Abstract

Bilirubin is a neurotoxic product responsible for neonatal jaundice, which is generally treated by phototherapy. The photoreaction involves ultrafast internal conversion via an elusive intermediate and Z-E isomerization with minor yield (less than 3% in solution). The structure of the intermediate remains unclear. Here, the combination of UV-vis and mid-IR ultrafast transient absorption spectroscopy reports a comprehensive picture of the mechanism and provides essential structural information about the intermediate species. Thus, spectral dynamics during the earliest ps unveils a wavepacket travelling from the Franck-Condon region to the crossing point with a dark state. The latter shows a tighter molecular skeleton than the ground state and decays with 15 ps time constant. Remarkably, the relative contribution of a non-decaying component increases linearly with pump energy, suggesting that Z-E isomerization could also be triggered by two-photon excitation. Implications for the photochemistry of protein-bound open tetrapyrroles are discussed.

Highlights

  • Tetrapyrroles constitute arguably one of the most relevant classes of molecules

  • A linear chain tetrapyrrole resulting from the degradation of the heme group.[7]

  • Ultrafast deactivation of Bilirubin IXa (BR) occurs via the dark intermediate Bcold

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Summary

Introduction

Tetrapyrroles constitute arguably one of the most relevant classes of molecules. Porphyrins, closed chain tetrapyrroles, are essential for oxygen transport and photosynthesis, whereas open chain bilins sense and trigger the biological response to light in phytochromes.[1,2] It is fascinating that this relatively simple structure fulfils effectively various essential functions in a vast variety of living organisms. The neighbourhood of the Franck–Condon region.[19] The observation was explained by consecutive mechanisms[16,18] invoking a ‘‘partially twisted’’ structure from a dark excited state. The latter could be localized in a single dipyrridone moiety,[18] have charge transfer character,[20] or result from pp*–np* mixing,[16] but experimental evidence is still lacking. BR has attracted considerable interest because of its central role in neonatal jaundice.[12] Light absorption in the UV region transforms the stable (4Z,15Z) isomer into the polar forms (4Z,15E) and (4E,15Z) The latter show enhanced solubility in blood serum[21] and can be eliminated in the liver. In contrast to previous proposals,[16,17,18] the intermediate shows stronger CQC bonds than the ground electronic state, indicating that the molecular structure is not twisted

Materials and methods
Results and discussion
Two-photon excitation
Findings
Conclusions
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