Improved sensitivity of laser-enhanced ¹H^α-¹³C^α-correlation via suppression of C^α-C' scalar-coupling evolution

Low-concentration photochemically induced dynamic polarization (LC-photo-CIDNP) enables the spectroscopic analysis of biomolecules containing the amino acids Trp and Tyr at sub-micromolar concentration in solution. Typical LC-photo-CIDNP pulse sequences involving ¹H-¹³C correlation, however, perform well in the case of aromatic resonances but display a relatively poor signal-to-noise ratio for ¹³C^α and ¹³C^β resonances. Here, we develop a novel pulse sequence denoted as ¹³C perturbation-recovered selective-pulse photo-CINDP enhanced reverse INEPT, or ¹³C PRESPRINT, tailored to the LC-photo-CIDNP analysis of ¹H-¹³C^α pairs. Our method, which is based on full suppression of 1-bond C^α-C' scalar-coupling evolution during the constant-time delay, results into a sensitivity improvement by a factor of 2. The enhanced performance of this pulse sequence enabled us to improve the analysis of LC-photo-CIDNP laser-power dependence at very low (200 nM) sample concentration. An improved theoretical model, developed to quantitatively describe this laser-power dependence, shows excellent agreement with our ¹³C PRESPRINT experimental data.