganese(II) dimer.

  • 4.
    Karaush, N. N.
    et al.
    Bohdan Khmelnytsky Natl Univ, Dept Chem & Nanomat Sci, UA-18031 Cherkassy, Ukraine..
    Minaeva, V. A.
    Bohdan Khmelnytsky Natl Univ, Dept Chem & Nanomat Sci, UA-18031 Cherkassy, Ukraine..
    Baryshnikov, Gleb V.
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Theoretical Chemistry and Biology. Bohdan Khmelnytsky Natl Univ, Dept Chem & Nanomat Sci, UA-18031 Cherkassy, Ukraine.
    Minaev, Boris F.
    Bohdan Khmelnytsky Natl Univ, Dept Chem & Nanomat Sci, UA-18031 Cherkassy, Ukraine.
    Ågren, Hans
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Theoretical Chemistry and Biology. Uppsala Univ, Dept Phys & Astron, Box 516, SE-75120 Uppsala, Sweden..
    Identification of tautomeric intermediates of a novel thiazolylazonaphthol dye - A density functional theory study2018In: Spectrochimica Acta Part A - Molecular and Biomolecular Spectroscopy, ISSN 1386-1425, E-ISSN 1873-3557, Vol. 203, p. 324-332Article in journal (Refereed)
    Abstract [en]

    The recently synthesized thiazolylazo dye, 1-(5-benzy1-1,3-thiazol-2-yl)diazenyllnaphthalene-2-ol called shortly BnTAN, is studied by density functional theory (DFT) in three tautomeric forms in order to explain the available H-1 NMR, UV-Vis and FTIR spectra. An experimentally observed IR band at 1678 cm(-1), assigned to the C=O bond stretching vibration, supports the notion that BnTAN retains in the less stable keto-form even in the solid state due to an ultrafast single-coordinate intramolecular proton transfer. This finding is also in a good agreement with an X-ray crystallography analysis which indicates an intermediate position of the proton between the OH and -N=N-groups. Calculations also show that some experimentally observed H-I NMR signals could be considered as being averaged values between theoretically calculated chemical shifts for the corresponding protons in the keto- and enol-tautomers. At the same time the UV-Vis spectra are almost insensitive to the tautomerization processes as the main single band absorption at 500 nm is present in all tautomers according to our TD DFT simulations. The minor differences in spectral features of the long-wavelength visible region are also noted and discussed with respect to the manifestation of the less stable tautomer form.

  • 5.
    Kloo, Lars A.
    et al.
    KTH, Superseded Departments, Chemistry.
    Taylor, M. J.
    Spectroscopic characterisation of indium(III) chloride and mixed ligand complexes2002In: Spectrochimica Acta Part A - Molecular and Biomolecular Spectroscopy, ISSN 1386-1425, E-ISSN 1873-3557, Vol. 58, no 5, p. 953-957Article in journal (Refereed)
    Abstract [en]

    Acidic solutions of InCl3 plus InBr3 contain mixed halide complexes and solvent extraction yields the four-coordinate anions InClnBr4-n- whose v(In-Cl) and v(In-Br) modes are reported. Two solid products of InCl3 with 15-crown-5 have been identified; crystallisation from aqueous HCl gave a molecular adduct containing InCl3(H2O)(2), whereas the ionic complex [InCl2(15-crown-5)][InCl4] was produced under anhydrous conditions. The vibrational spectra are assigned.

