Fluorescent J-Aggregates and Their Biological Applications - Advanced Fluorescence Reporters in Chemistry and Biology - page 153

concentration led to the reversion of the CD spectrum i.e., to the reversion of the
induced chirality of an aggregate. This was explained by the PTC J-aggregates
binding to HSA regions of different secondary structure, namely

-helix and
random-coil. The authors demonstrated the same chirality reversion to take place
for PTC in the presence of poly- L -glutamate at pH values 2.92 and 8.08 that was
explained by the poly- L -glutamate existence in

a

-helix and random-coil conforma-
tions respectively. Thus, it was concluded that increasing the HSA concentration
led to increase of the relative content of PTC J-aggregates interacting with the
random coil HSA regions. It was thus concluded that the interaction of J-aggregates
with the

a

-helix regions of HSA was less stable as compared to that with the
random-coil one. It should be noted that the CD spectrum of the J-aggregate
induced by HSA addition was also observed for MTC and ETC dyes differing
from PTC by the meso-substitution group (methyl and ethyl respectively instead
of phenyl one) [ 43 ]. But in the case of MTC and ETC, no chirality reversion was
observed indicating that these dyes form J-aggregates only with the random-coil
regions of HSA.
The porphyrin meso -tetrakis( p -sulfonatophenyl) porphine (TSPP) (Fig. 10 ) was
shown to demonstrate J-aggregates at acidic pH values lower than the p K a of the
porphyrin protonation (p K a ¼

a

4.8). Such protonation gives to TSPP molecules two
positive charges in addition to already existing four negative ones. Such strongly
zwitterionic nature of the porphyrin is believed to cause its aggregation in aqueous
solution upon increasing its concentration because of the electrostatic interac-
tions between negative and positive charges of the molecules, when the positively
charged nitrogen of one molecule is adjacent to the negatively charged sulphonate
group of another one [ 44 ]. At the same time, addition of HSA and -lactoglobulin

(BLG) proteins to the aqueous solution of nonaggregated TSPP at pH

b

2 also
causes the appearance of the J-aggregate spectra, the Soret band being shifted from
434 to 490 nm, and the longest wavelength Q-band from about 650 to 704 nm. The
J-aggregate bands were most intense at the porphyrin to protein concentration ratios
40 for HSA and 2 for BLG, while for the same ratios near 4 and 1 respectively the
J-aggregate bands disappear, being transformed to monomer emission. Therefore,
the observed J-aggregates were formed on the protein structure.
It is interesting that TSPP J-aggregates formed on both proteins revealed strong
CD signal in the region of Soret band (490 nm), but the shapes of these CD bands
significantly differed in the cases of HSA and BLG, pointing to the difference in
J-aggregate packing on these two proteins [ 44 ]. The exciton delocalization length
for TSPP J-aggregate formed on HSA was estimated to be about 6-8 porphyrin
molecules [ 45 ]. The study of the influence of the ionic strength on the J-aggregate
formation reveals the electrostatic nature of aggregation [ 44 , 45 ]. That is, the TSPP
molecules bearing two positive and four negative charges each, attract to each other
and to the protein that is positively charged at acidic pH [ 45 ]. J-aggregates of TSPP
were also shown to be formed on bovine serum albumin (BSA) and lysozyme at
pH

¼

1.3 [ 46 ]. The model for TSPP J-aggregate formation on BSA was proposed
in [ 47 ], though no assumptions about the site of their formation (e.g.,

¼

a -helix,

b -sheet, or random coil region) were made.