  Chemical exchange saturation transfer (CEST) is widely used for enhancing the
solution NMR signatures of magnetically-dilute spin pools; in particular
species at low concentrations undergoing chemical exchanges with an abundant
spin pool. CEST's main feature involves encoding and then detecting the weak
NMR signals of the magnetically dilute spin pools on a magnetically abundant
spin pool of much easier detection - for instance the protons of H2O. Inspired
by this method, we propose and exemplify a methodology to enhance the
sensitivity of magic-angle spinning (MAS) solid-state NMR spectra. Our proposal
uses the abundant 1H reservoir arising in organic solids as the magnetically
abundant spin pool, and relies on proton spin diffusion in lieu of chemical
exchange to mediate polarization transfer between a magnetically dilute spin
pool and this magnetically abundant spin reporter. As an initial test of this
idea we target the spectroscopy of naturally-abundant 13C, and rely on a
Fourier-encoded version of the CEST experiment for achieving broadbandness in
coordination with both MAS and heteronuclear decoupling - features normally
absent in CEST. Arbitrary evolutions of multiple 13C sites can thus be
imprinted on the entire 1H reservoir, which is subsequently detected.
Theoretical predictions suggest that orders-of-magnitude signal enhancements
should be achievable in this manner - on the order of the ratio between the 13C
and the 1H reservoirs' abundances. Experiments carried out under magic-angle
spinning conditions evidenced ca. 5-10x enhancements. Further opportunities and
challenges arising in this Fourier-Encoded Saturation Transfer (FEST) MAS NMR
approach are briefly discussed.
