Dynamic Nuclear Polarization of Amyloidogenic Peptide Nanocrystals:  GNNQQNY, a Core Segment of the Yeast Prion Protein Sup35p

Dynamic nuclear polarization (DNP) permits a ∼102−103 enhancement of the nuclear spin polarization and therefore increases sensitivity in nuclear magnetic resonance (NMR) experiments. Here, we demonstrate the efficient transfer of DNP-enhanced 1H polarization from an aqueous, radical-containing solvent matrix into peptide crystals via 1H−1H spin diffusion across the matrix−crystal interface. The samples consist of nanocrystals of the amyloid-forming peptide GNNQQNY7 - 13, derived from the yeast prion protein Sup35p, dispersed in a glycerol−water matrix containing a biradical polarizing agent, TOTAPOL. These crystals have an average width of 100−200 nm, and their known crystal structure suggests that the size of the biradical precludes its penetration into the crystal lattice; therefore, intimate contact of the molecules in the nanocrystal core with the polarizing agent is unlikely. This is supported by the observed differences between the time-dependent growth of the enhanced polarization in the solvent versus the nanocrystals. Nevertheless, DNP-enhanced magic-angle spinning (MAS) spectra recorded at 5 T and 90 K exhibit an average signal enhancement ε ≈ 120. This is slightly lower than the DNP enhancement of the solvent mixture surrounding the crystals (ε ≈ 160), and we show that it is consistent with spin diffusion across the solvent−matrix interface. In particular, we correlate the expected DNP enhancement to several properties of the sample, such as crystal size, the nuclear T 1, and the average 1H−1H spin diffusion constant. The enhanced 1H polarization was subsequently transferred to 13C and 15N via cross-polarization, and allowed rapid acquisition of two-dimensional 13C−13C correlation data.