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Bovine serum albumin (BSA) was dissolved in a mixture of deuterated glycerol and heavy water. The clusters formed by the 3300 proton spins in each BSA molecule were dynamically polarized up to P = 40%. Spin-contrast variation in small-angle neutron scattering was studied at several target polarizations. Zero contrast, and hence minimum polarized neutron small-angle scattering, is expected at PH = 60% from extrapolation of present data. The three basic scattering functions of spin-contrast variation look very similar because the shape of the BSA molecule and its proton distribution are congruent. Neutron smallangle scattering of BSA is similar to X-ray small-angle scattering at room temperature, indicating no deterioration of the molecular structure of BSA on solidification.

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.

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