Quantum yield and equilibrium position of the configurational photoisomerization of bilirubin bound to human serum albumin

Noninvasive, continuous fluorescence monitoring of bilirubin photodegradation

Nowadays phototherapy is widely used for treatment of various diseases. However, efficient application of phototherapy requires an understanding of light interactions with main endogenous chromophores (e.g., hemoglobin, bilirubin, and water) in tissue. In particular, bilirubin is the target chromophore in the treatment of neonatal jaundice, which is the most common disease affecting up to 80% of preterm infants. The most frequently recommended treatment technique for this disease is phototherapy with blue light in combination with conventional drug therapy. To follow threshold total serum bilirubin (TSB) concentration guidelines, it is essential to estimate TSB concentration accurately. The gold standard biochemical analysis is invasive and bulky. Moreover, noninvasive methods do not provide sufficient reproducibility and accuracy. In this research, the fluorescence sensing of bilirubin with human serum albumin complexes was studied. The fluorescence time course during light irradiation (central wavelength: 467 nm and power density: 12.13 mW cm^-2) was demonstrated to depend on the initial concentration. Specifically, for the bilirubin concentration C = 18.65 μM, an insignificant fluorescence signal increase was observed during the first 30 minutes of light irradiation, while for bilirubin concentration C = 373 μM, the fluorescence signal did not reach maximum during 2.5 hours of light irradiation. Thus, fluorescence sensing might show increased accuracy when used with other noninvasive bilirubin sensing methods.

Study of Model Systems for Bilirubin and Bilin Chromophores: Determination and Modification of Thermal and Photochemical Properties

Bilin chromophores and bilirubin are involved in relevant biological functions such as light perception in plants and as protective agents against Alzheimer and other diseases. Despite their extensive use, a deep rationalization of the main factors controlling the thermal and photochemical properties has not been performed yet, which in turn hampers further applications of these versatile molecules. In an effort to understand those factors and allow control of the relevant properties, a combined experimental and computational study has been carried out for diverse model systems to understand the interconversion between Z and E isomers. In this study, we have demonstrated the crucial role of steric hindrance and hydrogen-bond interactions in thermal stability and the ability to control them by designing novel compounds. We also determined several photochemical properties and studied the photodynamics of two model systems in more detail, observing a fast relaxation of the excited state shorter than 2 ps in both cases. Finally, the computational study allowed us to rationalize the experimental evidence.

Initial photochemistry of bilirubin probed by femtosecond spectroscopy

Bilirubin is a breakdown product from heme catabolism, and reduced excretion of bilirubin can lead to jaundice. Phototherapy is the most common treatment for neonatal jaundice, a condition frequently encountered in newborn infants. Knowledge of the photochemistry of bilirubin, which is dominated by (ultra)fast components, is necessary for the profound understanding of the processes in phototherapy. Here, we report results from femtosecond fluorescence upconversion measurements on bilirubin and half-bilirubin model compounds, as well as pump-probe absorption measurements on bilirubin. A fast component of ca. 120 fs in the multiexponential fluorescence decay, being only visible in the bilirubin molecule, is interpreted as exciton localization within the molecular halves. The slower components of several hundreds of femtoseconds and a few picoseconds, occurring in bilirubin and the half-bilirubin model, are interpreted as relaxation to a (twisted) intermediate, which decays further with ca. 15 ps to the ground state.

Bilirubin binding properties of pigeon serum albumin and its comparison with human serum albumin

Binding of bilirubin (BR) to pigeon serum albumin (PgSA) was studied by absorption, fluorescence and CD spectroscopy and results were compared with those obtained with human serum albumin (HSA). PgSA was found to be structurally similar to HSA as judged by near- and far-UV CD spectra. However, PgSA lacks tryptophan. Binding of BR to PgSA showed relatively weaker interaction compared to HSA in terms of binding affinity, induced red shift in the absorption spectrum of BR and CD spectral characteristics of BR-albumin complexes. Photoirradiation results of BR-albumin complexes also showed PgSA-bound BR more labile compared to HSA-bound BR.

