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An Ultrafast Spectroscopic and Quantum-Chemical Study of the Photochemistry of Bilirubin

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An Ultrafast Spectroscopic and Quantum-Chemical Study of the Photochemistry of Bilirubin An Ultrafast Spectroscopic and Quantum-Chemical Study of the Photochemistry of Bilirubin Initial Processes in the Phototherapy for Neonatal Jaundice Burkhard Zietz Department of Chemistry Biophysical Chemistry Umeå University Sweden, 2006 The front cover shows a newborn baby undergoing phototherapy for neonatal jaundice at Umeå's university hospital, overlaid with two molecular structures: the calculated HOMO (highest occupied molecular orbital) superimposed on the bilirubin molecule and the ge- ometry of a dipyrrinone model system, in the geometry of the conical intersection between the ground and first excited electronic states. Also shown is a plot of symbolic potential energy surfaces for the ground and excited electronic states. The picture of the Aurora Borealis on the back was taken just outside Umeå, and the light is caused by emission from nitrogen and oxygen molecules, as well as oxygen atoms, after having been excited by energetic particles (electrons and protons) from the sun. © 2006 Burkhard Zietz ISBN 91-7264-010-3 Printed by Solfjädern Offset, Umeå, Sweden TO SEE A WORLD IN A GRAIN OF SAND AND A HEAVEN IN A WILD FLOWER HOLD INFINITY IN THE PALM OF YOUR HAND AND ETERNITY IN AN HOUR William Blake (1757-1827) in "Auguries of Innocence" Table of contents Abstract vii Sammanfattning viii List of Papers ix Preface 1 1. Introduction 3 2. Bilirubin, Neonatal Jaundice and the Mechanism of Phototherapy 7 3. Experimental Methods 15 3.1 Femtosecond transient absorption spectroscopy 16 3.2 Fluorescence upconversion 21 3.3 Anisotropy measurements 24 4. Quantum Chemical Methods and Concepts 25 4.1 Basic quantum chemical concepts 25 4.2 The CASSCF method 31 4.3 Density functional methods (DFT) 32 4.4 Conical intersections 33 4.5 Exciton theory 35 5. Initial Photochemistry of Bilirubin (Summary of Results) 38 Conclusions 44 Future Outlook 45 References 46 Acknowledgements 51 v vi Abstract Bilirubin is a degradation product of haem, which is constantly formed in all mammals. Increased levels of bilirubin in humans lead to jaundice, a condi- tion that is very common during the first days after birth. This neonatal jaundice can routinely be treated by phototherapy without any serious side effects. During this treatment, bilirubin undergoes a photoreaction to iso- mers that can be excreted. The most efficient photoreaction is the isomerisa- tion around a double bond ( Z-E-isomerisation), which results in more solu- ble photoproducts. The work presented in this thesis shows results of a femtosecond optical spectroscopy study, combined with quantum-mechanical investigations, of the mechanism of isomerisation of bilirubin. The spectroscopic research was conducted with bilirubin in organic solvents, and in buffer complexed by human serum albumin. This albumin complex is present in the blood, and has thus medical importance. Quantum-chemical calculations (CASSCF) on a bilirubin model were used to explain experimental results. The fluorescence decay observed with femtosecond spectroscopy shows an ultrafast component (~120 fs), which is explained by exciton localisation, followed by processes with a lifetime of about 1-3 ps. These are interpreted as the formation of a twisted intermediate, which decays with a lifetime of 10-15 ps back to the ground state, as observed by absorption spectroscopy. CASSCF calculations, in combination with the experimental results, suggest the ca. 1-3 ps components to be relaxation to the twisted S 1 minimum, fol- lowed by the crossing of a barrier, from where further relaxation takes place through a conical intersection back to the ground state. Time-dependent DFT calculations were utilised to analyse the absorption spectrum of bilirubin. Good agreement with the measured spectrum was achieved, and low-lying states were observed, that need further investiga- tion. The theoretically obtained CD spectrum provides direct evidence that bilirubin preferentially binds to human serum albumin in the enantiomeric P-form at neutral pH. vii Sammanfattning Bilirubin är en nedbrytningsprodukt av hem som ständigt bildas hos alla däggdjur. En förhöjd bilirubinkoncentration i den mänskliga kroppen kan leda till gulsot, något som är mycket vanligt under de första dagarna efter födelsen (neonatal gulsot). Fototerapi används rutinmässigt som säker be- handlingsmetod, under vilken bilirubin genomgår en fotoreaktion till en isomer som kan utsöndras. Den mest effektiva fotoreaktionen är en Z-E- isomerisation, vilken leder till lösligare fotoprodukter. Arbetet som presenteras i denna avhandling visar resultaten av en kombin- erad femtosekund optisk-spektroskopisk och kvantmekanisk undersökning av mekanismen