Fluorescent J-Aggregates and Their Biological Applications - Advanced Fluorescence Reporters in Chemistry and Biology - page 149

explain their results as follows. At high temperatures (e.g., above 35 C) the DiSC 3+ (5)
dye molecules form the face-to-face, i.e., sandwich-type, dimers in the minor
groove of [poly(dI-dC)] 2 , as it was supposed to be the case for DiSC 2 (5) in [ 31 ].
As the temperature is high enough, the charged substituents of the opposite dye
molecules are held away one from another due to gauche -conformations of the
(CH 2 ) 3 groups. The decrease in the solution temperature results in all- trans con-
formations of the above mentioned groups that brings the charged N + (CH 3 ) 3 groups
close to one to another. The latter condition causes an increase of electrostatic
repulsion and a decrease in H-aggregate stability. This leads to the relative shift of
the opposite dye molecules in dimers, which leads to the change in molecule
packing geometry and hence to the transformation of H-aggregate to J-aggregate.
At the same time, increase in the ionic strength of the solution leads to the screening
of the dye electrostatic charges. This requires smaller distances between the
charged N + (CH 3 ) 3 groups for the H-aggregates to become unstable, and thus
leads to the decrease in the temperature of H- to J-aggregate transition. It should
be noted that the authors regard the H- and J-aggregates of DiSC 3+ (5) on [poly(dI-
dC)] 2 as a series of respective H- and J-dimers interacting one with another. This
interaction is believed to lead to the appearance of another permitted J-aggregate
absorption transition. At the same time, in our opinion, while for H-dimers the
splitting of the spectrum could be explained as the perturbation, in the case of
J-aggregates it would be more correct to regard the series of J-dimers as a single
J-aggregate with several molecules in an elementary cell. Such J-aggregate struc-
ture should lead to the Davydov splitting and thus to the appearance of several
separate bands in absorption spectrum. It should be noted that the aggregates of
DiSC 3+ (5) on [poly(dI-dC)] 2 were also studied in more detail in [ 33 ] experimentally
and in [ 34 ] via computer simulations.
Formation of an aggregate packed as a series of neighboring face-to-face dimers
in DNA groove was suggested for the trimethine cyanine dye L-21 (Fig. 7 ) studied
in [ 35 ]. Absorption spectrum of the above mentioned aggregate consisted of H-
and J- bands, shifted to short- and long-wavelength regions respectively as com-
pared to the monomer one. The aggregate revealed intensive narrow fluorescence
band situated close to the absorption J-band. One more absorption band of L-21,
called the D-band, was attributed to separate J-dimers formed in DNA groove.
The study of pseudoisocyanine absorption, LD and CD, in the presence of DNA
revealed the appearance of new bands in addition to the monomer one in CD and
LD spectra [ 36 ]. Increase in dye to DNA concentration ratio results in the generation
of primary short-wavelength, and then long-wavelength CD and LD bands. The first
one was attributed by authors to the dimer between the partly intercalated pseudoiso-
cyanine molecule and another one bound to it, while the second corresponded to the
J-aggregates of pseudoisocyanine molecules formed in the presence of DNA.
The J-aggregates of bichromophoric dye BCD (Fig. 7 ) formed on DNA were
studied in [ 37 ]. While this dye was shown to be able to form J-aggregates with a
rather broad absorption spectrum in water solution in the presence of NaCl, an
addition of DNA also caused fluorescent J-aggregate formation with the appearance
of narrow absorption band. Besides the fact of formation in the presence of DNA,