Chemical stability of prostacyclin (PGI2) in aqueous solutions.

The aqueous geochemistry of Zr and the solubility of some Zr-bearing minerals

Synthetic and mechanistic aspects of 1,3-diene photooxidation

Why Pentose- and Not Hexose-Nucleic Acids? Purine-Purine Pairing in homo-DNA: Guanine,Isoguanine, 2,6-Diaminopurine, and Xanthine This paper concludes the series of reports in this journal [1–4] on the chemistry of homo-DNA, the constitutionally simplifie dmodel system of hexopyranosyl-(6′ → 4′)-oligonucleotide systems stidued 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 III of the series [3], a survey of what we know today about purine-purine pairingin 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 Watson-Crick pair between guanine and isoguanine, as well as one between 2,6-diaminopurine and xanthinone, 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-containg duplexes in comparison with duplexes known to be paired in the reverse guanine is replaced by 7-carbauguanine. Isoguanine and 2,6-diaminopurine also have the capability of self-pariring in the reverse-Hoogsteen mode, as previously observed for adenine and guanine [3]. In this type of pairing, the interchangeably. Fig. 36 provides an overall survey of the relative strength of pairing in all possible purine-purine combinations. Watson-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 (cumulative) purine-purine pairing in DNA (reverse-Hoogsten or Hoogsteen) seems to occur in triplexes and tetrapalexes only, its occurrence in duplexes in a characteristic feature of homo-DNA chemistry. The occurrence of purine-purine Watson-Crick base pairs is probably a consequence of homo-DNA's quasi-linear ladder structure [1][4]. In a double helix, the distance between the two sugar C-atoms, on which a base pair is anchored, is expected to be constrained by the dimensions of the helix; in a linear duplex, however, there would be no restrictions with regard to base-pair length. Homo-DNA's ladder-like model also allows one to recognize one of the reasons why nucleic-acid duplexes prefer to pair in antiparallel, rather than parallel strand orientation: in homo-DNA duplexes, (averaged) backbone and base pair axes are strongly inclined toward one another [4]; the stronger this inclination, the higher the preference for antiparallel strand orientation is expected to be (Fig. 16). In retrospect, homo-DNA turns out to be one of the first artificial oligonucleotide systems (cf. Footnote 65) to demonstrate in a comprehensive way that informational base pairing involving purines and pyrimidines is not a capability unique to ribofuranosyl systems. Stability and helical shape of pairing complexes are not necessary conditions of one another; it is the potential for extensive conformational cooperativity of hte backbone structure with respect to the constellational demands of base pairing and base stacking that determines whether or nor a given type of base-carrying backbone structure is an informational pairing system. From the viewpoint of the chemical aetiology of nucleic-acid structure, which inspired our investigations on hexopyranosyl-(6′ → 4′)-oligonucleotide systems in the first place, the work on homo-DNA is only an extensive model study, because homo-DNA is not to be considered a potential natural-nucleic-acid altenratie. In retrospect, it seems fortunate that the model study was carried out, because without it we could hardly have comprehended the pairing behavior of the proper nucleic-acid alternatives which we have studied later and which will be discussed in Part VI of this series. The English footnotes to Fig. 1–49 provide an extension of this summary.

Warum Pentose‐ und nicht Hexose‐Nucleinsäuren??. Teil V. (Purin‐Purin)‐Basenpaarung in der homo‐DNS‐Reihe: Guanin, Isoguanin, 2,6‐Diaminopurin und Xanthin

A set of substituent parameters λd is proposed which permits the prediction of E/Z equilibrium constants for olefins, enamines, Schiff bases, hydrazones, semicarbazones, oximes, and nitrones. The set includes values for the „non-bonded” electron pairs and the nitrone oxygen as well as for amino or phenyl groups coplanar and orthogonal to the double bond. Ketone anils prefer a perpendicular phenyl group. Principal limitations of the very simple heuristic model are discussed. The model is also applied to the „cis”- and „ortho”-effects, enolates, and Schiff bases of cyclic and bicyclic ketones.



Empirische Substituentenparameter fur E/Z-Gleichgewichtskonstanten



Ein vorgeschlagener Satz von Substituentenparametern λd erlaubt die Vorhersage der E/Z-Gleichgewichtskonstanten bei Olefinen, Enaminen, Schiffschen Basen, Hydrazonen, Semicarbazonen, Oximen und Nitronen. Der Parametersatz enthalt Werte fur „nichtbindende” Elektronenpaare und Nitronsauerstoff sowie fur Amino- und Phenylgruppen, die koplanar oder orthogonal zur Doppelbindung gestellt sind. Phenylreste in Ketonanilen bevorzugen die Orthogonalkonformation. Grundsatzliche Beschrankungen des sehr einfachen heuristischen Modells werden besprochen. Es last sich anwenden auf den „cis”- und den „ortho”-Effekt, auf Enolate und Schiffsche Basen cyclischer und bicyclischer Ketone.

Empirical Substituent Parameters for E/Z Equilibrium Constants

/pdf/thermodynamics-of-vinyl-ethers-part-4-rotational-isomerism-368mkfz0l4.pdf

Thermodynamics of vinyl ethers part 4, rotational isomerism and the relative stabilities of the geometric isomers of alkyl 1-propenyl ethers

/pdf/the-fluoride-complexes-of-zirconium-iv-a-potentiometric-11957sp2ob.pdf

The Fluoride Complexes of Zirconium(IV). A Potentiometric Investigation.

/pdf/the-fluoride-complexes-of-hafnium-iv-i-a-solvent-extraction-1zulfojk9g.pdf

The Fluoride Complexes of Hafnium(IV). I. A Solvent Extraction and Potentiometric Investigation.

/pdf/the-fluoride-complexes-of-hafnium-iv-ii-determination-of-the-4g6mor9t4k.pdf

The Fluoride Complexes of Hafnium(IV). II. Determination of the Stability Constants by the Cation Exchange Method.

/pdf/thermodynamics-of-vinyl-ethers-part-10-the-stabilizing-3s47ojg65i.pdf

Esko Taskinen

Papers

Thermodynamics of vinyl ethers part 4, rotational isomerism and the relative stabilities of the geometric isomers of alkyl 1-propenyl ethers

The Fluoride Complexes of Zirconium(IV). A Potentiometric Investigation.

The Fluoride Complexes of Hafnium(IV). I. A Solvent Extraction and Potentiometric Investigation.

The Fluoride Complexes of Hafnium(IV). II. Determination of the Stability Constants by the Cation Exchange Method.

Thermodynamics of vinyl ethers part 10, the stabilizing effect of beta-phenyl substitution on alpha,beta-unsaturated ethers