Stroboscopic detection of nuclear resonance in an arbitrary scattering channel.

The theory of heterodyne/stroboscopic detection of nuclear resonance scattering is developed, starting from the total scattering matrix as a product of the matrix of the reference sample and the sample under study. This general approach holds for all dynamical scattering channels. In the forward channel, which has been discussed in detail in the literature, the electronic scattering manifests itself only in an energy-independent diminution of the scattered intensity. In all other channels, complex resonance line shapes of the heterodyne/stroboscopic spectra are encountered, as a result of the interference of electronic and nuclear scattering. The grazing-incidence case will be evaluated and described in detail. Experimental data of classical X-ray reflectivity and their stroboscopically detected resonant counterpart spectra on the [(nat)Fe/(57)Fe]10 isotope periodic multilayer and antiferromagnetic [(57)Fe/Cr]20 superlattice are fitted simultaneously.

Determination of the magnetic spin direction from the nuclear forward-scattering line intensities.

An expression is derived for the line intensities in a nuclear forward-scattering energy spectrum that is obtained via a Fourier transformation of the time dependence of the wavefield. The calculation takes into account the coherent properties of the nuclear forward-scattering process and the experimental limitations on the observable time window. It is shown that, for magnetic samples, the spin direction can be determined from the ratios between the different lines in the energy spectrum. The theory is complemented with experimental results on alpha-iron.

Controlling absorption of gamma radiation via nuclear level anticrossing.

A significant reduction of absorption for single gamma photons has been experimentally observed by studying Mössbauer spectra of 57Fe in a FeCO3 crystal. The experimental results have been explained in terms of a quantum interference effect involving nuclear level anticrossing due to the presence of a combined magnetic dipole and electric quadrupole interaction.

Solution conformation of virginiamycin S. II. The conformation of allohydroxy- and deoxyvirginiamycin S.

The experimental results of 1H- and 13C-NMR studies of allohydroxy-, and of deoxyvirginiamycin S strongly confirm the conformation that was proposed earlier for the parent virginiamycin S (Anteunis, Callens and Tavernier (1975) Eur. J. Biochem. 58, 259--268). The changing nature of dipole-induced dipole interaction is responsible for the specific gradually increasing libration of the N-MePhe side chain along the series virginiamycin S, allohydroxy-, deoxyvirginiamycin S. Previous methods for the estimation of rotameric populations around the alpha, beta bonds are critically evaluated and compared to the present results obtained from interpretation of geminal 2J (beta) coupling constants.

Solution conformation of virginiamycin S. III. Patricin A: A further model for cooperative effects of the Pro ring conformation and backbone.

Although the main characteristics of the parent virginiamycin S conformation i.e. a bend of type VI (Lewis, P.N., Momamy, F.A. and Scheraga, H.A. (1973) Biochim. Biophys. Acta 303, 211--229) formed by the Pro-N-MePhe-X-PhGly sequence is still present in patricin A, the substitution of X = pipecolic acid by proline in the latter results in a destabilization of the tertiary structure of the depsipeptide, since two isomeric states of a peptidic bond appear in C2HC13 solution. Addition of +/- 30% (v:v) (C2H3)2SO totally shifts this equilibrium in favor of the major parent isomer. These results completely fit with what is known up to now on the occurrence and structure of turns (Chou and Fasman (1977) J. Mol. Biol. 115,135--175).

Solution conformation of virginiamycins (staphylomycins).

The 1H (at 300 MHz) and 13C nuclear magnetic resonance spectra of virginiamycins S and S4 and vernamycin Balpha have been unravelled and analyzed. Together with model building and theoretical considerations, this allows the detailed description of their solution conformations. The depside bond can rotate and gives to the backbone some conformational mobility. The orientation of the depsicarbonyl bond depends on the surrounding. Apparent discrepancies between the different methods that are applicable for the disclosure of the nature of peptide H-bonding, have found a rational explanation.