Sensitivity enhancement in solid-state (13)C NMR of synthetic polymers and biopolymers by (1)H NMR detection with high-speed magic angle spinning.

Indirect detection of 13C and 15N nuclear magnetic resonance (NMR) spectra through 1H NMR signals offers large sensitivity advantages in studies of organic and biological molecules in solution and is almost universally employed.1-3 Although sensitivity enhancement by indirect detection was first demonstrated in NMR4-6 and nuclear quadrupole resonance7,8 of solids, direct detection has generally been preferred in solid-state NMR.9 This is because the broad 1H NMR lines of organic solids negate sensitivity enhancement under the most common conditions. We have recently shown10 that substantial sensitivity enhancements can in fact be achieved by indirect detection in one-dimensional (1D) solid-state 15N NMR spectroscopy of organic compounds and biopolymers under magic angle spinning (MAS) at speeds that greatly reduce the 1H NMR line widths.10-12 Here we demonstrate the feasibility of sensitivity enhancement in solidstate 13C NMR spectroscopy of general organic solids. We present experimental results both for the noncrystalline synthetic polymer poly(methyl methacrylate) (PMMA) and for the heptapeptide N-acetyl-Lys-Leu-Val-Phe-Phe-Ala-Glu-NH2 (Aâ16-22, representing residues 16 through 22 of the 40-residue Alzheimer’s â-amyloid peptide13) in the form of amyloid fibrils. We report enhancements in both 1D and two-dimensional (2D) experiments. Extension to 13C NMR, which forms the basis for many structural and dynamical studies in organic and biological systems, and to 2D spectroscopy significantly broadens the impact and generality of indirect detection methods in solid-state NMR. The sensitivity enhancement factor ê, defined as the ratio of frequency-domain signal-to-noise ratios for 1H-detected and 13Cdetected measurements, is given by