Magnetic-Field Dependence of LC-Photo-CIDNP in the Presence of Target Molecules Carrying a Quasi-Isolated Spin Pair

NMR spectroscopy is well known for its high atomic-scale resolution, especially at high applied magnetic field. However, the sensitivity of this technique is marginal. Liquid-state low-concentration photo-chemically induced dynamic nuclear polarization (LC-photo-CIDNP) is a promising emerging technology capable of enhancing NMR sensitivity in solution. LC-photo-CIDNP works well on solvent-exposed Trp and Tyr residues, either in isolation or within proteins. This study explores the magnetic-field dependence of the LC-photo-CIDNP experienced by two tryptophan isotopologs in solution upon in situ LED-mediated optical irradiation. Out of the two uniformly ¹³C,¹⁵N-labeled Trp (Trp-U-¹³C,¹⁵N) and Trp-α-¹³C-β,β,2,4,5,6,7-d₇ species employed here, only the latter bears a quasi-isolated ¹H^α-¹³C^α spin pair. Computer simulations of the predicted polarization due to geminate recombination of both species display a roughly bell-shaped field dependence. Specifically, Trp-U-¹³C,¹⁵N is predicted to show a fairly weak field dependence with a maximum polarization at ca. 500 MHz (11.7 T). In contrast, Trp-α-¹³C-β,β,2,4,5,6,7-d₇ is expected to show a much sharper field dependence with a polarization maximum at ca. 200 MHz (4.7 T). Experimental LC-photo-CIDNP studies on both Trp isotopologs (1 μM concentration) are consistent with the theoretical predictions. In summary, this study highlights the prominent field-dependence of LC-photo-CIDNP enhancements (ε) experienced by Trp isotopologs bearing a quasi-isolated spin pair.