Advances in contrast variation for macromolecular structure determination by polarized neutron scattering and anomalous dispersion of synchrotron X‐rays
Authors: Boulin C. , Büldt G. , Dauvergn F. , Gabriel A. , Goerigk G. , Knop W. , Krumpolc M. , Munk B. , Nierhaus K. , Niinikoski T. O. , Nowotny V. , Rieubland M. , Rijllart A. , Schärpf O. , Schink H.‐J. , Wagner R. , Stuhrmann H. B. 
Published: March 2011

Contrast variation for macromolecular structure determination is usually achieved by isomorphous replacement of 1‐H by 2‐H (D) using small‐angle neutron scattering (SANS). This is particularly easy in aqueous solvents. By adding heavy water the contrast of dissolved proteins, nucleic acids and membranes changes drastically. It is the region inaccesible to solvent molecules, which acts as a label. Measurements of the scattering intensity at three different scattering densities of a solvent yields the three basic scattering functions. The contrast dependence of the radius of gyration receives particular interest.

More recently smaller labels have been used. Their dimensions are smaller than those of the total particle by an order of magnitude. They are used for in situ structure determination of the labelled region. Contrast variation of the labelled region can be achieved by gradual deuteration of the label. Again, the measurements have to be done at three different contrasts in order to separate the scattering function of the label. ‐ By eliminating any intramolecular scattering density fluctuation (K. Nierhaus' principle of the ‘transparent’ ribosome) the structure of selectively labelled ribosomal proteins can be determined in situ by using only one derivative.

Spin contrast variation of biomolecules relies on the nuclear spin dependence of neutron scattering by protons and, to a smaller extent, by deuterons. It is particularly well suited for labels rich in protons embedded in a deuterated matrix. Variation of the target polarization yields the three basic scattering functions from the same sample.

In collaboration with CERN and ILL, a special set‐up for spin contrast variation has been installed at the reactor FRG1 of the GKSS Research Centre at Geesthacht. The dynamic nuclear polarization of proton spins in proteins, nucleic acids and ribosomes is achieved in the presence of Cr(V) at T = 0.4 K, H = 2.5 T and 4 mm microwave irradiation. Within two hours, the proton polarization will reach more than 70 % in favourable cases. Measurements have been carried out with various proteins, transfer ribonucleic acid (tRNA) and the large subunit of E.coli ribosomes, the latter also with its total protein deuterated (i.e. ribosomal ribonucleic acid (rRNA) acts as a label). There is an agreement between the nuclear spin polarization of protons and deuterons measured by NMR and the nuclear spin dependent change of polarized small‐angle neutron scattering.

The results of spin contrast variation experiments are compared with those obtained from near‐edge anomalous X‐ray scattering using synchrotron radiation. Particular attention is given to the anomalous dispersion of light elements, like sulfur, which tend to show a strong dependence on the chemical bonding of the atom. In purple membrane, the anomalous scattering of sulfur in protein, bound to methionine and sulfates in the lipid matrix has been determined separately.