Excited States of Bilirubin

Abstract

The excited states of bilirubin (BR) in a variety of environments have been studied by 347 nm laser flash photolysis. Quantum yields of formation of triplet BR have been shown to be less than 0.005 in solution in water @H 9-11), methanolic ammonia, 10% aqueous mulgofen and in cetyl trimethylammonium bromide. In benzene the quantum yield was 0.01 although this diminished to less than 0.005 on addition of triethylamine. Permanent products are formed with benzene and with 1% methanolic ammonia. With BR in HSA a transient decaying with k = 3.5 x lo5 s -l is formed by a monophotonic process together with a permanent product. Neither species is affected by oxygen or by iodide ion. Both originate from BR molecules in the strongest binding site in the HSA. The yields of both species are unaffected by salt but are temperature dependent. The decay of the transient is strongly temperature dependent corresponding to an activation energy of about 50-60 kJ mol-'. If this transient is a triplet it is formed with a quantum yield of 0.13 f 0.01. The relevance of these results to an understanding of the photo therapeutic process is discussed.

Picosecond primary photoprocesses of bilirubin bound to human serum albumin.

“…The quantum yield ofthe forward photoreaction was found to be the same value within experimental error, 0.20 ± 0.02. These observations were noted to be consistent with a simple mechanism in which photointerconversion ofthe isomers occurs via a common twisted excited singlet state intermediate.11 Other workers have also suggested that the photoisomerizations of BR and other pyrrole pigments are "singlet state" reactions (5,6,23 Fluorescence quantum yields for BR were calculated from the integrated fluorescence spectra in comparison with the spectra from solutions of acridine yellow in ethanol at 22°C, for which a yield of 0.86 was previously determined (24 Picosecond Time-Resolved Spectroscopy. The apparatus used in these experiments, described in detail elsewhere, is based on a continuous-wave-pumped, passively mode-locked picosecond dye laser, producing 0.5-ps pulses at 610 nm (25,26).…”

“…The quantum yield ofthe forward photoreaction was found to be the same value within experimental error, 0.20 ± 0.02. These observations were noted to be consistent with a simple mechanism in which photointerconversion ofthe isomers occurs via a common twisted excited singlet state intermediate.11 Other workers have also suggested that the photoisomerizations of BR and other pyrrole pigments are "singlet state" reactions (5,6,23 …”

“…This is evidenced by observations of low fluorescence yields (Of < 5 X 10-4) (2, 3), low yields of triplet states (<0.01) (4-6), and the absence of efficient bleaching of the chromophore (7). The same situation pertains to BR bound to the high-affinity site on human serum albumin (hereafter referred to as "albumin") (If = 0.002) (2, 3,5 (15,16). Principles governing the rate of twisting would be those determining diffusional motion of nuclei on an intramolecular potential surface (17)(18)(19).…”

QUANTUM YIELD and EQUILIBRIUM POSITION OF THE CONFIGURATIONAL PHOTOISOMERIZATION OF BILIRUBIN BOUND TO HUMAN SERUM ALBUMIN*

“…The low fluorescence yield of BR/HSA near room temperature, -0.001 (Greene et al, 1981) and still lower yield for PBR/HSA fluorescence, combined with the observations of low triplet state yields, <0.01 (Sloper and Truscott, 1980;Matheson et al, 1980), suggest that photoisomerization, which is evidently a high quantum yield process, proceeds from the excited singlet state and provides the major relaxation pathway for photoexcited BR/HSA. The simplest mechanistic scheme is presented below.…”

Spectral fluorescence and polarization characteristics of Z,Z-bilirubin IXα

“…A distinguishing feature of the fluorescence of Z,Z-BP IXα solutions at room temperature is its extremely low quantum yield: ϕ fl < 2⋅10 −4 for solutions of the pigment in chloroform, and ϕ fl = (1-3)⋅10 −3 for its complex with HSA in buffered aqueous medium [12][13][14][15][16][17][18]. And the low value of ϕ fl is not due to the high efficiency of intersystem crossing S 1 → T 1 : the quantum yield in the triplet state for solutions of bilirubin in chloroform is ϕ t < 0.005 [36], while for the BR-HSA complex ϕ t < 0.01 [18]. Hence neither emission of radiation as fluorescence nor intersystem crossing S 1 → T 1 are major deactivation pathways for the electronic excitation energy of the singlet state of the bilirubin molecule at room temperature.…”