[NMR paper] A Strategy for Enhancement of (13)C-photo-CIDNP NMR Spectra by Exploiting Fractional (13)C Labeling of Tryptophan.

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A Strategy for Enhancement of (13)C-photo-CIDNP NMR Spectra by Exploiting Fractional (13)C Labeling of Tryptophan.

A Strategy for Enhancement of (13)C-photo-CIDNP NMR Spectra by Exploiting Fractional (13)C Labeling of Tryptophan.

J Phys Chem B. 2015 Aug 5;

Authors: Eisenreich W, Joshi M, Illarionov B, Kacprzak S, Lukaschek M, Kothe G, Budisa N, Fischer M, Bacher A, Weber S

Abstract
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 polarization 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.


PMID: 26244593 [PubMed - as supplied by publisher]



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