The room-temperature fluorescence properties of DNA nucleoside and nucleotide aqueous solutions are studied by steady-state and time-resolved spectroscopy. The steady-state fluorescence spectra, although peaking in the near-UV region, are very broad, extending over the whole visible domain. Quantum yields are found to be mostly higher and the fluorescence decays faster than those reported in the literature. The fluorescence spectra of the 2‘-deoxynucleosides are identical to those of the 2‘-deoxynucleotides, with the exception of 2‘-deoxyadenosine, for which a difference in the spectral width is observed. The steady-state absorption and fluorescence spectra do not show any concentration dependence in the range 5 × 10-6 to 2 × 10-3 M. All fluorescence decays are complex and cannot be described by monoexponential functions. From the zero-time fluorescence anisotropies recorded at 330 nm, it is deduced that after excitation at 267 nm the largest modification in the electronic structure is exhibited by 2‘-deoxyguanosine. In the case of purines, the fluorescence decays and quantum yields are the same for 2‘-deoxynucleosides and 2‘-deoxynucleotides. In contrast, for pyrimidines, the fluorescence quantum yields of nucleotides are higher and the fluorescence decays slower as compared to those of the corresponding nucleosides showing that the phosphate moiety affects the excited-state relaxation.

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It is known that carcinogenic mutations may be caused by chemical reactions triggered by the absorption of ultraviolet light by DNA. [1,2] One of the main factors determining the efficiency of these reactions is evidently the lifetime of the excited states involved. The longer the lifetime, the higher the probability for the excitation energy to be transferred from base to base and reach a reactive site, as happens in photosynthesis. Furthermore, bimolecular reactions involving the excited bases and other nonexcited species become more probable, because the reactants have the time to reach the appropriate configuration by molecular diffusion or other local motions. However, the extremely short lifetimes ( 1 ps) of the lowest excited singlet states of the DNA units (adenine, cytosine, guanine and thymine nucleosides and nucleotides), [3±8] are thought to function as an intrinsic protection against DNA degradation by sunlight. Nevertheless, one would expect that the nature of the excited states and/or the deactivation pathways to be modified when going from individual units to organised multichromophoric systems. In particular, exciton states may be formed, delocalised over several molecular units. Consequently, the lifetime of the excited state of the organised system may be drastically altered.Previous fluorescence measurements performed for single and double-stranded oligonucleotides and polynucleotides have indeed revealed the existence of long-lived components decaying on the nanosecond timescale. [9±12] The major part of the fluorescence could not be resolved because of the limited time resolution.Herein, we report the first fluorescence measurements of DNA oligonucleotides obtained with femtosecond resolution using the upconversion technique. They concern a double-stranded oligomer (dA) 20 ¥ (dT) 20 consisting of twenty adenosine (dA) ± thymidine (dT) base pairs and its constitutive single-stranded oligomers (dA) 20 and (dT) 20 . The choice of this particular system was motivated from the fact that the photophysical properties of double-stranded oligomers, in which adenosine units are located on one single strand and thymidine on the other, have been the subject of several studies. [11±15] Fluorescence decays were recorded at 330 nm, corresponding to the emission maximum of (dA) 20 ¥ (dT) 20 , after excitation at 267 nm. All oligomer fluorescence disappeared more slowly than that of the monomers and, in addition, showed a weak tail persisting at times longer than the studied time window. This long tail was difficult to characterise due to the limited signal-tonoise ratio of our setup for such weak signals. In order to describe the decays in a quantitative way and make some phenomenological comparisons, we fitted the fluorescence decays by biexponential functions to which we added, when necessary, a constant c to account for the persisting long tail: a ¥ exp(À t/t 1 ) b ¥ exp(À t/t 2 ) c. The results of the fits of the decays are shown in Table 1. We have also determined an [17] M. Table 1. Characteristic...

Cytosine methylation, which determines the hot spots for DNA photo-damage, is shown to induce a red-shift of the nucleoside absorption spectrum, making the chromophore more vulnerable to solar radiation, and a tenfold increase of the fluorescence lifetime, making excited statereactions more probable. A femtosecond investigation of the excited state deactivation reveals a quite complex mechanism.

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The present study deals with the photophysical properties of triguanosine diphosphate in aqueous solutions, which are compared with those of the 2'-deoxyguanosine monophosphate. They are studied by steady-state absorption and fluorescence spectroscopy as well as by time-resolved fluorescence spectroscopy with femtosecond resolution. The temperature, salt and concentration dependence of the absorption and fluorescence spectra reveal that association of the trimers takes place. The resulting aggregates could correspond to a tetraplex structure. The aggregate fluorescence quantum yield is higher and the fluorescence lifetime much longer than those of the monomer. These results show the interaction between guanosine residues that may manifest itself via self-solvation, hydrogen bonding and/or delocalization of the excitation.

A series of biochip readers developed for gel-based biochips includes three imaging models and a novel nonimaging biochip scanner. The imaging readers, ranging from a research-grade versatile reader to a simple portable one, use wide-field objectives and 12-bit digital large-coupled device cameras for parallel addressing of multiple array elements. This feature is valuable for monitoring the kinetics of sample interaction with immobilized probes. Depending on the model and the label used, the sensitivity of these readers approaches 0.3 amol of a labeled sample per gel element. In the selective scanner, both the spot size of the excitation laser beam and the detector field of view match the size of the biochip array elements so that the whole row of the array can be read in a single scan. The portable version reads 50-mm long, 150-element, one-dimensional arrays in 5 s. With a dynamic range of 4000:1, a sensitivity of 1-5 amol of a labeled sample per gel element, and a data format facilitating online processing, the scanner is an attractive, inexpensive solution for biomedical diagnostics. Fluorophores for sample labeling were compared experimentally in terms of detection sensitivity, influence on duplex stability, and suitability for multilabel analysis and thermodynamic studies. Texas Red and tetracarboxyphenylporphyn proved to be the best choice for two-wavelength analysis using the imaging readers.