Fluorescent J-Aggregates and Their Biological Applications - Advanced Fluorescence Reporters in Chemistry and Biology - page 146

of pure DCP, the J-aggregates formation was observed to be less efficient than in
the case of 1:1 concentrations ratio. The complete absence of J-aggregates for
the pure DMPC vesicles was explained by zwitterionic nature of this lipid. Thus,
the negative charge of the vesicle (brought in by DCP) is important for the cationic
(i.e., positively charged in water solution) dye J-aggregate formation. It should be
noted that upon addition of the dye II to the aqueous solution of the vesicles in
which the dye I J-aggregates were already formed, dye II formed J-aggregates in the
vesicles as well, and resonance excitation energy transfer from dye I J-aggregates to
the dye II J-aggregates was observed.
Aggregation of pseudoisocyanine molecules in the supported lipid bilayers
formed by different lipid molecules was studied in [ 29 ]. In contrast to the results
described in [ 28 ], pseudoisocyanine J-aggregates were shown to be present in
zwitterionic lipid bilayers, while the charged (both positively and negatively)
ones did not induce aggregation [ 29 ]. It should be also noted that depending on
the lipid of which the bilayer consists, J-aggregates of two types were observed,
the I-type J-aggregates having the spectra close to those of pseudoisocyanine
J-aggregate in solution, and those of the II-type being shifted as compared to that
of I-type. Preferentially, J-aggregates of both types were formed in the gel-phase
regions of the bilayers. Moreover, J-aggregate formation was shown to occur at the
center of the gel-phase domain, and to depend substantially on the area per lipid
“head” (i.e., on the distance between the neighbor lipids).
The pseudoisocyanine J-aggregate formation in the lipid bilayer was suggested
to occur as follows. First the dye molecules bind to the bilayer because of electro-
static attraction and because of cation p interaction (i.e., interaction between the

dye -electron cloud and the lipid “head” cation); then reorientation of the lipids

bound to the dyes occurs so that the dye J-aggregate structures are formed. This is
supposed to be due to the energy gain upon dyes stacking, while the reorientation of
lipid molecules does not require energy [ 29 ].

p

5 J-Aggregates Formed on DNA

Many of organic dyes, e.g., the cyanine ones, are widely used for DNA fluorescent
detection [ 25 ]. Such dyes bind to the DNA, either intercalating between the
nucleotide base pairs, or via the groove-binding. They can strongly increase the
fluorescence intensity as a result of fixation of their structure. The dye molecules
often bind to DNA close one to another, forming thus an aggregate. It was
mentioned above that for the cationic dyes in aqueous solution, Coulombic repul-
sion between the dye charges hinders aggregation, while the addition of ions results
in screening of the dye electric charge and thereby promotes aggregation. Hence, as
the DNA sugar-phosphate backbone in aqueous solution is charged negatively,
the aggregation of positively charged dye molecules on negatively charged DNA
(as well as on other charged surfaces [ 17 ]) is accompanied by screening of the dye