No abstract

Ultraviolet and infrared absorption spectroscopy, in aqueous and non-aqueous media, have been employed to study the tautomerism of 9-substituted isoguanines, including the nucleoside iso­ guanosine. With the aid of a series of model compounds, it was shown that 9-substituted isogua­ nines, and isoguanosine, in aqueous medium are predominantly in the form N(1)H,2-keto-6-amino. In dioxane solution the tautomeric equilibrium is shifted in the direction of the enol form. The shift towards this form is accentuated for those analogues in which the exocyclic amino group is methylated. With the aid of N6,N6,9-trimethylisoguanine, and its 9-octyl analogue, the tautomeric constant was studied as a function of concentration, temperature, and solvent polarity, and the results applied to evaluate the tautomeric equilibria of 9-methylisoguanine and isoguanosine as a function of these variables. In general the enol form is favoured by a decrease in solvent polarity, by a decrease in concentration in dioxane, or an increase in temperature in chloroform solution. Syntheses are described for several N6-amino and methylamino derivatives of 2-methoxy-9-methylpurine, and 3-methyl-5-oxo-7,8-dihydroimidazo (2,1-i) purine, which served as an analogue of the unavailable 1,9-dimethylisoguanine.

Tautomerization is a fundamental process in chemistry and biology, where it plays a major role in vision and enzymatic reactions. Usually, extensive spectroscopic ensemble studies are required to identify a tautomeric equilibrium. For instance, indirect evidence for the fast motion of the two inner hydrogen atoms between the nitrogen atoms has been deduced for porphycene, 1 a constitutional isomer of porphyrin, from complex NMR 1,2 and fluorescence spectroscopy 3,4 studies in a solid host for both ground and excited singlet states.A fascinating aspect of single-molecule spectroscopy (SMS) is its capability to detect a chemical reaction in a single chromophore. This has been reported, for instance, for electron transfer, 5 conformational changes, 6 or photochromism. 7 These studies recorded variations in fluorescence decay curves, fluorescence intensity trajectories, or fluorescence spectra. The fast exchange of the hydrogens in porphyrin-like molecules, however, leads to a dynamic equilibrium of chemically equivalent structures and hence cannot be inferred from lifetime or spectral measurements. On the other hand, the two structures differ in the orientation of the transition dipole moments of the S 0 -S 1 transition. As determined from bulk fluorescence anisotropy studies, 4 the S 0 -S 1 transition moments in the two trans forms enclose an angle of about 70-80°( Figure 1A). We will exploit this information in the interpretation of the intensity patterns of fluorescence from single porphycene molecules.Single-molecule transition dipole moments have been used previously to probe the electric field distribution in the focal spot of a confocal microscope. [8][9][10] In this report, we show that the analysis of single-molecule fluorescence patterns in real space excited by an azimuthally polarized laser beam ( Figure 1B) provides an important alternative tool for analyzing structural changes in the large class of compounds where fluorescence resonance energy transfer (FRET), fluorescence decay curves, or fluorescence spectra cannot be used. Porphyrins and porphycenes are important representatives of such a reaction. We show for the first time the direct visualization of the two tautomers resulting from double hydrogen transfer in a single molecule of porphycene. Samples of porphycene immobilized in a 10-20-nm-thick PMMA layer have been prepared by spin coating of a 10 -9 M solution in toluene onto a quartz cover slide. Two-dimensional (2D) spatial images of the fluorescence intensities of individual molecules were obtained by raster scanning the sample in 20-nm steps per 5 ms. The fluorescence intensity is proportional to |p‚E| 2 , the square of the projection of the electric field E on the transition dipole moment p. Hence, the observed patterns reveal the orientations of p 8,9 and can be compared with numerical simulations taking into account the experimental parameters of our microscope.We observed a total of 66 spatially separated single-molecule patterns on one sample that revealed characteristic blinking and ...

No abstract

Laser-induced fluorescence and dispersed fluorescence spectra measured in supersonic jets for 9,10,19,20-tetra-n-methylporphycene and 9,10,19,20-tetra-n-propylporphycene reveal, for both compounds, the presence of two different species which are assigned to trans and cis tautomeric forms. Doublet splitting of lines is observed, disappearing upon deuteration of the inner nitrogen atoms. This finding is interpreted as an indication of double hydrogen tunneling. The values of tunneling splitting are obtained for both ground and lowest singlet excited states. The splitting is similar for cis and trans forms, and the barrier for tautomerization is larger in the excited state. Due to the coupling of hydrogen motion with rotation of alkyl substituents, tautomerization occurs in an asymmetric double minimum potential, with the ordering of energy minima reversed upon excitation. The second singlet excited state is found to lie very close to S(1), thus facilitating an efficient radiationless depopulation.

No abstract

No abstract

No abstract