On the modulation of photoinduced fluorescence enhancement and conformational stability of albumin-bound bilirubin: effect of epsilon-NH(2) groups blocking and chloroform binding

Photoinduced fluorescence enhancement of bilirubin bound to primary binding site on human serum albumin (HSA) was completely ceased when epsilon-NH(2) groups of its internal lysine residues were covalently blocked by acetylation or succinylation though the pigment bound to these derivatives in a folded conformation akin to that bound to HSA. These photoinduced fluorescence modulations cannot be ascribed to the binding of bilirubin to secondary low affinity sites as the CD spectrum of bilirubin bound to these derivatives showed complete inversion upon addition of chloroform which binds to subdomain IIA in HSA where high affinity bilirubin binding site is located. Presence of chloroform reconciled the photoinduced alterations in the CD spectrum observed in its absence, suggesting that chloroform stabilized the bound ligand against light but the fluorescence properties of bilirubin complexed with acetylated or succinylated derivatives remained unchanged. Guanidination of internal epsilon-NH(2) groups in HSA by O-methylisourea did not alter the spectral properties of the bound ligand. These results suggest that salt linkage(s) existing between epsilon-NH(2) groups of lysine residues in HSA and carboxyl groups of bilirubin, act(s) as a potential barrier during conformational rotation of the bound ligand assisted by photoactivation and their abolishment can alter its dynamics and stereoselectivity, a hitherto unnoticed implication of salt linkage(s) in BR-HSA complex.

Evaluation of the quantum yield for E-->Z isomerization of bilirubin bound to human serum albumin. Evidence of internal conversion processes competing with configurational photoisomerization

The quantum yield (phi zz) for the E--> photoisomerization of bilirubin (BR) bound to human serum albumin (HSA) under laser light irradiation has been evaluated with absorbance spectroscopy implemented by the high-performance liquid chromatography technique. The value at 458 nm is about 0.2, four times smaller than the value previously reported in the literature. As for the Z-->E configurational and the structural photoisomerization processes, the quantum yield varies with excitation wavelength, from 0.23 in the blue to 0.16 in the green. The sum of the quantum yields of the Z<-->E configurational photoisomerization reactions is 0.2-0.3 in the blue-green spectral region, thus giving direct evidence of the existence of internal conversion processes of the BR-HSA complex which compete with configurational photoisomerization. An evaluation of the quantum yields of the Z<-->E reactions for filtered broad-band light excitation in the 390-530 nm spectral region from data already published in the literature is also reported. Good agreement with our data of the quantum yields phi ZZ and phi ZE is found, despite the diversity of the experimental procedures in the two cases.

Wavelength-dependent quantum yield for Z----E isomerization of bilirubin complexed with human serum albumin

The quantum yield, phi ZE, for configurational photoisomerization (4Z,15Z----4Z,15E) of bilirubin bound non-covalently to human serum albumin was determined (at 23 +/- 2 degrees C) by laser excitation and chromatographic analysis of products. Values obtained for photoexcitation at 465 nm were about one-half those previously reported. The quantum yield was dependent on excitation wavelength, decreasing from a value of 0.109 +/- 0.010 for excitation at 457.9 nm to a value of 0.054 +/- 0.005 for excitation at 514.5 nm. The wavelength dependence is consistent with rapid transfer of excitation energy between the two non-identical pyrromethenone chromophores of bilirubin in the singlet excited state. Since the quantum yields for photoisomerization and luminescence of bilirubin bound to serum albumin at room temperature are both low, internal conversion processes, rather than Z----E configurational isomerizations, are probably the major pathways for deactivation of photo-excited bilirubin.

Quantum yields for the cyclization and configurational isomerization of 4E,15Z-bilirubin

Extraction of a solution of bilirubin configurational isomers in chloroform with an aqueous solution of human serum albumin was found to remove selectively the 4Z,15E-isomer. This phenomenon was used to develop a method for the purification of the 4E,15Z-isomer of bilirubin. The quantum yield for the cyclization and configurational isomerization of the 4E,15Z-isomer bound to a molar excess of human serum albumin was measured at 450 and 510 nm. The quantum yield for cyclization to form lumirubin was 0.12 and 0.19 at 450 and 510 nm, respectively. The quantum yield for configurational isomerization to form 4Z,15Z-bilirubin was 0.03 and 0.05 at 450 and 510 nm. An analysis of previously published data on the quantum yield for the formation of lumirubin from 4Z, 15Z-bilirubin bound to human serum albumin suggests that all of the formation of lumirubin may occur via consecutive photochemical processes with the 4E,15Z-isomer as an intermediate.