bakom bilirubins isomerisation. Den spektroskopiska studien genomfördes med bilirubin, löst i organiska lösningsmedel och i buffert i komplex med humant serumalbumin. Detta albuminkomplex finns i blodet, och är därför av medicinskt intresse. Kvantmekanistiska CASSCF- beräkningar på en bilirubinmodell användes för att förklara de experimen- tella resultaten. Det uppmätta fluorescence sönderfallet visar ultrasnabba komponenter (~120 fs). Dessa tolkas som excitonlokalisering, som följs av bildandet av ett vridet intermediat med en hastighetskonstant på ca. 1 ps -1(beroende på lösningsmedlet). Absorptionsmätningar visar att detta intermediat sönder- faller tillbaka till grundtillståndet med en livstid på 10-15 ps. CASSCF beräkningar, i kombination med de experimentella resultaten, ty- der på att sönderfallet med livslängden på ca. 1 ps är en relaxation till det vridna S 1-tillståndet. Reaktionsvägen därifrån antas passera en barriär till en konisk genomskärning, som möjliggör snabb relaxation till grundtillståndet. Tidsberoende DFT-beräkningar användes för att analysera bilirubins absorp- tionsspektrum, vilket gav bra överensstämmelse med uppmätta data. Dessu- tom hittades ett tillstånd med låg excitationsenergi, som kräver ytterligare studier. Med hjälp av det beräknade CD-spectret kunde det visas att biliru- bin binder till albumin i P-formen vid neutralt pH. viii List of Papers The present thesis is based on the following papers: I B. Zietz , A. N. Macpherson and T. Gillbro: Resolution of ultrafast ex- cited state kinetics of bilirubin in chloroform and bound to human se- rum albumin. Phys. Chem. Chem. Phys. 6 (2004) 4535-4537. II B. Zietz and F. Blomgren: Conical intersection in a bilirubin model - a possible pathway for phototherapy of neonatal jaundice. Revised ver- sion submitted to Chem. Phys. Letters III B. Zietz : Bilirubin's Optical Absorption and CD Spectra Analysed by Time-Dependent-DFT. Manuscript, revised version IV B. Zietz and T. Gillbro: Initial Photochemistry of bilirubin probed by femtosecond spectroscopy. Manuscript The following papers are not included in this thesis: – B. Zietz , V. I. Prokhorenko, A. R. Holzwarth and T. Gillbro: A com- parative study of the energy transfer kinetics in artificial BChl e aggre- gates containing a BChl a acceptor, and BChl e-containing chlorosomes of Chlorobium phaeobacteroides . J. Phys. Chem. B 110 (2006); DOI: 10.1021/jp053467a – W. Rettig, B. Zietz : Do twisting and pyramidalization contribute to the reaction coordinate of charge-transfer formation in DMABN and de- rivatives? Chem. Phys. Letters 317 (2000) 187–196 – T. Shutova, A. Villarejo, B. Zietz , V. Klimov, T. Gillbro, G. Samuels- son, G. Renger: Comparative studies on the properties of the extrinsic manganese-stabilizing protein from higher plants and of a synthetic peptide of its C-terminus. Biochim. Biophys. Acta 1604 (2003) 95– 104 ix x Preface This thesis is organised as follows: Chapter 1 (Introduction) gives an over- view over the subject, also relating it to adjacent research topics, as well as introducing the basic tools and concepts used in this work. Chapter 2 (Bilirubin, Neonatal Jaundice and Phototherapy) gives a more detailed presentation of the chemistry of bilirubin and phototherapy as well as the medical background. In the following Chapter 3 (Experimental Methods) , the methods of ultra-fast time-resolved spectroscopy are presented. The concepts of quantum chemistry, used to gain theoretical insight into the mechanism of phototherapy, are given in Chapter 4 (Quantum Chemical Methods) . The pages following (Acknowledgments) are used to express my gratitude to people who have joint me through the process of this work. The remainder is composed of published research papers and manuscripts, con- taining the main results of this work, in the following order: (I) " Resolution of ultrafast excited state kinetics of bilirubin in chloroform and bound to human serum albumin " describes the general phenomenon of the photo- chemistry of bilirubin on the ultrafast time scale. (II) " Conical intersection in a bilirubin model - a possible pathway for phototherapy of neonatal jaundice " tries to give an insight into the molecular mechanism on the atomic level of initial relaxation after photoexcitation. (III) " Bilirubin's Op- tical Absorption and CD Spectra Analysed by Time-Dependent-DFT " is aimed at understanding the nature of the absorption and the excited states of bilirubin, which form the basis for its photochemistry. More detailed results of the photochemistry of bilirubin and model compounds, both in solvent solution and bound to albumin, are presented in (IV) " Initial Photochemistry of bilirubin probed by femtosecond spectroscopy ". 1 2 Chapter 1 1. Introduction Walk into any children's ward in a hospital, and you're likely to find new- born babies treated with phototherapy. Lying in bright blue, white
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