5-(Pyren-1-yl)-2'-deoxyuridine (PydU) and 5-(Pyren-1-yl)-2'-deoxycytidine (PydC) were used as model nucleosides for DNA-mediated reductive electron transport (ET) in steady-state fluorescence and femtosecond time-resolved transient absorption spectroscopy studies. Excitation of the pyrene moiety in PydU and PydC leads to an intramolecular electron transfer that yields the pyrenyl radical cation and the corresponding pyrimidine radical anion (dU.- and dC.-. By comparing the excited state dynamics of PydC and PydU, we derived information about the energy difference between the two pyrimidine radical anion states. To determine the influence of protonation on the rates of photoinduced intramolecular ET, the spectroscopic investigations were performed in acetonitrile, MeCN, and in water at different pH values. The results show a significant difference in the basicity of the generated pyrimidine radical anions and imply an involvement of proton transfer during electron hopping in DNA. Our studies revealed that the radical anion dC.- is being protonated even in basic aqueous solution on a picosecond time scale (or faster). These results suggest that protonation of dC.- may also occur in DNA. In contrast, efficient ET in PydU could only be observed at low pH values (< 5). In conclusion, we propose--based on the free energy differences and the different basicities--that only dT.- but not dC.- can participate as an intermediate charge carrier for excess electron migration in DNA.

Fluorescent or luminescent probes that are sensitive to the local environment within DNA duplexes represent important tools for DNA hybridisation [1] and conformational changes caused by DNA±protein interactions, [1] or for the detection of physiologically important DNA base mismatches or lesions on DNA chips or microarrays.[2] As a consequence, there is a continuously increasing demand for new fluorophores that have a clear and specific range of spectral characteristics which are tunable to distinct excitation or emission wavelengths. One suitable and important way to create new emission properties is to attach chromophores covalently to natural DNA bases. Recently, we applied this modification strategy to the preparation of photoexcitable charge donors, which have been used for the investigation of DNA-mediated electron transport. [3,4] Herein, we report the properties of DNA duplexes bearing the 1-ethynylpyrene moiety (PyÀCC) covalently attached to the bases dX = dA, dC, dG, or dU. Three structural features of these PyÀCCÀdX-modified DNA duplexes are important: i) a clear steric separation of the pyrene moiety from the DNA base stack due to the rigid ethynyl group, ii) a strong electronic coupling between the pyrene and the base moiety provided by the acetylene bridge and iii) a partial stacking of the base moiety as part of the delocalised PyÀCCÀdX chromophore. Moreover, the incorporation of the PyÀCCÀdX moiety could influence only the local conformation, but should not perturb the overall B-DNA duplex conformation.The PyÀCCÀdX-modified oligonucleotides were synthesised by a semiautomated synthetic strategy with solid-phase Sonogashira-type cross-coupling conditions (Scheme 1). [4,5] It is important to note that a time-consuming synthesis of PyÀCCÀ modified phosphoramidites [6] can be avoided because this modification protocol is based on commercially available DNA building blocks. First, the oligonucleotide was synthesised by following standard protocols on a DNA synthesiser up to the position of the PyÀCCÀdX unit. At this position, either 8-bromo-2'-deoxyadenosine, 2'-deoxy-5-iodocytidine, 8-bromo-2'-deoxyguanosine, or 2'-deoxy-5-iodouridine was inserted automatically without the final deprotection of the terminal 5'-OH group. Subsequently, the CPG vials were removed from the synthesiser and a Sonogashira-coupling reagent mixture containing Pd(PPh 3 ) 4 (60 mm), 1-ethynylpyrene (120 mm) and CuI (60 mm) in DMF/Et 3 N (3.5:1.5) was added to the CPG vials under dry conditions with syringes. After a coupling time of 3 h at room temperature, the CPGs were washed with different solvents, dried and attached to the DNA synthesiser to finish the synthesis automatically. Modification of the standard procedures for deprotection and cleavage of the oligonucleotides from the solid phase, or during workup was not necessary. The PyÀCCÀdX-modified oligonucleotides were purified by semipreparative HPLC and identified by MALDI-TOF mass spectrometry. HPLC analysis of the unpurified oligonucleotides showed excellent coupling ...

Preparation of Pyrene-Modified Purine and Pyrimidine Nucleosides via Suzuki-Miyaura Cross-Couplings and Characterization of Their Fluorescent Properties. -(MAYER, E.; VALIS, L.; HUBER, R.; AMANN, N.; WAGENKNECHT*, H.-A.; Synthesis 2003, 15, 2335-2340; Inst. Org. Chem. Biochem., TU Muenchen, D-85747 Garching, Germany; Eng.) -Mais 11-188

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