New trends in photobiology (invited review). Recent advances in bilirubin photophysics

The high photosensibility of bilirubin represents the main reason why neonatal jaundice is treated with light. The relaxation processes of excited bilirubin appear to be governed by complex mechanisms which are not fully known. At room temperature, radiationless channels, such as Z----E configurational photoisomerizations and internal conversions, contribute mostly to the deactivation of singlet excited bilirubin. Exciton coupling processes between the two pyrromethenone chromophores constituting the molecule may play a crucial role in bilirubin photophysics.

Quantum yields for laser photocyclization of bilirubin in the presence of human serum albumin. Dependence of quantum yield on excitation wavelength

The quantum yield for laser photocyclization of bilirubin to lumirubin in the presence of human serum albumin (phi LR) was measured at five monochromatic excitation wavelengths in the range 450-530 nm. Solutions used were optically thin throughout the wavelength range and precautions were taken to exclude contributions from photocyclization of bilirubin XIII alpha impurities. The values obtained (7.2-18 x 10(-4] were lower than those previously reported and showed the following wavelength dependence: 457.9 less than 488.0 less than 501.7 less than 514.5 approximately equal to 528.7. However, the rate of lumirubin formation, normalized to constant fluence, decreased with wavelength over the same wavelength range and no evidence was found that photoisomerization of bilirubin to lumirubin is faster with green (514.5 or 528.7 nm) than with blue (457.9 or 488.0 nm) light. The stereoselectivity of the configurational isomerization of bilirubin to 4Z,15E and 4E,15Z isomers also was studied. This reaction became less regioselective for the 4Z,15E isomer with increasing wavelength. The observed wavelength dependence of phi LR and of the [4E,15Z]: [4Z,15E] ratio at photoequilibrium are consistent with an exciton coupling model in which intramolecular energy transfer can occur between the two pyrromethenone chromophores of the bilirubin molecule in the excited state. Relative rates of lumirubin formation in vivo at different excitation wavelengths and constant fluence were estimated for different optical thicknesses and for different skin thicknesses. These estimates suggest that the recently reported clinical equivalence of blue and green phototherapy lights probably reflects the marked variation of skin transmittance with wavelength more than wavelength-dependent photochemistry. The calculations also indicated that the optimal wavelength for phototherapy is probably on the long wavelength side of the bilirubin absorption maximum.

UV excitable fluorescence of lumirubin

Solutions of bilirubin in different solvents show negligible fluorescence when they are excited with UV light. After irradiation, blue-violet fluorescence (400-450 nm) is observed, whose intensity and peak position depend on the irradiation time. The isolation of pure lumirubin led us to attribute this fluorescence specifically to this photoisomer, even if small contributions from other photoproducts cannot be ruled out. The emission peak of the pure lumirubin in aqueous solution (phosphate buffer) is at 415 nm while the main excitation peak is at 315 nm. Finally, an interpretation of the observed fluorescence is proposed on the basis of the similarity of the present results with previously reported experimental data.

Phototherapy in a new light

Visible light phototherapy has been used in the treatment of neonatal jaundice for more than 25 years. This article reviews the current state of knowledge of how phototherapy works and provides a framework for understanding why green may soon become the color of choice for phototherapy lamps.

Wavelength-dependence of the relative rate constants for the main geometric and structural photoisomerization of bilirubin IX alpha bound to human serum albumin. Demonstration of green light at 510 nm as the most effective wavelength in photochemical changes from (ZZ)-bilirubin IX alpha to (EZ)-cyclobilirubin IX alpha via (EZ)-bilirubin

