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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
Author(s) -
Prunkl Christa,
Pichlmaier Markus,
Winter Rainer,
Kharlanov Vladimir,
Rettig Wolfgang,
Wagenknecht HansAchim
Publication year - 2010
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200902823
Subject(s) - chromophore , ethidium bromide , substituent , chemistry , fluorescence , titration , dna , photochemistry , quenching (fluorescence) , redox , stereochemistry , organic chemistry , biochemistry , physics , quantum mechanics
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.