  • 6.
    Li, Junfeng
    et al.
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Theoretical Chemistry and Biology.
    Luo, Yi
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Theoretical Chemistry and Biology.
    Zhang, Jinglai
    Henan Univ, Coll Chem & Chem Engn, Kaifeng 475004, Henan, Peoples R China..
    A theoretical study on vibronic spectra and photo conversation process of protonated naphthalenes2018In: Spectrochimica Acta Part A - Molecular and Biomolecular Spectroscopy, ISSN 1386-1425, E-ISSN 1873-3557, Vol. 205, p. 520-527Article in journal (Refereed)
    Abstract [en]

    The equilibrium structures and vibrational frequencies of the ground state and several singlet low-lying excited states of alpha-and beta-protonated naphthalenes (alpha-and beta-HN+) have been studied by time -dependent density -functional theory (TD-DFT). Within the Franck -Condon approximation, vibronic absorption spectra of alpha-HN+ and beta-HN+, together with the vibronic emission spectrum of alpha-HN+, have been calculated. The obtained good agreement between the theoretical and experimental spectra enables to correctly assign vibronic features in both absorption and emission spectra. Moreover, the non -radiative deactivation pathway from the low-lying excite states to the ground state in alpha-HN+ and beta-HN+, as well as the photo-induce proton transfer pathway, are investigated at the CASPT2/CASSCF/6-31G* level. Our study is helpful for understanding the photochemical behavior of these important polycyclic aromatic hydrocarbon molecules.

  • 7. Minaev, B.
    et al.
    Loboda, O.
    Vahtras, Olav
    KTH, Superseded Departments, Biotechnology.
    Ågren, Hans
    KTH, Superseded Departments, Biotechnology.
    Bilan, E.
    Physical properties and spectra of IO, IO- and HOI studied by ab initio methods2002In: Spectrochimica Acta Part A - Molecular and Biomolecular Spectroscopy, ISSN 1386-1425, E-ISSN 1873-3557, Vol. 58, no 5, p. 1039-1053Article in journal (Refereed)
    Abstract [en]

    Structure and properties of the IO, IO- and HOI species, which are of potential importance for the ozone destruction catalytic cycle in the troposphere, have been calculated together with the EPR, NMR and UV-visible spectra by ab initio methodology with account of spin-orbit coupling (SOC) effects. Multi-configuration self-consistent field calculations with linear and quadratic response techniques and the multi-reference configuration interaction method have been employed. Photodissociation of these species, crucial for the catalytic ozone-destruction cycle, is critically reviewed and analyzed. Calculations predict that the singlet-triplet (S-T) transition to the lowest triplet state (X(1)A' --> (3)A) should be responsible for the weak long-wavelength tail absorption ( similar to450-560 nm) and photodissociation of the HOI molecule. The second, more intense, band around 400 nm is produced by two overlapping S-S and S-T transitions. In order to check this assignement of the HOI photodissociation the isoelectronic IO- anion and IO radical have been studied by the same methods. Comparison with the EPR spectrum of the 10 radical indicates that the methods are reliable which gives credit to the accuracy of the HOI spectral interpretation. NMR spectra of HOI and IO- molecules,and some other properties are calculated for the first time.

  • 8.
    Minaev, Boris
    KTH, School of Biotechnology (BIO), Theoretical Chemistry (closed 20110512).
    Ab initio study of low-lying triplet states of the lithium dimer2005In: Spectrochimica Acta Part A - Molecular and Biomolecular Spectroscopy, ISSN 1386-1425, E-ISSN 1873-3557, Vol. 62, no 4-5, p. 790-799Article in journal (Refereed)
    Abstract [en]

    Observation of Bose-Einstein condensation in Li-7(2) initiated the interest in the scattering length of two ground state lithium atoms when they approach each other as a radical pair triplet a(3)Sigma(u)(+) state. But some properties of this state are still unknown. In present work, a number of low-lying triplet states of lithium molecule are calculated by multi-configuration self-consistent field (MCSCF) and response techniques with account of spin-orbit coupling, spin-spin coupling and some other magnetic perturbations. The singlet-triplet transition probabilities to the ground state are also presented. Most results are connected with the weakly bound lowest triplet a(3)Sigma(u)(+) state, whose radiative lifetime and spin-splitting are unknown so far in spite of its great importance in Bose-Einstein condensation. Calculations indicate that this state has a very small spin-splitting, lambda(ss) = -0.01 cm(-1), which is negligible in comparison with the line-width in experimental Fourier transform C spectra published so far. Similar splitting is obtained for the upper state of the 1(3)Sigma(g)(+)-a(3)Sigma(u)(+) transition. This is in agreement with experimental rovibronic analysis of the 1(3)Sigma(g)(+)- a(3)Sigma(u)(+) band system in which the triplet structure was not resolved. The radiative lifetime of the a(3)Sigma(u)(+) state is predicted to exceed 10 h.

