A new polarized target for neutron scattering studies on biomolecules: first results from apoferritin and the deuterated 50S subunit of ribosomes

A new polarized target for neutron scattering has been designed by CERN and tested successfully using the reactor FRG-1 at the GKSS Research Centre. The nuclear spins are aligned with respect to the external field - parallel or antiparallel - by dynamic nuclear polarization (DNP). To avoid absorption of neutrons by ³He, the frozen solutions of biomolecules are immersed in liquid ⁴He which in turn is thermally coupled to the cooling mixture of ³He/⁴He of the dilution refrigerator. Compared with earlier experiments where the sample had been cooled directly by ³He, the rate of detectable neutrons increased by a factor of 30. Another factor of 30 is due to the installation of the cold source and the beryllium reflector in FRG-1. Polarized neutron scattering from apoferritin in deuterated solvent shows that the proton spin polarization is homogeneous in apoferritin molecules. After saturation of proton nuclear magnetic resonance (NMR), polarized neutron scattering is dominated by deuteron spin contrast. With the deuterated large subunit of E. coli ribosomes, three different basic scattering functions are derived from spin-contrast variation, reflecting the known scattering-length-density distribution of the architecture of rRNA and ribosomal proteins. The planned in situ structure determination of a mRNA fragment is discussed in the light of the present results.