Guanine nonplanarity

Razskazovskiy et al. employ ESR spectroscopy at low temperatures to investigate electron transfer within brominated DNA [8]. The brominated DNA base electron traps were introduced by bromination of DNA in ice-cooled aqueous solution. The procedure is shown by NMR and GC/MS techniques to modify thymine, cytosine, and guanine, transforming them into 5-bro-mo-6-hydroxy-5,6-dihydrothymine, T(OH)Br, 5-bromocytosine, CBr, and 8-bromoguanine, GBr, derivatives. The bromination products formed in molar ratio close to T(OH)Br/CBr/GBr 0.2 1 0.23 and serve as internal electron scavengers on y-irradiation. Structurally the CBr and GBr are planar, but T(OH)Br is quite nonplanar with the bromine directly above the molecular plane. This disrupts the DNA base stack. Paramagnetic products that result... 

The S1(hh ) excited state geometry of the keto-N9H tautomer of guanine at the CIS/6-31 lG(d,p) level was found to be strongly nonplanar around the C6N1C2N3... 

An interesting feature of the gas-phase guanine tautomers is that their amino group is nonplanar. The sum of the angles about the amine nitrogen is less than 348° for the tautomers 14a-i, and specifically 340.4° and 337.4° in 14a and 14b, respectively. This is more pyramidal than for any of the other nucleic acid bases. 

The examples of hydroxyl radical addition products identified in single crystals of adenine and guanine base derivatives confirm that water can play an important role in the radiation damage mechanism. It should be noted that the new mechanism has attracted some criticism [38]. The main objection is that nonplanar geometries were calculated for the gas phase radicals whereas planar structures are expected experimentally due to the hydrogen-bonding scheme... 

Nonplanarity of the guanine amino group is substantially more pronounced than the nonplanarity of adenine and cytosine. 

Let us start the analysis of the influence of the hydration on the nonplanarity phenomena from the nonplanarity of the first type. There are two important observations here. First of all one may see that the nonplanarity of the first type (sp- hybradization of the amino group) does not depend significantly either on chemical structure of the tautomers or on the interaction with water molecules in the case of guanine and cytosine. 

Guanine, in addition to being the largest NAB, also has the most complex tautomeric equilibria these have been the subject of a number of ab initio studies. " At least three different tautomers have been observed in IR isolated matrix experiments. Furthermore, the amino tautomers of guanine exhibit the most pronounced nonplanarity among the NABs. 

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