Determination of Association Constants (Ka) from Solution NMR Data

Direct proportionality of the hydrogen bonding energy and the intensification of the stretching ν(XH) vibration in infrared spectra

The aromatic character of porphyrins, which has significant chemical and biological consequences, can be substantially altered by judicious modifications of the parent ring system. Expansion of the macrocycle, which is achieved by introducing additional subunits, usually increases the so-called free curvature of the ring, leading to larger angular strain. This strain is reduced by a variety of conformational changes, most notably by subunit inversion and p surface twisting. The latter effect creates a particularly convenient access to Mobius aromatic molecules, whose properties, predicted over 40 years ago, are of considerable theoretical importance. The conformational processes occurring in porphyrin analogues are often coupled to other chemical phenomena, and can thus be exploited as a means of constructing functional molecular devices. In this Review, the structural chemistry of porphyrinoids is discussed in the context of their conformational dynamics and p-electron conjugation.

Figure Eights, Möbius Bands, and More: Conformation and Aromaticity of Porphyrinoids

Publisher Summary The chapter discusses the effects of some chemical mutagens and carcinogens on nucleic acids. Consideration of the known chemical action of alkylating agents on deoxyribonucleic acid (DNA) suggested two principal possible mechanisms. First, anomalous base pairing in DNA could be envisaged to result from either ionization or tautomeric change of alkylated guanine. Second, removal of alkylated purines from the DNA template, by hydrolysis, could cause alterations in dual-energy x-ray absorptiometry (DXA) base sequence, possibly by the deletion of a base pair. The carcinogenic dialkylnitrosamines are shown to yield effective alkylating agents, by oxidative metabolism, and, as previously mentioned, these can alkylate both ribonucleic acid (RNA) and DNA in vivo. The alkylation of guanine residues in DNA can occur in mammalian cells in vivo following the administration of chemical carcinogens. Also, some quantitative correlation is emerging between the extent of this reaction and the incidence of tumors in the affected tissues. The possibility that degradation of RNA, as observed to follow from the action of diazomethane, may be a significant reaction in vivo that could account for their observation that molecular ribonucleic acid (mRNA) is lost from the ribosomes of dimethylnitrosamine treated rats.

Effects of Some Chemical Mutagens and Carcinogens on Nucleic Acids

NMR Nomenclature: Nuclear Spin Properties and Conventions for Chemical Shifts. IUPAC Recommendations 2001.

Proton magnetic resonance studies of hydrogen bonding between the S--H proton of benzenethiol and the hydrogen acceptors dimethylformamide, pyridine, N- methylpyrazole, tributyl phosphate, and benzene, all in CCl/sub 4/ solution, are reported. Data on the variation of the chemical shift of the S --H proton as a function of the concentration of hydrogen acceptor are treated graphically in a manner which permitted the determination of both the hydrogen bond shift, DELTA / sub c/, and the equilibrium constant for formation of a 1: 1 complex, K. The erthalpy change on hydrogen bond formation was determined from the variation of K with temperature. For the benzenethiol-dimethylformamide system, for which the most complete results were obtained, DELTA /sub c/ appears to be independent of temperature over the range --12 to 68 deg C. (auth)

Proton magnetic resonance studies of hydrogen bonding of benzenethiol with several hydrogen acceptors

Nitrogen-15 nuclear magnetic resonance spectroscopy. Substituent effects on 15N-H coupling constants and nitrogen chemical shifts in aniline derivatives

Robert B. Bradley

Papers

Proton magnetic resonance studies of hydrogen bonding of benzenethiol with several hydrogen acceptors

Nitrogen-15 nuclear magnetic resonance spectroscopy. Substituent effects on 15N-H coupling constants and nitrogen chemical shifts in aniline derivatives

Effects of ``Ring Currents'' on the NMR Spectra of Porphyrins

Nuclear magnetic resonance studies of methyl derivatives of cytosine.

Stereochemical dependence of 15NCH and 13CH coupling constants in oxaziridines