HELVETICA zyxwvut CHIMICA ACTA zyxwvu - Vol. 81 (1998) 375 z L Warum Pentose- und nicht Hexose-Nucleinsauren? Teil V zyxwvu '1 (Purin-Purin)-Basenpaarung in der homo-DNS-Reihe: Guanin, Isoguanin, 2,6-Diaminopurin und Xanthin von Katrin Groebke2), Jiirg Hunziker2), William Fraser '). Ling Peng *), UIF Diederichsen'). Kaspar Zimmermann4). Armin Holzner5). Christian Leumann und Albert Eschenmoser* .aboratorium fur Organische Chemie der Eidgenossischen Technischen Hochschule. Universititstrasse 16, CH-8092 Zurich zyxwvu In Me~noriurn zyxwvuts Vludo PreIog Why Pentose- and Not Hexose-Nucleic Acids? Purine-Purine Pairing in homo-DNA: Guanine, Isoguanine, 2,CDiaminopurine, and Xanthine This paper concludes the series of reports in this journal [l-41 on the chemistry of homo-DNA, the constitutionally simplified model system of hexopyranosyl-(6' + 4)-oligonucleotide systems studied in our labo- ratory as potentially natural-nucleic-acid alternatives in the context of a chemical aetiology of nucleic-acid structure. The report describes the synthesis and pairing properties of homo-DNA oligonucleotides which contain as nucleobases exclusively purines, and gives, together with part 111 of the series [3]. a survey of what we know today about purine-purine pairing in homo-DNA. In addition. the paper discusses those aspects of the chemistry of homo-DNA which, we think, influence the way how some of the structural features of DNA (and RNA) are to be interpreted on a qualitative level. Purine-purine pairing occurs in the homo-DNA domain in great variety. Most prominent is a novel tridentate zy Wutson-Crick pair between guanine and isoguanine, as well as one between 2.6-diaminopurine and xanthine, both giving rise to very stable duplexes containing the all-purine strands in antiparallel orientation. For the guanine-iso- guanine pair. constitutional assignment is based on temperature-dependent UV and CD spectroscopy of various guanine- and isoguanine-containing duplexes in comparison with duplexes known to be paired in the reverse- Hoogs/rm mode. The assignment is supported by the characteristic changes observed in pairing behavior when guanine is replaced by 7-carbaguanine. Isoguanine and 2,6-diaminopurine also have the capability of self-pairing in the reverse-Hoc~gstrc.n mode, as previously observed for adenine and guanine [3]. In this type of pairing. the purine bases that contain an amino group in the 6-position (adenine. 2,6-diammopurine. and isoguanine) behave interchangeably. Fig. 36 provides an overall survey of the relative strength of pairing in all possible purine-purine combinations. Wutson-Crick pairing of isoguanine with guanine demands the former to participate in its 3H-tautomeric form; hitherto this specific tautomer had not been considered in the pairing chemistry of isoguanine. Whereas Teile I -IV der Reihe 'Warum Pentose- und nicht Hexose-Nucleinsiuren?'. vgl. [1-4]. Die vorliegende Arbeit gilt als 9. Mitteilung in der Reihe 'Chemie von cr-Aminonitden' (8. Mitteilung dieser Reihe vgl. [4]). Teile der hier publizierten Ergebnisse sind von J.H. an der Herbstversammlung der Schweizerischen Chemischen Gesellschaft in Bern am 16.10.1991 vorgetragen sowie in mehreren von A.E. gehaltenen und im Druck erschienenen Vortrigen erwihnt worden; vgl. [5-81. Vgl. die Promotionsarbeiten [9- 121. Postdoktorat ETH. 1989-91. Postdoktorat ETH. 1989-92. Postdoktorat ETH. 1992-93.