Intrinsic DNA fluorescence
Fluorescence studies combined to theoretical computations[11][12][13] and transient absorption measurements[14][15] bring information about the relaxation of the electronic excited states and, thus, contribute to understanding the very first steps of a complex series of events triggered by UV radiation, ultimately leading to DNA damage.
[20][21] The development of such optoelectronic devices for certain applications would have the advantage of bypassing thew step of chemical synthesis or avoiding the uncertainties due to non-covalent biding of fluorescent dyes to nucleic acids.
Due to the weak intensity of the intrinsic DNA fluorescence, specific cautions are necessary in order to perform correct measurements and obtain reliable results.
For measurements using laser excitation, the circulation of the DNA solution by means of a peristaltic pump is recommended; the reproducibility of successive fluorescence signal needs to be checked.
[31][32][33] Due to the fluorescence dependence on the secondary structure, it is possible to follow the formation[34] and the melting[35] of G-Quadruplexes by monitoring their emission; and also to detect the occurrence of hairpin loops in these systems.
[41] A limited number of measurements were also performed with UVA excitation (330 nm), where DNA single and double strands, but not their monomeric units, absorb weakly.
As a result, photons emitted at short wavelengths are reabsorbed by the DNA solution (inner filter effect) and the blue part of the spectrum is truncated.
[57][58][59] Although they discovered the existence of nanosecond components exclusively for multimeric nucleic acids, they failed to obtain a full picture of the fluorescence dynamics.
[77] Among others, the conformational disorder characterizing the nucleic acids modulates the coupling values,[78][79] giving rise to a large number of Franck-Condon states.
[86] The decrease of fluorescence anisotropy observed for all the DNA multimers on the femtosecond time-scale was explained by an ultrafast transfer of the excitation energy among the nucleobases.
[94] Their specificity is that their emission appears at short wavelengths (λ<330 nm) and represents the longest-living components of the overall duplex fluorescence, decaying on the nanosecond timescale.
The contribution of the high energy emitting states to the total fluorescence increases with the local rigidity of the duplex (depending on the number of the Watson-Crick hydrogen bonds or the size of the system) and the excitation wavelength.
The latter point, associated with the very weak spectral width observed for the most representative example (polymeric duplex with alternating guanine-cytosine sequence) is reminiscent of the emission stemming from J-aggregates.
Thus, target DNA in human serum,[97] Pb2+ ions in water,[98] aptamer binding,[99] as well as the interaction of quinoline dyes (commonly used in the food and pharmaceutical industries)[100] were detected.
[105] This potential application could leverage the short wavelength emission of duplexes, associated with collective excited states whose properties are highly sensitive to the geometrical arrangement of the nucleobases.
