This paper concludes the series of reports in this journal [l-41 o n the chemistry of homo-DNA, the constitutionally simplified model system of hexopyranosyl-(6' + 4)-oligonucleotide systems studied in our laboratory 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 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-isoguanine 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

A series of 5-spirocyclopropanobarbiturates bearing alkyl and aryl su bstituents on the cyclopropane ring has been synthesized. Dihydro-orotate dehydrogenase from Clostridium oroticum was shown to be inhibited by these compounds. A related series of 5-membered-ring compounds (hydantoins and pyrazoles) was prepared but all the compounds were found to be inactive. In order to correlate these observations with previous resu Its concerning 5arylmet hy I hydantoi ns and 5aryl idenehydantoins as inhibitors, 5-arylidenebarbiturates were also assessed as inhibitors and found t o be the most active of the compounds investigated. The results are interpreted in the context of molecular recognition by this enzyme and the possibility of using substrate surrogates as templates for constructing latent inhibitors of enzymes.

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