Slobodan Macura

Analysis of laboratory-frame and rotating-frame cross-relaxation buildup rates from macromolecules

Abstract A quadratic approximation of cross-relaxation buildup rates is described for identifying and correcting for indirect magnetization transfer effects in macromolecular spectra. This approach has permitted the accurate determination of interproton distances in a protein of molecular weight 6000, turkey ovomucoid third domain. An illustration is given of the analysis of a three-spin system to yield a distance between two protons that lacked resolved direct NOESY and ROESY cross peaks.

Strategies for eliminating unwanted cross-relaxation and coherence-transfer effects from two-dimensional chemical-exchange spectra

Abstract A method for eliminating cross relaxation and for suppressing coherence-transfer effects from exchange spectra of macromolecules that permits a more rigorous study of chemical-exchange processes is demonstrated. Cross relaxation is suppressed when the effects of longitudinal and transverse relaxation are forced to compensate each other. For macromolecules undergoing slow rigid-body isotropic motion ( ω 0 τ c ⪢ 1), the ratio of the longitudinal to transverse cross-relaxation rates is equal to − 1 2 . If the mixing period of an exchange experiment is designed so that the magnetization components alternately cross relax along the longitudinal axis twice as long as they cross relax along the transverse axis, the direct cross-relaxation peaks cancel. Several mixing pulse sequences are proposed, based on average Hamiltonian theory taken to zeroth order, for the elimination of cross relaxation and suppression of TOCSY coherence transfer in exchange spectra. Both simulated and experimental results demonstrate the elimination of cross relaxation and suppression of TOCSY transfer by these mixing pulse sequences. Clean exchange spectra of turkey ovomucoid third domain (6015 Da) obtained by this method enabled the study of a ring flip exchange processes that is slow on the NMR time scale near 0°C.

Sesquiterpene lactones from Achillea abrotanoides

Abstract The isolation of desacetoxymatricarin, desacetyl matricarin and 1β, 10β-epoxydesacetoxymatricarin (a new guaianolide) from Achillea abrotanoides Vis. is reported. The configuration of the epoxy ring in this lactone was determined by comparison of its 13 C and 1 H NMR data to those of diastereomeric 1α,10α-epoxydesacetoxymatricarin, the epoxidation product of desacetoxymatricarin. A flavonoid centaureidin was also detected in this extract.

Sesquiterpene lactones from Inula helenium

Abstract The isolation of 11(13)-dehydrocriolin [a germacranolide also known as 11(13)-dehydroivaxillin], 2α-hydroxyalantolactone, 4α,5α-epoxy-10α,14-H-inuviscolide (the major component) and carabrone from the aerial parts of Inula helenium is reported. The first two lactones were isolated for the first time from this plant.

Heteronuclear correlation 2D NMR spectroscopy of macromolecules

Abstract The relaxation properties of macromolecules present problems which require modification of existing experimental procedures for 2D heteronuclear cross-correlation NMR of small molecules. In macromolecules, the large 1 H linewidths for hydrogens bound directly to 13 C strongly limit achievable resolution for correlations over one bond. For correlations utilizing n J , n > 1, resolution is improved at the expense of sensitivity, but explicit consideration must be given to the effect of transverse relaxation on the efficiency of heteronuclear magnetization transfer. For macromolecules with correlation times in the spin-diffusion regime, long-range correlations were obtained via 1 H 1 H cross relaxation followed by transfer to 13 C. This experiment shows improved resolution and sensitivity, permits the observation of 1 H 1 H cross relaxation in some magnetically equivalent spin systems and may be useful for indirect detection of 1 H 1 H cross relaxation in crowded 1 H NMR spectra.

Evaluation of errors of interproton distances and correlation time determined from NMR cross-relaxation rates

SummaryWe have analyzed a combined use of the two-dimensional nuclear Overhauser effect in the laboratory frame (NOESY) and in the rotating frame (ROESY) to determine interproton distances and correlation time in medium-sized rigid molecules (Davis, 1987). This method can be applied in the intermediate motional regime, 0.2 < ωoτc, < 5, (τc, correlation time, (ωo resonance frequency). Error limits depend on the motional regime and are smallest near ωoτc=1.14.The method was tested on six geminal proton pairs in the bicyclic octapeptide (S-deoxo-γ-[R]-OH-Ile3 amaninamide, Mw =870) for which at 297 K in DMSO, a correlation time of 1.0 ns, with a standard deviation of 0.12 ns, and an interproton distance of 1.87 Å, with standard deviation of 0.04 Å, are obtained.