Natural-abundance C nuclear magnetic resonance spectra of medium-molecular-weight organic compounds

Combined multi-band decoupling in biomolecular NMR spectroscopy

Multi-resonance NMR experiments are powerful analytical and structural tools. Their conceptualization assumes that RF fields may be combined independently to manipulate spin interactions. However, practical implementation can compromise performance. One limitation is the generation of combination bands when two or more RF fields are applied simultaneously within the NMR probe. The combination bands can lead to significant interference with the detection circuitry. A facile approach to combined multi-band decoupling can resolve these problems and increase sensitivity two-fold (or more), by time sharing the application of the individual frequencies rather than time sharing decoupling and data acquisition.

ORSAT and modifications of SEFT and APT

Up to now, three subspectra are usually needed for the complete assignment of all signals in 13C spectrum: two DEPT spectra (phi = 90 degrees and phi = 135 degrees) and a proton decoupled spectrum. In this paper, we present a method in which a complete assignment becomes possible with merely two spectra. For this purpose, a new pulse sequence (ORSAT) has been elaborated by using off-resonance irradiation. The method described here is a further development of SEFT and APT. The second required spectrum is a DEPT (phi = 135 degrees). Signal assignment of cholesteryl acetate is demonstrated as an example. 13C routine spectroscopy can be significantly accelerated by applying this method.

Biosynthesis of natural products

The experimental verification of a proposed biosynthetic pathway for a given natural product is often difficult to obtain with the use of the whole organism (permeability factors) or, in the case of higher plants, a cell-free system. Until the purified enzyme for each step of biosynthesis is available, biosynthetic studies can, however, be carried out, albeit with modest incorporation values, by means of either hydroponic or injection methodology. Where viable seed sources are available it is suggested that improvements of several orders of magnitude in incorporation can be achieved by a short-term incubation (small pool size of precursors; trapping of reactive intermediates) or a long-term feeding (equilibration of precursor with the compartmentalized or induced synthetases). In bacterial, fungal, mammalian, and plant systems, where incorporation efficiencies provide the opportunity to study (13)C enrichment (at least equal to natural abundance of the isotope), we can expect a rapid expansion since the method removes the tedium of carbon-by-carbon degradation. For the next few years, however, the prognosis would seem to favor parallel studies of (13)C and (2)H, and of (14)C/(3)H ratio techniques since the last-mentioned method provides more information concerning the stereoselectivity of labeling processes on the microgram scale.

Fourier transform spectroscopy

The recent introduction of Fourier transform methods is revolutionizing IR and NMR spectroscopy. The application of IR interferometry or of NMR pulse methods, followed by Fourier transformation of the resultant interferogram, permits acquisition of spectral data in a time that is orders of magnitude less than by conventional spectroscopic methods. This reduction in time permits the study of transient species, or by "time-averaging" procedures S/N may be improved without the expenditure of inordinate amounts of time. The FT methods finds especially important application in the study of NMR spectra of nuclei of low sensitivity and low abundance, such as (13)C.

Carbon-13 Fourier transform nuclear magnetic resonance. II. Ribonuclease

High-resolution Fourier transform nuclear magnetic resonance was used to observe the natural abundance (13)C spectrum of a 0.02 M solution of bovine pancreatic ribonuclease A. Peaks assignable to only three carbons were easily observed after 10 hr of signal averaging. A number of tentative assignments were made. As expected, the (13)C spectrum was appreciably richer in detail than the corresponding proton spectrum. Natural abundance (13)C Fourier transform nmr appears to be a practical tool for the study of biopolymers in solution.

Natural-abundance 13C nuclear magnetic resonance spectra of terpenes and carotenes

Natural-abundance (13)C nuclear magnetic resonance spectra are reported for some simple terpenes and carotenes. The techniques involved in the assignment of the resonances to specific carbons are outlined. The potential of this nondegradative procedure for structural analysis is demonstrated for the investigation of carbon atoms in chemical and biochemical systems.

Carbon-13 nuclear magnetic resonance: naturally occurring nucleosides

The natural-abundance carbon-13 nuclear magnetic resonance spectra of the naturally occuring pyrimidine and purine nucleosides have been determined and completely analyzed. The carbon-13 chemical shifts obtained provide information on the electronic structure and qualitatively at least correlate well with the known chemical reactivity of these molecules. The carbon-13 shifts are in general far more informative than proton shifts for structural analysis of the nucleosides.

Nuclear magnetic resonance spectroscopy: 13C spectra of some common nucleotides

Natural-abundance (13)C nuclear magnetic resonance spectra of ten common nucleotides in neutral, aqueous solutions have been determined and interpreted. The spectra of two of these substances were also determined in acidic solutions, and several of the carbon chemical shifts were found to depend markedly on pH. Within the limited range of concentrations employed, there were observed no carbon chemical-shift changes which could be ascribed to base-stacking or base-pairing phenomena.

Nuclear magnetic resonance spectroscopy: 13C spectra of unsubstituted inositols

The chemical shifts of (13)C in natural abundance have been obtained from nuclear magnetic resonance spectra of scyllo-, myo-, chiro-, and epi-inositols at 15.1 MHz. A set of empirical substitutent parameters were derived and shown to be moderately successful in correlating the (13)C resonance-line positions for these substances. The substituent parameters conform generally to the idea that steric hindrance or porximity effects are of special importance in influencing chemical shifts of carbons in cyclohexane systems.