Squaraine Dyes Derived from Indolenine and Benzo[e]indole as Potential Fluorescent Probes for HSA Detection and Antifungal Agents

Abstract

Four squaraine dyes derived from 2,3,3-trimethylindolenine and 1,1,2-trimethyl-1H-benzo[e]indole with different combinations of barbituric groups attach to the central ring, having ester groups and alkyl chains in the nitrogen atoms of heterocyclic rings were synthesized. These dyes were fully characterized, and their photophysical behavior was studied in ethanol and phosphate-buffered solution. Absorption and emission bands between 631 and 712 nm were detected, with the formation of aggregates in aqueous media, which is typical of this class of dyes. Tests carried out with 1,3diphenylisobenzofuran allowed us to verify the ability of the dyes to produce singlet oxygen. The interaction of synthesized dyes with human serum albumin (HSA) was also evaluated, being demonstrated a linear correlation between fluorescence intensity and protein concentration. The antifungal potential of the dyes against the yeast Saccharomyces cerevisiae was evaluated using a broth microdilution assay. In order to test the photosensitizing capacity of the synthesized dyes, tests were carried out in the dark and with irradiation, using a custom-built light-emitting diode that emits close to the absorption wavelength of the studied dyes. The results showed that the interaction of dyes with HSA and the antifungal activity depends on the different structural modifications of the dyes.

Evaluation of Antifungal Activity and Potential Application as Fluorescent Probes of Indolenine and Benzo[e]Indole‐Based Squarylium Dyes

“…Once again, the results for these dyes are very similar to each other, highlighting the dye SQ3 with a decrease in DPBF absorbance of more than 80%. In aqueous medium, the SQ3 dye obtained a result similar to that already reported for the SQ8 dye (25), being the dyes most efficient in terms of singlet oxygen production.…”

“…was plotted in an attempt to obtain the dissociation constant and the Hill constant, which was not possible due to the fact that the graph (Fig. 7d) obtained is not linear, as already reported in other articles by some of us (23–25). However, it is possible to draw other conclusions about the mode of binding of the dye to the protein that is related to the various binding sites that HSA makes available for the dye.…”

“…This dye demonstrated a light response similar to the benzo[ e ]indole‐derived dye SQ6 and indolenine derived dye SQ8 whose photostability was already reported by Gomes et al . (25). The lower photostability is attributed to dye SQ1 and SQ4 , which was almost completely degraded, losing their ability to emit color, as observed for dyes SQ5 and SQ9 .…”

“…1 and their synthesis has already been reported by our research group (22)(23)(24)(25). The activity as antifungal agents against Saccharomyces cerevisiae of some of the presented dyes (SQ5, 6 and SQ8, 9) has also been evaluated in previously published studies (25). Thus, in this work, the evaluation of antifungal activity of dyes SQ1-4 and SQ7 against S. cerevisiae yeast will be carried out, as well as other complementary studies such as photosensitivity and singlet oxygen production, in order to be able to compare the different response caused by changes in the structure of the dyes.…”

“…The dyes presented in this work are shown in Fig. 1 and their synthesis has already been reported by our research group (22–25). The activity as antifungal agents against Saccharomyces cerevisiae of some of the presented dyes ( SQ5 , 6 and SQ8 , 9 ) has also been evaluated in previously published studies (25).…”