Optical, Redox, and DNA‐Binding Properties of Phenanthridinium Chromophores: Elucidating the Role of the Phenyl Substituent for Fluorescence Enhancement of Ethidium in the Presence of DNA

Abstract

The phenanthridinium chromophores 5-ethyl-6-phenylphenanthridinium (1), 5-ethyl-6-methylphenanthridinium (2), 3,8-diamino-5-ethyl-6-methylphenanthridinium (3), and 3,8-diamino-5-ethyl-6-(4-N,N-diethylaminophenyl)phenanthridinium (4) were characterized by their optical and redox properties. All dyes were applied in titration experiments with a random-sequence 17mer DNA duplex and their binding affinities were determined. The results were compared to well-known ethidium bromide (E). In general, this set of data allows the influence of substituents in positions 3, 6, and 8 on the optical properties of E to be elucidated. Especially, compound 4 was used to compare the weak electron-donating character of the phenyl substituent at position 6 of E with the more electron-donating 4-N,N-diethylaminophenyl group. Analysis of all of the measurements revealed two pairs of chromophores. The first pair, consisting of 1 and 2, lacks the amino groups in positions 3 and 8, and, as a result, these dyes exhibit clearly altered optical and electrochemical properties compared with E. In the presence of DNA, a significant fluorescence quenching was observed. Their binding affinity to DNA is reduced by nearly one order of magnitude. The electronic effect of the phenyl group in position 6 on this type of dye is rather small. The properties of the second set, 3 and 4, are similar to E due to the presence of the two strongly electron-donating amino groups in positions 3 and 8. However, in contrast to 1 and 2, the electron-donating character of the substituent in position 6 of 3 and 4 is critical. The binding, as well as the fluorescence enhancement, is clearly related to the electron-donating effect of this substituent. Accordingly, compound 4 shows the strongest binding affinity and the strongest fluorescence enhancement. Quantum chemical calculations reveal a general mechanism related to the twisted intramolecular charge transfer (TICT) model. Accordingly, an increase of the twist angle between the phenyl ring in position 6 and the phenanthridinium core opens a nonradiative channel in the excited state that depends on the electron-donating character of the phenyl group. Access to this channel is hindered upon binding to DNA.

Restriction of Photoinduced Twisted Intramolecular Charge Transfer

“…Different from the common concepts of intramolecular rotation/twisting and intramolecular charge transfer (ICT), TICT is an unusual photophysical phenomenon, usually defined by a molecular rotamerism accompanied by intramolecular separation of charges. [27] Investigations on TICT were usually performed in molecularly dispersed solutions, [28][29][30][31][32][33][34] but the photophysics involved in aggregates or in the solid state are scarcely investigated. In addition, the mechanism of restriction of photoinduced TICT was seldom reported in AIE or AIEE organic systems.…”

Biophysical studies of mutated K562 DNA (erythroleukemic cells) binding to adriamycin and daunomycin reveal that mutations induce structural changes influencing binding behavior

“…There is also the possibility of grove binding where electrostatic, hydrogen bonding, or hydrophobic interactions are involved in bringing the drug molecules closer to the sugar phosphate backbone. These interactions also lead to decrease in fluorescence intensity and in turn reduction in K SV values (Li, Xu, Guo, Zhu, & Zhao, 1997;Prunkl et al, 2010). The electron transfer property of anthracyclines (Chaires, 1996) may also be one reason for decrease in fluorescence intensity when bound to DNA.…”

Expanding the Palette of Phenanthridinium Cations

“…[7] The fluorescence of the 3,8-diamino-6-aryl derivatives, such as ethidium bromide (1) and propidium iodide (2), is hugely enhanced upon intercalation, and both the amino groups and the 6-aryl substituent contribute significantly to this effect ( Figure 1). [8] As a result, in spite of its toxicity, 1 is widely used in molecular biology laboratories to detect DNA on agarose gels following electrophoresis, [9] and its displacement from DNA is often used to assess the binding of novel ligands to DNA. [10] Propidium iodide is also a valuable DNA stain employed in flow cytometry and histochemistry.…”