Photo-CIDNP experiment

Photo-CIDNP experiments using anthraquinones as photosensitizers for oxidation of a variety of uracil- and thymine-derived cyclobutane dimers, e.g. c,s-l, t,s-l, c,a-1, t,a-l, and c,s-2, 4, and 5, demonstrated the existence of both Pyr +oPyr and its dissociation product, the monomer radical cation Pyr + [6, 7]. 

The majority of the earlier photo-CIDNP experiments were carried out in a cw spectrometer with the sample subjected to continuous uv irradiation. The first consideration was therefore how to get light into the nmr probe as a relatively high light-absorbed intensity was required to generate the radicals at a... 

Figure 6. Schematic diagram of the irradiation arrangement in photo-CIDNP experiments.

Figure 7. "Temperature effect" in a photo-CIDNP experiment using a high-power light source. The upper trace represents polarization observation without adequate cooling of the sample to offset the heat generated by the light source. The lower trace represents polarization observation with air cooling.

All Photo-CIDNP experiments need an NMR spectrometer and a light source. 

There are two basic variants of photo-CIDNP experiments. 

Despite the fact that CIDNP spectroscopy has reached 40 years of age, novel pulse sequences were devised only recently that greatly improve the performance of these two classes of photo-CIDNP experiments with respect to their specific key problems. 

Figure 8 Principle of a photo-CIDNP experiment without time resolution. Further explanation, see text.

Figure 9 A time-resolved photo-CIDNP experiment. Further explanation, see text. Adapted from Ref. 75 with permission copyright (2006) Taylor 8t Francis Ltd, http / www.tandf.co.uk/journals.

Figure 11 Background suppression by a phase cycle in a photo-CIDNP experiment with gated illumination. The actual pulse sequence, for example that of Figure 10 follows directly after the final delay A2. On the even scans, the receiver phase is inverted. The block in parentheses can be repeated as desired. Further explanation, see text. Adapted from Ref. 73 with permission copyright (2005) Elsevier Inc.

Although the first case puts no demands on the energetics because escape is always a feasible process, no direct observation by a time-resolved CIDNP experiment seems to have been successful so far instead, it has repeatedly been reported that time-resolved photo-CIDNP experiments showed olefin radical cations or anions to be configurationally stable. The second and third cases are obviously only possible if the energy of the olefin triplet or the biradical lies below the energy of the radical ion pair. 

In this way we can treat a flash photo-CIDNP experiment in which G-pairs are formed very rapidly (within 10 7 s of light absorption) by a light pulse of short duration (< 1 ys). This argument can be generalized later to apply to the more conventional steady state experiment, for example with continuous photolysis. 

Tryptophan. A typical photo-CIDNP experiment goes as follows. A solution of the substrate in the presence of 0.2 to 0.4 mM flavin is irradiated in the NMR probe by an argon ion laser (0.6 s, 5 W light pulses) prior to the rf pulse and acquisition of the free induction decay (FID). Alternating "light" and "dark" FID s are taken, which after... 

He attributed the positively enhanced lines In Figure 6b to the active site His 119 on the basis of Its known chemical shifts (Markley, 1975a). This Is nicely confirmed by a photo-CIDNP experiment In the presence of the competitive Inhibitor cytldlne 2 -monophosphate, which Is known to bind with Its phosphate group to His 119. In figure 7 the normal photo-CIDNP difference spectrum of 1.5 mH RNase A Is compared with that In the presence of 8 mM 2 -CMP. It can be seen In Figure 7b that the positive lines have disappeared from the spectrum. Thus, the Inhibitor blocks accessto His 119 or otherwise Inactivates this residue towards the photoreaction. Since histidines play an In rtant role In the catalytic mechanism of many enzymes, the enhancement of histidine resonances Toy CIDNP Is of particular Interest. 

Ru(tap)3] and [Ru(tap)2(phen)] with guanosine-5-monophosphate or N-acetyl-tyrosine gives rise to photo-CIDNP signals [125], that is, non-Boltzmann nuclear spin state distributions that has been detected by NMR spectroscopy as enhanced absorption or emission signals. However, the interpretation framework must be confirmed. In order to validate the experimental predictions. Density Functional Theory (DFT) calculations can be performed. These calculations are based on the determination of the electronic structure of the mono-reduced form of Ru(II) complexes in gas phase and aqueous solution. Recently, some of us showed that the electron spin density and the isotropic Fermi contact contribution to the hyperfine interactions with the nuclei agree remarkably well with the observed photo-CIDNP enhancements [34]. Thus, combined photo-CIDNP experiments and DFT calculations open up new important perspectives for the study of polyazaaromatic Ru(II) complexes photoreactions. 

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