The photophysical properties of stacked G-tetrads in diverse systems, including concentrated solutions of 59-guanosine monophosphate (59-GMP), polyguanylic acid (poly(G)) and the G-rich oligodeoxynucleotide sequence characteristic of human telomeric DNA, are probed by ps-TRIR and compared to those of the monomeric 59-GMP.Electronic excited states of individual nucleic acid bases and nucleotides are typically characterised by ultrafast relaxation processes (,1 ps). 1-3 However, in higher order structures, such as polymeric strands and aggregated species, longer-lived transients are observed, 2a and these present a means to promote chemical modifications leading to genetic mutation. The proposed mechanisms for these extended lifetimes include base-stacking interactions, excimer/exciplex formation and hydrogen bonding. 4 To fully understand the nature of the molecular specific interactions, it is essential to employ techniques that can interrogate the structural modifications underpinning these processes, and here we apply ultrafast time-resolved infrared spectroscopy (TRIR) for this purpose. ps-TRIR is now being demonstrated as a powerful technique for probing the photophysical properties of DNA, [5][6][7][8] and its strength stems from its ability to yield kinetic and structural information that goes beyond that obtainable from fluorescence or transient UV/visible absorption techniques.In this communication, we describe how ps-TRIR can be employed to investigate conformationally dependent photophysical properties of base-base interactions. Guanine is known to aggregate to form stacked species at high concentrations. 9 It is also interesting as this base absorbs more strongly at longer wavelengths and is the most sensitive of the nucleobases to singlet oxygen and photodynamic oxidation. In this paper, we initially consider 59-GMP at concentrations where it is (a) monomeric and (b) stacked. These results are then compared to the polymer poly(G) (which is known to spontaneously form structures composed of stacked guanine tetrads) and, finally, to the biologically relevant G-rich telomeric sequence dAGGG(TTAGGG) 3 which is vital for protection against genome degradation and plays an important role in aging and cancer. 10 Fig . 1a shows the ps-TRIR spectra of a neutral solution of 59-GMP (10 mM, pH 7.0) at various delays after excitation with 267 nm radiation. The spectrum contains regions of transient absorption and regions where depletion ('bleaching') of the ground state bands is dominant. The 59-GMP bleaches occur at 1581 and 1669 cm 21 and are attributed predominantly to ground state ringbased and carbonyl vibrations respectively. 11 The shifting of the transient absorption bands to higher wavenumbers in the first y5 ps is consistent with cooling of the vibrationally hot ground state following electronic excitation. 5 This band-shifting coupled with spectral overlap of the ground state and transient bands means that kinetics obtained from intensity changes at a single pixel position produces a distribution of dec...

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The transient species formed following excitation of fac-[Re(CO)(3)(F(2)dppz)(py)](+) (F(2)dppz = 11,12-difluorodipyrido[3,2-a:2',3'-c]phenazine) bound to double-stranded polynucleotides [poly(dA-dT)](2) or [poly(dG-dC)](2) have been studied by transient visible and infra-red spectroscopy in both the picosecond and nanosecond time domains. The latter technique has been used to monitor both the metal complex and the DNA by monitoring the regions 1900-2100 and 1500-1750 cm(-1) respectively. These data provide direct evidence for electron transfer from guanine to the excited state of the metal complex, which proceeds both on a sub-picosecond time scale and with a lifetime of 35 ps, possibly due to the involvement of two excited states. No electron transfer is found for the [poly(dA-dT)](2) complex, although characteristic changes are seen in the DNA-region TRIR consistent with changes in the binding of the bases in the intercalation site upon excitation of the dppz-complex.

Photolysis of dipyrido-[3,2-a:2',3'-c]-phenazine (dppz) (1) in ethanol solution leads to the formation of 9,14-dihydrodipyridophenazine (2), which has been characterised by detailed NMR analysis, UV/VIS absorption spectroscopy, and theoretical calculations which reveal that its red colour is due to a low-lying intramolecular charge transfer state.

The photophysical properties of 5¢-guanosine monophosphate (5¢-GMP) and polyguanylic acid {poly(G)} in D 2 O solutions of varying pH have been studied using picosecond transient infrared absorption spectroscopy. Whereas in neutral or weakly alkaline solution only the vibrationally excited electronic ground state of 5¢-GMP is observed, in acidic solution the relatively long-lived (229 ± 20 ps) electronic excited state of protonated 5¢-GMP, which possesses strong absorptions at 1517 and 1634 cm -1 , could be detected. The picosecond transient behaviour of polyguanylic acid in acidic solution is also very different from that of the polynucleotide in neutral solution due not only to the protonation of guanine moieties yielding the protonated excited state but because of the disruption of the guanine stacks which are present in the species in neutral solution.