Geometry guanine: cytosine

What would be the geometry of the guanine-cytosine complex were there no hydrogen bonds or, more to the point, were chemists not to know about the benefits of hydrogen bonding More than likely, it would be a more closely-packed geometry, such as the following. 

Figure 25-22 Hydrogen bonding postulated between DNA strands involving guanine-cytosine and adenine-thymine. In each, case, the distance between the C1 of the two deoxyribose units is 11 A and the favored geometry has the rings coplanar.

The implication is that under the conditions of deep UV irradiation, which likely existed on the early earth, selective chemistry may have taken place in favor of species with the shortest excited state lifetimes. Benner and coworkers, for example, have proposed a molecular lexicon of 12 alternate bases that can produce 6 base pairs with virtually identical geometries as the guanine-cytosine (GC) triply hydrogen bonded base pair [1], Several of these alternate bases have been observed as products in simulation experiments that test the feasibility of synthesis of such compounds under primitive conditions [77, 78], This raises the... 

The anionic Watson-Crick guanine-cytosine base pair behaves in a different manner [75], Namely, out of several possible configurations, differing with the position of proton(s), the geometry with proton transferred from the N1 atom of guanine to the N3 atom of cytosine turned out to be the global minimum. This structure is more stable than the Watson-Crick anion by 2.9 (B3LYP) and... 

The geometry of a platinated guanine-guanine-cytosine triplet was computed at the Hartree-Fock level [99IC1481 ]. The mechanism of cisplatin antitumor activity... 

There is a comparatively rich literature on Cu complexes with nucleic acid bases and nucleotides. Mono- and bis-adenine complexes with Cu exhibit N7 coordination (Figure 14)7 as does the bis-guanine complex." With cytosine, both N3 and 02 positions are found to coordinate Cull 80,81 [Cu (NH3)2] has also been observed in square planar geometry bound to... 

Several square planar Pt complexes with bases, nucleosides, and nucleotides have been characterized. A series of guanine-derived complexes have been prepared with both cis- and trani-geometries. These complexes generally show Pt coordination to the N7 position except when that position is blocked and coordination to the N1 position can be observed. In the case of adenine, Pt coordination to both N7 and N1 is feasible. The crystal structure of bis-adenine Pt bound to the N7 position with irans-geometry has been reported. The mono-, bis-, and tris-cytosine Pt complexes all exhibit coordination to the N3 position. " " A number of mixed-base compounds have been prepared. ... 

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