  • 9.
    Minaev, Boris
    KTH, Superseded Departments, Biotechnology.
    Theoretical study of the external heavy atom effect on phosphorescence of free-base porphin molecule2004In: Spectrochimica Acta Part A - Molecular and Biomolecular Spectroscopy, ISSN 1386-1425, E-ISSN 1873-3557, Vol. 60, no 13, p. 3213-3224Article in journal (Refereed)
    Abstract [en]

    The radiative lifetime of phosphorescence of free-base porphin (H2P) molecule and its complexes with noble-gas atoms are calculated by time-dependent density functions theory (TD DFT) with quadratic response functions for account of spin-orbit coupling and electric dipole activity. The complexes with Ne, Ar, Kr, and Xe are used to simulate the external heavy atom (EHA) effect on phosphorescence of the H2P molecule in the corresponding noble gas matrices. The 133LYP functional and small basis set (3-21G) are used throughout the study and comparison of all complexes but other basis sets are also utilized to support the chosen approach. A slow radiative rate constant of free-base porphin phosphorescence (about 10(-3) S-1) is obtained with all basis sets being in the order of magnitude agreement with experimental estimations. A strong enhancement of the H2P phosphorescence rate (by 360 times) is calculated for Xe complex; while for Ne, Ar, and Kr complexes, the enhancement is equal to 1.1, 1.3, and 10.3 times, respectively. In these complexes, the noble gas atom is disposed at 3.6 Angstrom above the center of the porphin ring. In spite of shortcomings of the chosen simple model, the TD DFT calculations explain the most important features of the EHA effect on phosphorescence of free-base porphin. Calculations of the hypertine coupling tensors for all magnetic nuclei in the lowest triplet state of H2P molecule and its complexes with noble-gas atoms indicate an appreciable penetration of the spin density to the EHA region. This can be connected with the enhancement of spin-orbit coupling in the H2P molecule.

  • 10.
    Minaev, Boris F.
    KTH, Superseded Departments, Biotechnology.
    Ab initio study of the ground state properties of molecular oxygen2004In: Spectrochimica Acta Part A - Molecular and Biomolecular Spectroscopy, ISSN 1386-1425, E-ISSN 1873-3557, Vol. 60, no 5, p. 1027-1041Article in journal (Refereed)
    Abstract [en]

    The electric and magnetic properties of the ground state of oxygen molecule are calculated by multiconfiguration self-consisted field (MCSCF) method and compared with experimental data: the quadrupole moment, polarizability, the O-17 nuclear quadrupole coupling constant, magnetizability tensor, nuclear spin-rotation Coupling constant and rotational g factor. The last two constants are calculated for all possible isotope modifications. The rotational, ESR and NMR spectra are discussed. Fermi-contact hyperfine coupling parameter is additionally estimated by different methods. The NMR chemical shielding tensor for (OO)-O-17-O-16 species at high temperature limit (without electron spin contribution) is predicted. Potential energy curves for 10 excited bound states and the internuclear distance dependence of the studied properties are also presented.

