Quantum Yields for Laser Photocyclization of Bilirubin in the Presence of Human Serum Albumin. Dependence of Quantum Yield on Excitation Wavelength

Abstract

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.

The effect of hematocrit on the efficacy of phototherapy for neonatal jaundice

“…In addition, the calculations allowed us to predict that a narrow wavelength-band emitting light source, such as light-emitting diodes (LEDs), at 476 nm would be the most efficient for phototherapy and would also minimize heating of the neonate by absorbed light. Previous observations (5,15,16) that the optimal wavelength range for phototherapy is at longer wavelengths than the maximum of the bilirubin absorption spectrum (460 nm) are strongly supported by these calculations. …”

“…The two main transformations are configurational isomerization (to Z,E-bilirubin) and structural isomerization (to lumirubin). It is suggested that the rapidly excreted lumirubin is a key product regards phototherapy (16,26). Both the formation of Z,E-bilirubin and lumirubin exhibit wavelengthdependent yields.…”

“…Bilirubin undergoes various photochemical transformations (15,16,25). The two main transformations are configurational isomerization (to Z,E-bilirubin) and structural isomerization (to lumirubin).…”

“…Both the formation of Z,E-bilirubin and lumirubin exhibit wavelengthdependent yields. Using reported (16,25) wavelength dependencies to modify the "action spectra" accounts for the efficiency of the photochemistry. When the yield of lumirubin is included, λ max shifts to 480 nm (Figure 3).…”

Randomized controlled trial of light-emitting diode phototherapy

“…1 At these wavelengths, light penetrates the skin well and is absorbed strongly by bilirubin. 1,2 Thus, special blue fluorescent (BB) tubes that have their output primarily in the blue spectrum (430 to 490 nm) are recommended by the American Academy of Pediatrics (AAP) as the most effective light source for lowering the serum bilirubin. 3 Prototype light-emitting diode (LED) lights have been developed and tested in clinical and laboratory studies, [4][5][6] and a commercially available LED device is being used, currently, in nurseries in the United States.…”