Frank A. L. Anet

Carbon-13 Nuclear Magnetic Resonance Spectroscopy

Partial table of contents: Introduction, Theory, and Methods. 13C NMR Spectral Characteristics. Aliphatic Compounds. Aromatics. Organic Functional Groups. Ions, Radicals, and Complexes. Polymers. Relaxation Studies. Biomolecules. Special Techniques and Applications. Appendix. Index.

Glyoxylate as a backbone linkage for a prebiotic ancestor of RNA

The origin of the first RNA polymers is central to most current theories for the origin of life. Difficulties associated with the prebiotic formation of RNA have lead to the general consensus that a simpler polymer preceded RNA. However, polymers proposed as possible ancestors to RNA are not much easier to synthesize than RNA itself. One particular problem with the prebiotic synthesis of RNA is the formation of phosphoester bonds in the absence of chemical activation. Here we demonstrate that glyoxylate (the ionized form of glyoxylic acid), a plausible prebiotic molecule, represents a possible ancestor of the phosphate group in modern RNA. Although in low yields (∼ 1%), acetals are formed from glyoxylate and nucleosides under neutral conditions, provided that metal ions are present (e.g., Mg2+), and provided that water is removed by evaporation at moderate temperatures (e.g., 65 ∘C), i.e. under “drying conditions”. Such acetals are termed ga-dinucleotides and possess a linkage that is analogous to the backbone in RNA in both structure and electrostatic charge. Additionally, an energy-minimized model of a gaRNA duplex predicts a helical structure similar to that of A-form RNA. We propose that glyoxylate-acetal linkages would have had certain advantages over phosphate linkages for early self-replicating polymers, but that the distinct functional properties of phosphoester and phosphodiester bonds would have eventually lead to the replacement of glyoxylate by phosphate.

Limiting equations for exchange broadening in two-site NMR systems with very unequal populations

Abstract The exchange broadening which occurs in a two-site (A and B) situation in the absence of coupling is examined for very unequal ( > 10:1) populations of the two sites. For moderate exchange rates the observed spectrum consists essentially of a single broadened line. The broadening goes through a maximum as the exchange rate is varied. The value of the maximum broadening (width at half-height) is given by pδ, where p is the fractional population of the minor component and δ (in hertz) is the chemical shift difference between the two sites. The sum of the rate constants (kA + kB) at the point of maximum broadening is given as 2πδ sec−1.

Force-field calculations for some unsaturated cyclic hydrocarbons

Abstract Boyds iterative force-field computer program, with modified parameters, has been used to calculate strain energies in the following molecules: trans-cyclooctene, cyclohexene, melhylenecyclohexane, 1,3,5- and 1,3,6-cyclooctatriene, and cyclononyne. Both static and dynamic aspects of the conformational properties of these molecules have been investigated in detail, and the results are discussed in conjunction with extant experimental data and previous force-field calculations.

Detection of menaquinone-6 and a novel methyl-substituted menaquinone-6 in Campylobacter jejuni and Campylobacter fetus subsp. fetus.

Menaquinone-6 (2-methyl-3-farnesyl-farnesyl-1,4-naphthoquinone) and a methyl-substituted menaquinone-6 (2,[5 or 8]-dimethyl-3-farnesyl-farnesyl-1,4-naphthoquinone) were the major isoprenoid quinones found in membrane preparations of Campylobacter jejuni and Campylobacter fetus subsp. fetus. By reverse-phase high-performance liquid chromatography (HPLC) and thin-layer chromatography (TLC) the faster-eluting menaquinone-6 co-chromatographed with a menaquinone-6 standard. The identity of menaquinone-6 was confirmed by UV spectrophotometry, mass spectrometry and nuclear magnetic resonance (NMR) analysis. The slower-eluting methyl-substituted menaquinone-6 co-chromatographed with a menaquinone-7 standard by reverse-phase TLC but eluted between menaquinone-6 and menaquinone-7 standards by HPLC. The UV spectrum of the methyl-substituted menaquinone-6 did not correlate with either authentic menaquinone or demethylmenaquinone. Mass spectra showed an increase of 14 mass units when compared to menaquinone-6, and indicated that a methyl substituent was on the naphthoquinone nucleus. NMR spectra confirmed the presence of a methyl substituent at a peri position (carbon-5 or -8) on the benzenoid ring.

NMR relaxation by the antisymmetric component of the shielding tensor: a longer transverse than longitudinal relaxation time

Abstract The first experimental detection of an antisymmetrical component (σa) in the NMR shielding tensor of a nucleus is reported. The 13C NMR relaxation rates (1/T1 and 1/T2) of the ethylenic carbons in tetrachlorocyclopropene in toluene-d8 at about −90°C are dominated at high fields by shielding tensor modulated by random molecular rotation. We find that T2 is circa 1.1T1 in contrast to that in 13CS2, where σa is zero by symmetry and where the measured T2 is not greater than the maximum (0.857T1) allowed for this case. The present results are in qualitative agreement with previous ab initio IGLO calculations showing that the ethylenic carbons in cyclopropenes have unusually large antisymmetric components in their shielding tensors.

H-D coupling constants and deuterium isotope effects on the proton chemical shifts in partially deuteriated methanes

Abstract Very accurate frequency measurements on 500 MHz 1 H NMR spectra of CH 4 , CH 3 D, CH 2 D 2 , and CHD 3 are reported. There are relatively large (up to 10%) solvent and temperature effects on the isotope shifts ( 2 Δ = −13.998 to −15.555 ppb for CH 3 D), which are non-additive by ca 0.2 ppb/deuterium; 2 J HD (−1.929 ± 0.001 Hz, sign assumed, equivalent to 2 J HH = −12.57 Hz) is almost independent of solvent, temperature, and the number of deuterons present.

Conformational analysis of medium-ring ketones

Abstract The conformations of cyclooctanone, cyclooctane-1,5-dione, cyclononanone, ayclodecanone, cyclodecane-1,6-dione, and cycloundecanone are discussed.

Determination of the relative signs of coupling constants in some organomercuric compounds by internuclear double and triple magnetic resonance

Abstract INDOR techniques are employed in determining the relative signs of coupling constants in the following three organomercuric compounds: ethylmercuric chloride, JHgCH2 = ±222 cps, and JHgCH3 = ±300 cps; methylmercuric nitrate, JHgCH3 = ±258 cps, and JHgC13 = ± 1800 cps; and trans-2-chlorovinylmercuric chloride, JHgαH = ± 115.0 cps, and JHgβH = ± 160.5 cps. For a fourth compound, methylethynylmercury, which has a virtually zero value for the proton-proton coupling constant (JCHCH3 ≈ 0 cps), triple resonance experiments show that JHgCH3 = ±149 cps and JHgcH = ±69 cps. A table of coupling constants, with probable absolute signs, is given for all organomercuric compounds where such data are available.

A comparison of empirical force field parameters for molecular mechanics calculations on saturated hydrocarbons

Nine empirical force fields used in molecular mechanics calculations are tested for their abilities to reproduce experimental geometric and energy data on cis-syn-cis-perhydroanthracene.