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Unformatted text preview: Human Serum Albumin-flavonoid Interactions Monitored by Means of Tryptophan Kinetics O LAF J. R OLINSKI , A NDREW M ARTIN , AND D AVID J. S. B IRCH Department of Physics, University of Strathclyde, Scottish Universities Physics Alliance, Photophysics Group, Glasgow, United Kingdom A nonextensive model of decay kinetics has been used to describe fluorescence behavior of trypto- phan in human serum albumin on binding two flavonoids, quercetin and morin. We demonstrate that this approach, alternative to multiexponential representation of usually complex decays of tryptophan, is more adequate and can be beneficial in noninvasive lifetime sensing based on intrinsic fluorescence. Key words: HSA; tryptophan; fluorescence decays; nonextensive kinetics; lifetime distribution; flavonoids Introduction Noninvasive biomedical sensing based on intrinsic fluorescence of biomolecules is a subject of extensive research in biology and medicine. A new class of nonin- vasive probes based on lifetime measurements would be especially advantageous, however this would re- quire fine modeling of fluorescence decays reflecting usually complex excited-state kinetics of the intrinsic fluorophores. The most common intrinsic dyes, the fluorescent amino acids tryptophan ( Trp ), tyrosine ( Tyr ), and phenylalanine ( Phe ), occur in most proteins and are involved in a large number of bioactivities. Their flu- orescence responses, usually altered by these activities or analytes, provide valuable information on the events occurring on nanometer scale. For example, conforma- tional changes of proteins are usually reflected by shifts in fluorescence spectra, changes in quantum yield, and alterations in fluorescence decay kinetics. One of processes where noninvasive sensing would be beneficial for medical and biological applications, is binding analytes to proteins like human serum al- bumin (HSA), and this has been the subject of many studies based on steady-state 1 , 2 and, more recently, time-resolved 3 fluorescence of Trp . Trp is an especially promising potential intrinsic probe, not only due to its dual role as a participant Address for correspondence: Olaf J. Rolinski, University of Strathclyde, Department of Physics, 107 Rottenrow, Glasgow 64 0HG, UK. Voice: + 141 548 4239; fax: + 141 552 2891. email@example.com and a fluorescence probe in numerous bioactivities, but also because it can be optically excited and its de- cay kinetics monitored without interfering with Tyr or Phe , as they absorb photons of higher energy. In this extended abstract we demonstrate that using a specially selected light-emitting diode (LED) excita- tion source for Trp in a lifetime experiment 46 and an alternative approach to lifetime data analysis 3 , 7 can make Trp a valuable noninvasive probe. We report time-resolved studies of free HSA and HSA complexed with two flavonoids, quercetin and morin, and propose a nonexponential model of the decay functions....
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