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
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Media type:
E-Article
Title:
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
Description:
<jats:title>Abstract</jats:title><jats:p>The phenanthridinium chromophores 5‐ethyl‐6‐phenylphenanthridinium (<jats:bold>1</jats:bold>), 5‐ethyl‐6‐methylphenanthridinium (<jats:bold>2</jats:bold>), 3,8‐diamino‐5‐ethyl‐6‐methylphenanthridinium (<jats:bold>3</jats:bold>), and 3,8‐diamino‐5‐ethyl‐6‐(4‐<jats:italic>N</jats:italic>,<jats:italic>N</jats:italic>‐diethylaminophenyl)phenanthridinium (<jats:bold>4</jats:bold>) 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 (<jats:bold>E</jats:bold>). In general, this set of data allows the influence of substituents in positions 3, 6, and 8 on the optical properties of <jats:bold>E</jats:bold> to be elucidated. Especially, compound <jats:bold>4</jats:bold> was used to compare the weak electron‐donating character of the phenyl substituent at position 6 of <jats:bold>E</jats:bold> with the more electron‐donating 4‐<jats:italic>N</jats:italic>,<jats:italic>N</jats:italic>‐diethylaminophenyl group. Analysis of all of the measurements revealed two pairs of chromophores. The first pair, consisting of <jats:bold>1</jats:bold> and <jats:bold>2</jats:bold>, lacks the amino groups in positions 3 and 8, and, as a result, these dyes exhibit clearly altered optical and electrochemical properties compared with <jats:bold>E</jats:bold>. 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, <jats:bold>3</jats:bold> and <jats:bold>4</jats:bold>, are similar to <jats:bold>E</jats:bold> due to the presence of the two strongly electron‐donating amino groups in positions 3 and 8. However, in contrast to <jats:bold>1</jats:bold> and <jats:bold>2</jats:bold>, the electron‐donating character of the substituent in position 6 of <jats:bold>3</jats:bold> and <jats:bold>4</jats:bold> is critical. The binding, as well as the fluorescence enhancement, is clearly related to the electron‐donating effect of this substituent. Accordingly, compound <jats:bold>4</jats:bold> 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.</jats:p>