Excitation energy transport and trapping in concentrated solid solutions
, L. Kul
, H. Grajek
, A. Kamin
, C. Bojarski
Institute of Experimental Physics, University of Gdan
sk, 80-952 Gdan
sk, Wita Stwosza 57, Poland
Department of Technical Physics and Applied Mathematics, Technical University of Gdan
sk, 80-952 Gdan
sk, Narutowicza 11/12, Poland
Institute of Physics and Biophysics, Warmia and Masuria University, M. Oczapowskiego 7, 10-719 Olsztyn, Poland
Received 30 July 2002; received in revised form 5 November 2002; accepted 6 November 2002
Excitation energy transport and trapping is studied for monomer –fluorescent dimer system of flavomononucleotide (FMN) in polyvinyl
alcohol films (PVA). It is shown that the theory neglecting reverse energy transfer (RET) from dimers to monomers does not allow for the
explanation of concentration quenching and concentration depolarization results presented herein. Much better agreement has been obtained
using generalized energy transport theory in which fluorescent dimers are treated as imperfect traps for excitation energy. Such parameters
like the dimer quantum yield and its emission anisotropy are estimated.
D 2002 Elsevier Science B.V. All rights reserved.
Keywords: Excitation energy; Flavomononucleotide; Polyvinyl alcohol film
Flavins act as coenzymes and photoreceptors in many
biological systems . Flavin mononucleotide (FMN)
seems to be especially important among these due to its
role in the photoreception process. It has been shown
previously that the properties of FMN in polyvinyl alcohol
films (PVA) change dramatically at high dye concentrations
[2–4]. It has been found that FMN monomers at such high
concentrations form aggregates which could be identified as
dimers able to emit their own fluorescence in rigid polymer
films [3,4]. These fluorescent dimers can act as imperfect
traps for excitation energy, which means that they can
transfer part of the excitation energy back to the set of
monomers. The quantitative description of such forward and
reverse transport is complicated, nevertheless, it has been
elaborated theoretically [5–10] and confirmed experimen-
tally for simpler model systems [11 –18].
The main difficulty in the quantitative description of
reverse energy transport (RET) in the system studied is the
lack of critical concentrations for energy transfer from
dimers-to-monomers and between dimers. These quantities
could not be obtained by the usually applied spectroscopic
methods because of several reasons. First, it is very difficult
to obtain pure dimer fluorescence spectrum [3,4]. Second,
even if the dimers’ fluorescence spectrum could be known
exactly, one should take into account that dimers can be
excited both directly by light absorption and by receiving
nonradiative energy from the monomers. Additionally,
dimers can emit their own fluorescence and transfer energy
back to the set of monomers. Therefore, the dimer quantum
yield obtained by simple spectra separation is not, in
principle, its ‘‘true’’ quantum yield. Finally, the spectra of
monomers shift to the red with concentration due to inho-
mogeneous broadening of monomer energy levels [3,4].
This effect also limits the possibility of determining the
dimer quantum yield using the spectroscopic method .
The aim of this paper is to describe quantitatively the
behaviour of FMN concentrated PVA solutions. In partic-
ular, we propose a method to estimate the dimer quantum
yield based on the best fit of concentration quenching and
concentration depolarization of FMN in PVA to the theory
0304-4165/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.
Dedicated to Professor Alfons Kawski on the occasion of his 75th
* Corresponding author. Tel.: +48-5855-29244; fax: +48-5834-13175.
E-mail address: email@example.com (P. Bojarski).
Biochimica et Biophysica Acta 1619 (2003) 201 –208