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.
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.
Prunkl
Christa
C
Institute for Organic Chemistry, University of Regensburg, Universitätstrasse 31, 93053 Regensburg, Germany.
Pichlmaier
Markus
M
Winter
Rainer
R
Kharlanov
Vladimir
V
Rettig
Wolfgang
W
Wagenknecht
Hans-Achim
HA
D001554:Benzene / Q000737:chemistry*; D001665:Binding Sites; D004247:DNA / Q000737:chemistry*; D004996:Ethidium / Q000737:chemistry*; D005456:Fluorescent Dyes / Q000737:chemistry*; D010084:Oxidation-Reduction; D010617:Phenanthridines / Q000138:chemical synthesis / Q000737:chemistry*; D011789:Quantum Theory; D013056:Spectrophotometry, Ultraviolet
D005456:Fluorescent Dyes; D010617:Phenanthridines; C014740:phenanthridinium; D004247:DNA; D004996:Ethidium; D001554:Benzene