TNAs [(L)-alpha-threofuranosyl oligonucleotides] containing vicinally connected (3'-->2') phosphodiester bridges undergo informational base pairing in antiparallel strand orientation and are capable of cross-pairing with RNA and DNA. Being derived from a sugar containing only four carbons, TNA is structurally the simplest of all potentially natural oligonucleotide-type nucleic acid alternatives studied thus far. This, along with the base-pairing properties of TNA, warrants close scrutiny of the system in the context of the problem of RNA's origin.

The cover picture shows the synthesis of formylporphyrins by Umpolung reactions that use 1,3‐dithian‐2‐ylporphyrins as masked synthons. This method offers a new pathway for the preparation of free‐base and metallo formylporphyrins under mild conditions. Grignard and Wittig reactions have been performed to demonstrate the wide range of applications for formylporphyrins. Details are discussed in the article by M. O. Senge et al. on p. 3833 ff.

Dieter Seebach zum 65. Geburtstag gewidmet Our studies of a-l-Threofuranosyl-(3' 3 2')-oligonucleotides (−TNA×) are part of a systematic experimental inquiry into the base-pairing properties of potentially natural nucleic acid alternatives taken from RNA×s close structural neighborhood. TNA is an efficient Watson-Crick base-pairing system and has the capability of informational cross-pairing with both RNA and DNA. This property, together with the system×s constitutional and (presumed) generational simplicity, warrants special scrutiny of TNA in the context of the search for chemical clues to RNA×s origin.

Replacement of adenine by 2,6-diaminopurine-two nucleobases to be considered equivalent from an etiological point of view-strongly enhances the stability of TNA/TNA, TNA/RNA, or TNA/DNA duplexes and efficiently accelerates template-directed ligation of TNA ligands. [reaction: see text]