Influences of protonation and hydrogen bonding on intramolecular charge transfer compounds possessing different spacers

8-(p-CF3-cinnamyl)-modified purine nucleosides as promising fluorescent probes

Natural nucleotides are not useful as fluorescent probes because of their low quantum yields. Therefore, a common methodology for the detection of RNA and DNA is the application of extrinsic fluorescent dyes coupled to bases in oligonucleotides. To overcome the many limitations from which fluorescent nucleotide-dye conjugates suffer, we have developed novel purine nucleosides with intrinsic fluorescence to be incorporated into oligonucleotide probes. For this purpose we synthesized adenosine and guanosine fluorescent analogues 7-25, conjugated at the C8 position with aryl/heteroaryl moieties either directly, or via alkenyl/alkynyl linkers. Directly conjugated analogues 7-14, exhibited high quantum yields, φ >0.1, and short λ(em) (<385 nm). Alkynyl conjugated analogues 22-25, exhibited low quantum yields, φ <0.075, and λ(em)<385 nm. The alkenyl conjugated analogues 15-21, exhibited λ(em) 408-459 nm. While analogues 15,16, and 20 bearing an EDG on the aryl moiety, exhibited φ <0.02, analogues 17, and 21 with EWG on the aryl moiety, exhibited extremely high quantum yields, φ ≈ 0.8, suggesting better intramolecular charge transfer. We determined the conformation of selected adenosine analogues. Directly conjugated analogue 8 and alkynyl conjugated analogue 22, adapted the syn conformation, whereas alkenyl conjugated analogue 15 adapted the anti conformation. Based on the long emission wavelengths, high quantum yields, anti conformation and base-paring compatibility, we suggest analogues 17 and 21 for further development as fluorescent probes for the sensitive detection of genetic material.

Intermolecular hydrogen bond complexes by in situ charge transfer complexation of o-tolidine with picric and chloranilic acids

A two new charge transfer complexes formed from the interactions between o-tolidine (o-TOL) and picric (PA) or chloranilic (CA) acids, with the compositions, [(o-TOL)(PA)(2)] and [(o-TOL)(CA)(2)] have been prepared. The (13)C NMR, (1)H NMR, (1)H-Cosy, and IR show that the charge-transfer chelation occurs via the formation of chain structures O-H⋯N intermolecular hydrogen bond between 2NH(2) groups of o-TOL molecule and OH group in each PA or CA units. Photometric titration measurements concerning the two reactions in methanol were performed and the measurements show that the donor-acceptor molar ratio was found to be 1:2 using the modified Benesi-Hildebrand equation. The spectroscopic data were discussed in terms of formation constant, molar extinction coefficient, oscillator strength, dipole moment, standard free energy, and ionization potential. Thermal behavior of both charge transfer complexes showed that the complexes were more stable than their parents. The thermodynamic parameters were estimated from the differential thermogravimetric curves. The results indicated that the formation of molecular charge transfer complexes is spontaneous and endothermic.