An Ultrafast Spectroscopic and Quantum-Chemical Study of the Photochemistry of Bilirubin Initial Processes in the Phototherapy for Neonatal Jaundice

Abstract: Bilirubin is a degradation product of haem, which is constantly formed in allmammals. Increased levels of bilirubin in humans lead to jaundice, a conditionthat is very common during the first days after birth. This neonataljaundice can routinely be treated by phototherapy without any serious sideeffects. During this treatment, bilirubin undergoes a photoreaction to isomersthat can be excreted. The most efficient photoreaction is the isomerisationaround a double bond (Z-E-isomerisation), which results in more solublephotoproducts.The work presented in this thesis shows results of a femtosecond opticalspectroscopy study, combined with quantum-mechanical investigations, ofthe mechanism of isomerisation of bilirubin. The spectroscopic research wasconducted with bilirubin in organic solvents, and in buffer complexed byhuman serum albumin. This albumin complex is present in the blood, andhas thus medical importance. Quantum-chemical calculations (CASSCF) ona bilirubin model were used to explain experimental results.The fluorescence decay observed with femtosecond spectroscopy shows anultrafast component (~120 fs), which is explained by exciton localisation,followed by processes with a lifetime of about 1-3 ps. These are interpretedas the formation of a twisted intermediate, which decays with a lifetime of10-15 ps back to the ground state, as observed by absorption spectroscopy.CASSCF calculations, in combination with the experimental results, suggestthe ca. 1-3 ps components to be relaxation to the twisted S1 minimum, followedby the crossing of a barrier, from where further relaxation takes placethrough a conical intersection back to the ground state.Time-dependent DFT calculations were utilised to analyse the absorptionspectrum of bilirubin. Good agreement with the measured spectrum wasachieved, and low-lying states were observed, that need further investigation.The theoretically obtained CD spectrum provides direct evidence thatbilirubin preferentially binds to human serum albumin in the enantiomericP-form at neutral pH.