Fluorine-Labeled N-Boc-L-proline as a Marker for Solid-State NMR Characterization of Biofunctionalizations on Paper Substrates

An efficient approach employing 4-dimethylamino-pyridine and 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hy-drochloride as the coupling reagent is presented, which enables the functionalization of cotton linter paper substrates with the 19F spin label N-boc-cis-4-fluoro-L-proline. This spin label can be easily quantified by 19F magic angle spinning (MAS) NMR experiments to determine its loading on paper substrates. During the functionalization, the spin label stays intact, as confirmed by the 1H-19F heterocorrelation (1H -> 19F CP MAS FSLG HETCOR) experiments. In combination with dynamic nuclear polarization (19F MAS DNP), the N-boc-cis-4-fluoro-L-proline spin label allows us to inspect 1 mu mol/g and even lower molecule loadings on paper substrates, providing a highly sensitive local probe to analyze the structure of biofunctionalizations at the nanoscale on paper substrates in the future.