  • 11.
    Minaev, Boris
    et al.
    KTH, School of Biotechnology (BIO), Theoretical Chemistry.
    Wang, Yanhua
    KTH, School of Biotechnology (BIO), Theoretical Chemistry.
    Wang, Chuankui
    KTH, School of Biotechnology (BIO), Theoretical Chemistry.
    Luo, Yi
    KTH, School of Biotechnology (BIO), Theoretical Chemistry.
    Ågren, Hans
    KTH, School of Biotechnology (BIO), Theoretical Chemistry.
    Density functional theory study of vibronic structure of the first absorption Qx band in free-base porphin2006In: Spectrochimica Acta Part A - Molecular and Biomolecular Spectroscopy, ISSN 1386-1425, E-ISSN 1873-3557, Vol. 65, no 2, p. 308-323Article in journal (Refereed)
    Abstract [en]

    Harmonic vibrational frequencies and vibronic intensities in the first S-0 -> S-1 (pi pi*) absorption band of free-base porphin (H2P) are investigated by hybrid density functional theory (DFT) with the standard B3LYP functional. The S-0-S-1 transition probability is calculated using time-dependent DFT with account of Franck-Condon (FC) and Herzberg-Teller (HT) contributions to the electric-dipole transition moments including displacements along all 108 vibrational modes. Two weak wide bands observed in the gas phase absorption spectra of the H2P molecule at 626 and 576 nm are interpreted as the 0-0 band of the X(1)A(g)-> B-3u transition and the 0-1 band with largest contributions from the nu(10)(a(g)) = 1610 cm(-1) and nu(19)(b(1g)) = 1600 cm(-1) modes, respectively, in agreement with previous tentative assignments. Both bands are induced by the HT mechanism, while the FC contributions are negligible. A number of fine structure bands, including combination of two vibrational quanta, are obtained and compared with available spectra from supersonic jet and Shpolskij matrices. Both absorption and fluorescence spectra are interpreted on ground of the linear coupling model and a good fulfillment of the mirror-symmetry rule.

  • 12.
    Minaev, Boris
    et al.
    KTH, School of Biotechnology (BIO), Theoretical Chemistry (closed 20110512).
    Yaschuk, L.
    Kukueva, V.
    Calculation of the fine structure and intensity of the singlet-triplet transitions in the imidogen radical2005In: Spectrochimica Acta Part A - Molecular and Biomolecular Spectroscopy, ISSN 1386-1425, E-ISSN 1873-3557, Vol. 61, no 6, p. 1105-1112Article in journal (Refereed)
    Abstract [en]

    The singlet-triplet transition moments are calculated for the NH radical by multiconfiguration self-consistent field (MCSCF) method with a quadratic response (QR) technique. The band systems in the visible region (b(l) Sigma(+)-> X-3 Sigma(-) and a(1) Delta -> X-3 Sigma(-)) of the NH radical are analyzed in comparison with previous ab initio treatments and with the recent experimental data in attempt to solve some discrepancies. The b(1)Sigma(+)-> X-3 Sigma(-)(Omega) transition moments ratio for the two spin sublevels Omega = 1 and Omega = 0 of the ground state is well reproduced and the radiative lifetime of the b(1) Sigma(+) state (tau(b) = 58 ms) is obtained in a good agreement with the experimental value tau(b)= 53((+1 7)(-13)) 3[1 3 ms. The A(3) Pi X-3 Sigma-transition some improvement is achieved in comparison with the previous ab initio results, but the calculated radiative lifetime (tau(a) = 3.9 s) is still much lower than the recent measurement provides (tau(a)= 12.5 s). The zero field splitting and spin-rotation coupling constants are calculated for the ground state by different methods and advantage of the density functional theory is stressed.

