Strategy for Enhancement of ¹³C‑Photo-CIDNP NMR Spectra by Exploiting Fractional ¹³C‑Labeling of Tryptophan

The photo-CIDNP effect has proven to be useful to strongly enhance NMR signals of photochemically active proteins simply by irradiation with light. The evolving characteristic patterns of enhanced absorptive and emissive NMR lines can be exploited to elucidate the photochemistry and photophysics of light-driven protein reactions. In particular, by the assignment of ¹³C NMR resonances, redox-active amino acids may be identified and thereby electron-transfer pathways unraveled, in favorable cases, even with ¹³C at natural abundance. If signal enhancement is weak, uniform ¹³C isotope labeling is traditionally applied to increase the signal strength of protein ¹³C NMR. However, this typically leads to cross relaxation, which transfers light-induced nuclear-spin polarization to adjacent ¹³C nuclei, thereby preventing an unambiguous analysis of the photo-CIDNP effect. In this contribution, two isotope labeling strategies are presented; one leads to specific but ubiquitous ¹³C labeling in tryptophan, and the other is based on fractional isotope labeling affording sets of isotopologs with low probability of next-neighbor isotope accumulation within individual tryptophan molecules. Consequently, cross relaxation is largely avoided while the signal enhancement by ¹³C enrichment is preserved. This results in significantly simplified polarization patterns that are easier to analyze with respect to the generation of light-generated nuclear-spin polarization