Date Published: May 01, 2020
Publisher: International Union of Crystallography
Author(s): Dasharath Kondhare, Simone Budow-Busse, Constantin Daniliuc, Frank Seela.
7-Iodo-5-aza-7-deazaguanosine represents a base-modified nucleoside with an entirely different recognition pattern as the parent purine compound. It shows an anti conformation at the glycosylic bond and an N conformation (O4′-endo) for the ribose moiety. Packing is controlled by hydrogen bonds and by a 7-iodo contact to 2′-oxygen of the ribose moiety.
The specific recognition of complementary nucleobases in the DNA coding system is based on the combination of purine and pyrimidine bases (Watson & Crick, 1953 ▸). Adenine pairs with thymine and guanine with cytosine. Nevertheless, an alternative coding system entirely composed of purines (‘all-purine’ DNA) was suggested by Wächtershäuser (1988 ▸). In this, purines glycosylated at nitrogen-9 (adenine and guanine) base pair with purines glycosylated at nitrogen-3 (motif I xanthine and motif II isoguanine, see Fig. 1 ▸; purine numbering is used throughout this article). The resulting purine–purine base pairs are isoelectronic and isogeometric to standard purine–pyrimidine Watson–Crick pairs. However, purines glycosylated at nitrogen-3 are unstable and difficult to handle (Leonard, 2020 ▸).
The single-crystal X-ray structure of 7-iodo-5-aza-7-deazaguanosine (1) was determined and the geometric parameters of this RNA constituent were analyzed. This includes an anti conformation of the 5-aza-7-deazaguanine base, an N conformation (O4′-endo) of the ribose moiety and an antiperiplanar orientation of the 5′-hydroxy group.