In memoriam Rolf Scheffold (1. IX.95) Base pairing in p-RNA @ -o-ribopyranosyl-(4 + 2)-oligonucleotides) is not only stronger than in DNA and RNA, but also more selective in the sense that it is strictly confined to the Watson-Crick mode. Homopurine sequences (tested up to decamers) exist as single strands under conditions where they undergo reverse-Hoogsteen self-pairing in homo-DNA or Hoogsfeen self-pairing in DNA. This exceptional pairing selectivity is rationalized as hinging on two structural features of p-RNA: the large inclination between backbone axis and base-pair axes in p-RNA duplexes, and the higher rigidity of the p-RNA backbone compared with RNA, DNA, and homo-DNA. The most important consequence of the pairing selectivity refers to the potential of p-RNA to replicate. Replicative copying of sequence information by nonenzymatic template-controlled ligation is not hampered by self-pairing of guanine-rich templates, as it is known to be the case in the RNA series. We have demonstrated two replicative cycles in which G-rich p-RNA-octamer templates induce sequence-selective ligation of tetramer-2'-phosphate derivatives to complementary C-rich octamer sequences, and in which the latter, with comparable efficiency, induce corresponding ligation reactions back to the original G-rich octamers. Ligation is most satisfactorily achieved after pre-activation of the 2'-phosphate groups as 2',3'-cyclophosphate derivatives; in this version, the process does not proceed as oligocondensation, but as a genuine oligomerization. This is of considerable promise for the search for potentially natural conditions under which homochiral p-RNA strands might self-assemble and self-replicate.
Dulio Arigoni zu seinem 75. Geburtstag gewidmet, in Erinnerung an gute alte Zeiten Pyranosyl-RNA (−p-RNA× ) is an oligonucleotide system isomeric to natural RNA and composed of the very same building blocks as RNA. Its generational, chemical, and informational properties are deemed to be those of an alternative nucleic acid system that could have been a candidate in Nature×s evolutionary choice of the molecular basis of genetic function. We consider the study of the chemistry of p-RNA as etiologically relevant in the sense that knowledge of its structural, chemical, and informational properties on the chemical level offers both a perspective and reference points for the recognition of specific structural assets of the RNA structure that made it the (supposedly) superior system among possible alternatives and, therefore, the system that became part of biology as we know it today. The paper describes the chemical synthesis of b-d-(and l)ribopyranosyl-(4' 3 2')-oligonucleotide sequences, presents a resume of their structural and chemical properties, and cautiously discusses what we may and may not have learned from the pyranosyl isomer of RNA with respect to the conundrum of RNA×s origin. see [1 ± 8]. The label −Chemistry of Pyranosyl-RNA× previously used for the series of papers on p-RNA [1 ± 8] has been changed (see [9]) into −Pentopyranosyl Oligonucleotide Systems× as a consequence of the extension of our work on p-RNA to a whole family of diastereoisomeric pentopyranosyl oligonucleotides. In [9], the present paper had been assigned No. 9 in this series. The follow-up papers No. 10 [9], 11 [10], 12 [11], and 13 [12] in the series have already appeared. The present paper also is communication No. 30 in the series −Chemistry of a-Aminonitriles×. 4271 hexopyranosyl systems. Such hindrance is expected to occur between an equatorial 2'-OH group of a pyranosyl unit and the neighboring (downstream-positioned) nucleobase; it is clearly to be inferred from model considerations based on homo-DNA×s two types of pairing conformations (À g/ À g and t/ À g), which were derived by qualitative conformational analysis [29] 5 ) ( Fig. 2), and were found to co-exist in a homo-DNA duplex by an NMR structure analysis by Otting et al [26]. The results of experimental studies on 2'-deoxy-and 3'-deoxyallopyranosyl model systems [24] [46] [47] (Table 1) corroborated this interpretation.It was this insight into the consequences of the steric bulk of fully hydroxylated (6' 3 4')-hexopyranosyl building blocks that led us to ask whether base pairing might be Helvetica Chimica Acta ± Vol. 86 (2003) 4275 8 ) See Scheme 1 in [1]. It seems worth pointing out that an analogously built hexopyranosyl-(4' 3 2')oligonucleotide system, e.g., the b-allopyranosyl member depicted below, by the same reasoning, would not be expected to be a base-pairing system. The (repetitive) pairing conformation would suffer serious steric hindrance (see arrow) and, therefore, not become populated in competition with less strained (nonrepetitive) conformations. 9 ) ...
Die zu natürlicher RNA isomere Pyranosyl‐RNA (p‐RNA) – Ausschnitt schematisch im Bild rechts – zeigt höchst bemerkenswerte Eigenschaften. Homochirale Stränge mit entgegengesetztem Chiralitätssinn bilden quasiracemische Duplexe nach Basenauswahlregeln, die von jenen der isochiralen Paarung verschieden sind.
Dedicated to Professor Andre¬ M. Braun on the occasion of his 60th birthday We prepared two new linkers, S-functionalized adamantane derivatives 2 and 3, which bind as monolayers on polycrystalline gold. From these surface anchors, both l-and d-isomers of alanine can be grown as thin films of a-helical polypeptides directed from the gold surface by using the appropriate N-carboxyalanine anhydride. FT-IR Studies show that these layers are roughly 1000-ä thick and that, under the same growth conditions, the l-polypeptide layers grow at a rate ca. 30% greater than that of the non-natural d-amino acid. X-Ray photoelectron spectroscopy studies show that, upon equilibration, all three S-atoms of the sulfide moieties of 2 are bound to the gold surface, and that, on average, three of the four thiols of 3 are chemoadsorbed. The essential role of H 2 O on the surface of these films as a necessary component in these gas-phase polymerization reactions is demonstrated.
Pyranosyl‐RNA, an isomer of natural RNA (constitution and pairing conformation shown on the right), displays remarkable properties. Homochiral pyranosyl‐RNA strands with opposite senses of chirality are found to pair to give quasi‐racemic duplexes. Correspondingly altered pairing rules result.
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