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QUANTUM YIELDS FOR LASER PHOTOCYCLIZATION OF BILIRUBIN IN THE PRESENCE OF HUMAN SERUM ALBUMIN. DEPENDENCE OF QUANTUM YIELD ON EXCITATION WAVELENGTH
Author(s) -
McDonagh Antony F.,
Agati Giovanni,
Fusi Franco,
Pratesi Riccardo
Publication year - 1989
Publication title -
photochemistry and photobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.818
H-Index - 131
eISSN - 1751-1097
pISSN - 0031-8655
DOI - 10.1111/j.1751-1097.1989.tb04164.x
Subject(s) - chemistry , chromophore , quantum yield , excited state , photoisomerization , wavelength , rydberg formula , excitation , analytical chemistry (journal) , reaction rate constant , fluence , molecular physics , atomic physics , photochemistry , isomerization , optics , kinetics , fluorescence , physics , chromatography , organic chemistry , catalysis , ion , quantum mechanics , ionization
— The quantum yield for laser photocyclization of bilirubin to lumirubin in the presence of human serum albumin (tHL R ) 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 XHIa impurities. The values obtained (7.2‐18 10 ‐4 ) were lower than those previously reported and showed the following wavelength dependence: 457.9 < 488.0 < 501.7 < 514.5 528.7. 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,15 E and 4 E ,15 Z isomers also was studied. This reaction became less regioselective for the 4 Z ,15 E isomer with increasing wavelength. The observed wavelength dependence of tH LR and of the [4 Z ,15 E ]: [4 Z ,15 E ] 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.