The kinetics for the quantitatively important reaction: (Formula: see text) that is, the photochemical interconversion between bilirubin and its geometric and structural photoisomers bound to human serum albumin in aqueous solution when various wavelengths of monochromatic light were used, were assayed by h.p.l.c. In order to clarify the wavelength-dependence of the relative rate constants in the individual steps, a light-source with a half-bandwidth of 10 nm was used at increments of 20 nm, in the range from 410 nm to 550 nm. We describe for the first time studies on the wavelength-dependence of rate constants in geometric and structural photoisomerization reactions in vitro of (ZZ)-bilirubin or (EZ)-bilirubin bound to human serum albumin, especially the relative rate constants of cyclization of (EZ)-bilirubin into (EZ)-cyclobilirubin. Because studies in vitro have demonstrated that the wavelengths from 350 to 450 nm are mutagenic, the results obtained indicated that the safest and ideal light-source for phototherapy is green light of 510 nm, which keeps (ZE)-bilirubin concentrations as low as possible, as shown by a maximal value of k2 at 510 nm and a relatively low value of k1 at 510 nm. This light-source still ensures the substantial absorption of (ZZ)-bilirubin, which is the precursor of (EZ)-bilirubin, the intermediate in (EZ)-cyclobilirubin formation and, furthermore, as shown by the maximal value of k5 and a considerable value of k4 at 510 nm, promotes the cyclization of (EZ)-bilirubin derived from (ZZ)-bilirubin even though k3 at 510 nm also shows a peak value.

Light effects on transport and excretion of bilirubin in newborns

There is now firm evidence that phototherapy has the following effects on bilirubin metabolism in humans with unconjugated hyperbilirubinemia. It rather rapidly converts a substantial fraction of the normal toxic 4Z, 15Z form of bilirubin to the 4Z, 15E form, which probably is less toxic. Simultaneously it enhances the overall excretion of bilirubin by converting it to oxidation products and structural and configurational isomers that are excretable in bile and urine without the need for glucuronidation. We know that these reactions occur in vivo because we have synthesized the compounds involved and have identified them unambiguously in vivo in the tissues of jaundiced babies and rats undergoing phototherapy. It is unlikely that these photobiological effects on bilirubin metabolism and transport are restricted to babies undergoing purposeful phototherapy. All babies are exposed to visible light and all develop hyperbilirubinemia during early life, with many exhibiting jaundice. Because there is no lower intensity threshold for photochemical reactions, it seems probable that the photobiological effects described in this paper occur in most newborns to some degree. Furthermore, similar photoprocesses would be expected to occur in the approximately 2-5% of the population who have the benign condition known as Gilbert's syndrome, which is characterized by chronic mild unconjugated hyperbilirubinemia, particularly when they sunbathe. Clearly, in the particular instance of phototherapy of neonatal jaundice, blue light is therapeutic. In some respects it acts like a drug, almost like the ideal magic bullet, because it is specific for the target molecule and safe. The main limitation of phototherapy is that it is inefficient, a limitation that seems to be imposed by transport processes in the body and the optics of skin rather than by the photochemical reactions on which it depends.

Phototherapy for neonatal jaundice: optimal wavelengths of light

Phototherapy results in transformation of bilirubin to more water-soluble isomers. The efficacy of monochromatic visible light from 350 to 550 nm in the fastest photoisomerization reaction was quantitated by high-pressure liquid chromatography. The most effective wavelengths in vitro (i.e., leading to greater than 25% photoisomer) were in the blue spectrum from approximately 390 to 470 nm. Green light (530 nm) was not only ineffective for production of photoisomer, but capable of reversing the reaction. The results indicate that any clinically useful phototherapy unit must include the blue portion of the visible spectrum, and suggest that the effectiveness of phototherapy may be increased by elimination of green light.

Binding of photobilirubin to human serum albumin. Estimate of the affinity constant

Results of experiments based upon circular dichroic spectra suggest that configurationally (Z leads to E) isomerized bilirubin (photobilirubin) binds to human serum albumin at the primary bilirubin binding site with an affinity only 2-3 times lower than that of bilirubin. The high affinity of photobilirubin for albumin, comparable to that of bilirubin, supports the roles of albumin in the stabilization and transport of the isomerized pigment in vivo and strongly suggests that albumin also functions to sequester photobilirubin effectively, reducing its toxic potential. The high affinity of photobilirubin for albumin predicts that the isomerized pigment, formed in large amounts during phototherapy for neonatal hyperbilirubinemia, should not appear in the fast-diazo-reacting ('direct') bilirubin pool nor should it interfere with nonspectroscopic bilirubin binding tests. These predictions were confirmed for the Evelyn and Malloy diazo assay for 'direct' bilirubin and a Sephadex chromatography method for assessing 'loosely bound' plasma bilirubin.