Porphyrins guanine

H-atom abstraction has been demonstrated to be the mechanism of action of excited uranyl ions, and in this case negligible base oxidation is found. Nucleo-base (especially guanine) oxidation is the principal reaction caused by singlet oxygen and this reactive species can be generated by a number of the complexes (e.g. many Ru(II)polypyridyls and porphyrins). It is worth pointing out, however, that the yield of may be lower when the sensitiser is bound to DNA, and it is the authors view that some of the reactions claimed to proceed via 62 may be caused by direct reaction of the photo-oxidised sensitiser with the DNA. 

The tautomerism of functional derivatives of azines (e.g., pyridones) and other more complex systems (guanines, porphyrins, etc.) has been much studied both experimental and theoretically [40, 76-82], Our contribution to this field has been modest using a combination of 13C and 15N NMR in solution and in the solid state, together with GIAO//B3LYP/6-311++G calculations, the tautomerism of 2-pyri-done, 4(37/)-pyrimidone, uracil, and cytosine were determined [83],... 

Type B PCET systems of Fig. 17.3 may also be assembled using the three-point hydrogen bond of Watson-Crick base pairs such as guanine (G) and cytosine (C). Sessler and coworkers provided the first example of this assembly with 5, for which only energy transfer is observed [101,102]. A Type B PCET is realized when the cytosine of the GC base pair is appended with an Ae functional group. In 6, a Zn porphyrin serves as De and p-benzoquinone as Ae [103]. Time resolved fluorescence quenching experiments reveal that the rate of ET across the GC interface... 

Purine bases have received much less interest from CIDNP spectro-scopists [135]. The photoreaction of guanine with synthetic water-soluble porphyrins was addressed in [141]. Besides an earlier study of the single-stranded region of tRNA [142], the only reported examples of CIDNP from the corresponding biopolymers are two investigations of oligonucleotide duplexes [143]. 

Porphyrins substituted with a guanine side chain interact in aprotic solvents or liquid crystals with cytosine-substituted quinones or another porphyrin to form a 1 1 lateral heterodimeric system (Scheme 6.6.2). Such pairs showed photoinduced charge separations with lifetimes of a few microseconds (Sessler et al, 1995). 

An amusing nucleic acid derivative is also known a PEG-solubilized zinc porphyrin with an guanine side chain combines with a cytosine derivative of benzoquinone (Scheme 7.2.8). Electron transfer occurs in dichloromethane solutions (4 X 10 s ), but lifetime of the charge-separated pair is short (10 s) (Harri-man et al., 1992 Sessler et al., 1993). 

Guanine and guanosine are, of course, commercially available and can be derivatized in numerous ways. Along these lines, Sessler has used [32] a palladium-catalyzed Stille coupling to connect a porphyrin system to 8-bromoguanosine for use in a self-assembling, photosynthetic model system... 

The Mn =0 entity of manganese porphyrin is a powerful chemical oxidant capable of abstracting electrons from guanine. However, the products of the reaction clearly result from a two-electron oxidation mechanism. 

The use of the classical guanine-cytosine (G-C) Watson-Crick base pairing to form heterocomposite porphyrin systems has been established for some time, through the early work of the Sessler [30-32], Hamilton [33,34] and... 

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