Computational Characterization of “Dark” Intermediates in the Ultrafast Deactivation of Photoexcited Bilirubin

Abstract

The characterization of various intermediates in the ultrafast deactivation of photoexcited (Z,Z)-bilirubin-IXα was carried out using different computational methods. Various excited states of (Z,Z)-bilirubin-IXα and their respective vertical excitation energies were calculated using time-dependent density functional theory (TD-DFT) employing the Coulomb-attenuating method (CAM) combined with the B3LYP functional, which is known to predict accurate results on the charge transfer excitation process. Optimized geometries and absorption spectra were determined in chloroform solvent using the polarizable continuum model incorporating the integral equation formalism. The optimized geometries of different conformers of bilirubin (ZZ, ZE, EZ, and EE) along with their relative energies and vertical excitation energies were obtained. The geometry of the first excited state, S 1 , for the ZZ conformer was optimized using TD-DFT. The computational study suggests that excited-state intramolecular proton transfer (ESIPT) plays a major role in the deactivation process of (Z,Z)-bilirubin-IXα on a shorter time scale. The lactam−lactim tautomerism that arises from the ESIPT process gives rise to various intermediates of (Z,Z)-bilirubin-IXα. The computational results nicely corroborate the experimental findings available in the literature.

Conformational Control of the Photodynamics of a Bilirubin Dipyrrinone Subunit: Femtosecond Spectroscopy Combined with Nonadiabatic Simulations

“…A straightforward comparison of the photochemistry of 1 and bilirubin is not possible. The two dipyrrinone subunits of bilirubin exhibit excitonic coupling, and an intramolecular H-bonding in unconjugated bilirubin results in the fast formation of dark states, attributed to the lactam-lactim tautomers . In our study, we did not observe any proton transfer process that would be manifested by dual emission or formation of dark states, because the stimulated emission (SE) band after the excitation of Z - 1 with a 387 nm pump concomitantly decayed with GSB and ESA bands.…”

“…The overall transient data are characterized by multiexponential kinetics. A density functional theory (DFT) study predicted that the dark states are lactam-lactim tautomers that are produced via excited-state intramolecular proton transfer (ESIPT) within an initial, intramolecularly H-bonded bilirubin . The photophysics of bilirubin bound to UnaG fluorescent protein was investigated by time-correlated single-photon counting (TCSPC) and fs-TA spectroscopy .…”

“…The chemistry and photophysics of bilirubin-related compounds were widely studied with static ab initio methods. The conformational sampling of bilirubin was studied several times in both the gas phase and solution. − A time-dependent DFT (TDDFT) method was used for the characterization of bilirubin excited states, absorption spectra, and the character of the photochemical intermediates. , Particular attention was paid to the excitonic coupling in these systems. − , Zietz and Blomngren have briefly analyzed the topography of the potential energy surface of a dipyrrinone model system finding a conical intersection attributed to the photoisomerization . Later, Garcı́a-Iriepa and co-workers calculated the analogous topography for a similar bilirubin model, benzalpyrrolinone, connecting it with the lifetimes from fs-TA spectroscopy mentioned above .…”

“…A density functional theory (DFT) study predicted that the dark states are lactamlactim tautomers that are produced via excited-state intramolecular proton transfer (ESIPT) within an initial, intramolecularly H-bonded bilirubin. 11 The photophysics of bilirubin bound to UnaG fluorescent protein was investigated by time-correlated single-photon counting (TCSPC) and fs-TA spectroscopy. 10 This work compared the behavior of bilirubin rigidly bound to wild-type UnaG protein and its N57A mutant, where the binding is much looser, and furthermore, bilirubin in this protein is surrounded by solvent molecules, permitting its greater flexibility.…”

“…24−32 A time-dependent DFT (TDDFT) method was used for the characterization of bilirubin excited states, absorption spectra, and the character of the photochemical intermediates. 11,33 Particular attention was paid to the excitonic coupling in these systems. [12][13][14]34 Zietz and Blomngren have briefly analyzed the topography of the potential energy surface of a dipyrrinone model system finding a conical intersection attributed to the photoisomerization.…”