Strategy for Enhancement of 13C-Photo-CIDNPNMR Spectra by Exploiting Fractional 13C-Labelingof Tryptophan.

Thephoto-CIDNP effect has proven to be useful to strongly enhanceNMR signals of photochemically active proteins simply by irradiationwith light. The evolving characteristic patterns of enhanced absorptiveand emissive NMR lines can be exploited to elucidate the photochemistryand photophysics of light-driven protein reactions. In particular,by the assignment of 13C NMR resonances, redox-active aminoacids 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 labelingis 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 thephoto-CIDNP effect. In this contribution, two isotope labeling strategiesare presented; one leads to specific but ubiquitous 13Clabeling in tryptophan, and the other is based on fractional isotopelabeling affording sets of isotopologs with low probability of next-neighborisotope accumulation within individual tryptophan molecules. Consequently,cross relaxation is largely avoided while the signal enhancement by 13C enrichment is preserved. This results in significantlysimplified polarization patterns that are easier to analyze with respectto the generation of light-generated nuclear-spin polarization. [ABSTRACT FROM AUTHOR]