  • 13. Sajan, D.
    et al.
    Fischer, Andreas
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Inorganic Chemistry.
    Hubert Joe, I.
    Jayakumar, V. S.
    Surface-enhanced Raman scattering and the adsorption behavior of (RS)-phenylsuccinic acid2006In: Spectrochimica Acta Part A - Molecular and Biomolecular Spectroscopy, ISSN 1386-1425, E-ISSN 1873-3557, Vol. 64, no 3, p. 580-585Article in journal (Refereed)
    Abstract [en]

    The surface geometry of (RS)-phenylsuccinic acid molecule was studied by analysis of the SERS spectra of aromatic dicarboxylic acid adsorbed on silver colloid surfaces. For a reliable analysis of the SERS spectrum, we also performed density functional theoretical calculations. The SERS spectral features indicated that the RSPSA molecules should bound to the silver as dicarboxylate, with a strongly tilted orientation with respect to the normal to the surface. Such a tilted orientation was presumed to occur by the simultaneous sigma and pi-type coordination of carboxylate groups to silver surface caused by the steric hindrance and electrostatic repulsion between the two carboxylate groups, and thereby RSPSA on silver was easily displaced with aromatic carboxylic acids. A sigma-type coordination therefore seemed to be more important than a pi-type coordination for aromatic carboxylic acid derivatives to assemble on a silver surface. The large enhancement of in-plane bending, out of plane bending and ring breathing modes in the surface-enhanced Raman scattering spectrum indicates that the molecule is adsorbed on the silver surface in a 'atleast vertical' configuration, with the ring perpendicular to the silver surface.

  • 14. Sarveswari, S.
    et al.
    Srikanth, A.
    Murugan, N. Arul
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Vijayakumar, V.
    Jasinski, Jerry P.
    Beauchesne, Hanna C.
    Jarvis, Ethan E.
    Synthesis, characterization of (3E)-1-(6-chloro-2-methyl-4-phenyl quinolin-3-Yl)-3-aryl prop-2-en-1-ones through IR, NMR, single crystal X-ray diffraction and insights into their electronic structure using DFT calculations2015In: Spectrochimica Acta Part A - Molecular and Biomolecular Spectroscopy, ISSN 1386-1425, E-ISSN 1873-3557, Vol. 136, p. 1010-1017Article in journal (Refereed)
    Abstract [en]

    3E-1-(6-Chloro-2-methyl-4-phenylquinolin-3-yl)-3-arylprop-2-en-1-ones were synthesized and characterized by FTIR, H-1 NMR, C-13 NMR, HSQC, DEPT-135. In addition the compound 3E-1-(6-chloro-2-methyl-4-phenylquinolin-3-yl)-3-(2,5-dimethoxyphenyl)prop-2-en-1-one was subjected to the single crystal X-ray diffraction studies. Density functional theory calculations were carried out for this chalcone and its derivatives to investigate into their electronic structure, chemical reactivity, linear and non-linear optical properties. The structure predicted from DFT for chalcone is in good agreement with the structure from XRD measurement. (C) 2014 Elsevier B.V. All rights reserved.

  • 15. Segerie, Audrey
    et al.
    Lin, Li-Li
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Liegeois, Vincent
    Luo, Yi
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Champagne, Benoit
    Effects of the basis set and of the exchange-correlation functional on the Inelastic Electron Tunneling signatures of 1,4-benzenedithiol2014In: Spectrochimica Acta Part A - Molecular and Biomolecular Spectroscopy, ISSN 1386-1425, E-ISSN 1873-3557, Vol. 119, p. 34-41Article in journal (Refereed)
    Abstract [en]

    The effects of the atomic basis set and of the exchange-correlation (XC). functional on the Inelastic Electron Tunneling (LET) spectra have been investigated by considering the prototypical 1,4-benzenedithiol molecule. These studies have been completed by tackling the reliability of the same methods for predicting the IR absorption spectrum of the same molecule. The main conclusions are (i) the B3LYP XC functional is suitable to predict the relative vibrational frequencies, (ii) provided a scaling factor is used, the root mean square error on the vibrational frequencies goes down to 18 cm(-1), (iii) triple-zeta basis sets and in particular the cc-pVTZ basis set is a good compromise between accuracy and computational needs, (iv) basis set effects on the IET intensities are larger than those of the XC functional, and (v) the cc-pVTZ, cc-pVQZ, and aug-cc-pVDZ basis sets provide consistent IET intensities.

