Adenine hydrochloride, structure

The determination of the structure of adenine hydrochloride (see Volume 2, Section IV,K, of article IV by Katritzky and Lagowski) is an example of extremely accurate X-ray crystallography whereby the positions of individual hydrogen atoms were located. An example of the deduction of structure from bond lengths between heavy atoms is provided by Penfold s investigation of pyrid-2-thione. ... 

After inconclusive early work,-" structure 218 was demonstrated for adenine hydrochloride using X-ray diffraction (cf. also reference 256) and was later supported by nuclear magnetic resonance evidence. Four possible structures have been advanced for guanine... 

The adenine radical cation was observed in a single crystal of adenine hydrochloride hemihydrate [43]. In this crystal, the adenine is protonated at Nl. After electron loss, the molecule deprotonates at Nl, giving Ade(Nl -l-H, Nl-H). This produces a radical that is structurally equivalent to the cation of the neutral adenine molecule with spin density on C8 and N6 [p(C8) = 0.17 and p(N6) = 0.25]. The adenine radical cation is strongly acidic (pi a< 1) [22]. This strong driving force makes the reaction independent of environmental conditions. In single crystals of adenosine [42] and anhydrous deoxyadenosine [44], the N6 deprotonated cation [Ade(N6-H) ] is observed which is characterized by p(C8) = 0.16 and p(N6) = 0.42. The experimental isotropic hyperfine couplings are N6-H = 33.9 MHz and C8-H = 12.4 MHz. 

Fig. 12. A drawing of the crystal structure of adenine hydrochloride hemi-hydrate. Hydrogen bonds are indicated by dashed lines. The dimensions were calculated from the coordinates given by Cochran (11).

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