Towards Structure Determination of Self-Assembled Peptides Using Dynamic Nuclear Polarization Enhanced Solid-State NMR Spectroscopy
Supra-sensitivity: Dynamic nuclear polarization (DNP) enhanced solid-state NMR spectroscopy was performed on self-assembled peptide nanotubes. This approach yields significant experimental time savings (about five orders of magnitude; see picture) and was used to exemplify the feasibility of supramolecular structural studies of organic nanoassemblies at an atomic scale using DNP-enhanced solid-state NMR spectroscopy.
A New Tool for NMR Crystallography: Complete 13C/15N Assignment of Organic Molecules at Natural Isotopic Abundance Using DNP-Enhanced Solid-State NMR
NMR crystallography of organic molecules at natural isotopic abundance (NA) strongly relies on the comparison of assigned experimental and computed NMR chemical shifts. However, a broad applicability of this approach is often hampered by the still limited 1H resolution and/or difficulties in assigning 13C and 15N resonances without the use of structure-based chemical shift calculations. As shown here, such difficulties can be overcome by 13C-13C and for the first time 15N-13C correlation experiments, recorded with the help of dynamic nuclear polarization. We present the complete de novo 13C and 15N resonance assignment at NA of a self-assembled 2′-deoxyguanosine derivative presenting two different molecules in the asymmetric crystallographic unit cell. This de novo assignment method is exclusively based on aforementioned correlation spectra and is an important addition to the NMR crystallography approach, rendering firstly 1H assignment straightforward, and being secondly a prerequisite for distance measurements with solid-state NMR.
Welcoming natural isotopic abundance in solid-state NMR: probing π-stacking and supramolecular structure of organic nanoassemblies using DNP
The self-assembly of small organic molecules is an intriguing phenomenon, which provides nanoscale structures for applications in numerous fields from medicine to molecular electronics. Detailed knowledge of their structure, in particular on the supramolecular level, is a prerequisite for the rational design of improved self-assembled systems. In this work, we prove the feasibility of a novel concept of NMR-based 3D structure determination of such assemblies in the solid state. The key point of this concept is the deliberate use of samples that contain 13C at its natural isotopic abundance (NA, 1.1%), while exploiting magic-angle spinning dynamic nuclear polarization (MAS-DNP) to compensate for the reduced sensitivity. Since dipolar truncation effects are suppressed to a large extent in NA samples, unique and highly informative spectra can be recorded which are impossible to obtain on an isotopically labeled system. On the self-assembled cyclic diphenylalanine peptide, we demonstrate the detection of long-range internuclear distances up to ∼7 Å, allowing us to observe π-stacking through 13C–13C correlation spectra, providing a powerful tool for the analysis of one of the most important non-covalent interactions. Furthermore, experimental polarization transfer curves are in remarkable agreement with numerical simulations based on the crystallographic structure, and can be fully rationalized as the superposition of intra- and intermolecular contributions. This new approach to NMR crystallography provides access to rich and precise structural information, opening up a new avenue to de novo crystal structure determination by NMR.