  • 16.
    Song, Ce
    et al.
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology. Yanshan University, China.
    Li, L.
    Duan, Sai
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Luo, Yi
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology. University of Science and Technology of China, China.
    Tian, G.
    Theoretical simulations for vibrationally-resolved absorption spectra of naphthalenediimide cyclophane derivatives2017In: Spectrochimica Acta Part A - Molecular and Biomolecular Spectroscopy, ISSN 1386-1425, E-ISSN 1873-3557, Vol. 183, p. 339-347Article in journal (Refereed)
    Abstract [en]

    In the present work we systematically investigated the vibrationally-resolved absorption spectra of three core substituted naphthalenediimide cyclophane derivatives. It has been performed by time-dependent density functional theory calculations using three different exchange-correlation functionals, including the conventional B3LYP functional and two long-range corrected functionals: CAM-B3LYP and ωB97XD. The solvent effects were also considered with the polarizable continuum model. Calculation results showed that long range corrections are needed to correctly describe the optical properties of the three molecules because of the strong charge transfer characteristic of the excited states. The core substitution induced red shift to the first absorption band is nicely explained by the theoretical calculations. It is found that this band mainly involves the transitions within the core substituted naphthalenediimide chromophore. The high energy absorption band, on the other hand, is generated mainly from the un-substituted chromophore. These characters result in different substitution dependence for those two main absorption bands. Furthermore, the simulated vibrational profiles of the first two absorption bands nicely reproduce the observed vibrational features in the measured spectra. The accuracy of the calculated results from different functionals and basis sets has been discussed.

  • 17. van den Berg, P. A. W.
    et al.
    Widengren, Jerker
    Hink, M. A.
    Rigler, R.
    Visser, Ajwg
    Fluorescence correlation spectroscopy of flavins and flavoenzymes: photochemical and photophysical aspects2001In: Spectrochimica Acta Part A - Molecular and Biomolecular Spectroscopy, ISSN 1386-1425, E-ISSN 1873-3557, Vol. 57, no 11, p. 2135-2144Article in journal (Refereed)
    Abstract [en]

    Fluorescence Correlation Spectroscopy (FCS) was used to investigate the excited-state properties of flavins and flavoproteins in solution at the single molecule level. Flavin mononucleotide (FMN), flavin adenine dinucleotide (FAD) and lipoamide dehydrogenase served as model systems in which the flavin cofactor is either free in solution (FMN, FAD) or enclosed in a protein environment as prosthetic group (lipoamide dehydrogenase). Parameters such as excitation light intensity, detection time and chromophore concentration were varied in order to optimize the autocorrelation traces. Only in experiments with very low light intensity ( < 10 kW/cm(2)), FMN and FAD displayed fluorescence properties equivalent to those found with conventional fluorescence detection methods. Due to the high triplet quantum yield of FMN, the system very soon starts to build up a population of non-fluorescent molecules, which is reflected in an apparent particle number far too low for the concentration used. Intramolecular photoreduction and subsequent photobleaching may well explain these observations. The effect of photoreduction was clearly shown by titration of FMN with ascorbic acid. While titration of FMN with the quenching agent potassium iodide at higher concentrations ( > 50 mM of I-) resulted in quenched flavin fluorescence as expected, low concentrations of potassium iodide led to a net enhancement of the de-excitation rate from the triplet state., thereby improving the fluorescence signal. FCS experiments on FAD exhibited an improved photostability of FAD as compared to FMN: As a result of stacking of the adenine and flavin moieties, FAD has a considerably lower triplet quantum yield. Correlation curves of lipoamide dehydrogenase yielded correct values for the diffusion time and number of molecules at low excitation intensities. However, experiments at higher light intensities revealed a process which can be explained by photophysical relaxation or photochemical destruction of the enzyme. As the time constant of the process induced at higher light intensities resembles the diffusion time constant of free flavin, photodestruction with the concomitant release of the cofactor offers a reasonable explanation.

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