Abstract
While atomic scale structural and dynamic information are hallmarks of nuclear magnetic resonance (NMR) methodologies, sensitivity is a fundamental limitation in NMR studies. Fully exploiting NMR capabilities to study membrane proteins is further hampered by their dilution within biological membranes. Recent developments in dynamic nuclear polarization (DNP), which can transfer the relatively high polarization of unpaired electrons to nuclear spins, show promise for overcoming the sensitivity bottleneck and enabling NMR characterization of membrane proteins under native-like conditions. Here we discuss fundamental aspects of DNP-enhanced solid-state NMR spectroscopy, experimental details relevant to the study of lipid assemblies and incorporated proteins, and sensitivity gains which can be realized in biomembrane-based samples. We also present unique insights which can be gained from DNP measurements and prospects for further development of the technique for elucidating structures and orientations of membrane proteins in native lipid environments.
Highlights
Solid-state nuclear magnetic resonance (NMR) spectroscopy is distinctly capable of determining membrane protein structure and dynamics at high resolution within native lipid environments [1,2,3,4]; it can provide unique insights into lipid organization and dynamics [5,6]
We have found that 10% (v/v) DMSO provides sufficient cryoprotection and preserves polarizing agents (PAs) distribution for optimal MAS-dynamic nuclear polarization (DNP) NMR measurements using multilamellar lipid vesicle (MLV) preparations of membrane active peptides, described below, while 30–60% (v/v) glycerol leads to alteration of KL4 peptide structure [65]
In order to assess the viability of DNP samples for more complex NMR experiments requiring longer transverse relaxation times, we assessed the impact of PA on 13C T2 relaxation times in the samples with well-dispersed PA (Table 3)
Summary
Solid-state NMR spectroscopy (ssNMR) is distinctly capable of determining membrane protein structure and dynamics at high resolution within native lipid environments [1,2,3,4]; it can provide unique insights into lipid organization and dynamics [5,6]. The application range of NMR spectroscopy is sensitivity restricted due to the small transition energies involved resulting in low detectable polarization In this context, membrane protein structural biology via ssNMR spectroscopy is challenging due to protein dilution within lipid membranes, the smaller protein yields typically observed compared to soluble proteins, and the structural heterogeneity often observed even under optimal sample conditions further restricting the sensitivity. Recent mechanistic studies characterizing polarization transfer pathways within large spin systems under MAS-DNP conditions have provided insights into how further gains might be realized [25,26] Rapid progress in these areas have widely benefited biological applications of MAS-DNP-enhanced ssNMR spectroscopy. Signal enhancements can be utilized to study more challenging NMR-active nuclei such as 17O [31] and 43Ca [32,33], providing novel insights into membrane protein mechanisms
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