Sulfur is critical
for the correct structure and proper function
of proteins. Yet, lacking a sensitive enough isotope, nuclear magnetic
resonance (NMR) experiments are unable to deliver for sulfur in proteins
the usual wealth of chemical, dynamic, and structural information.
This limitation can be circumvented by substituting sulfur with selenium,
which has similar physicochemical properties and minimal impact on
protein structures but possesses an NMR compatible isotope (77Se). Here we exploit the sensitivity of 77Se NMR to the
nucleus’ chemical milieu and use selenomethionine as a probe
for its proteinaceous environment. However, such selenium NMR spectra
of proteins currently resist a reliable interpretation because systematic
connections between variations of system variables and changes in 77Se NMR parameters are still lacking. To start narrowing this
knowledge gap, we report here on a biological 77Se magnetic
resonance data bank based on a systematically designed library of
GB1 variants in which a single selenomethionine was introduced at
different locations within the protein. We recorded the resulting
isotropic 77Se chemical shifts and relaxation times for
six GB1 variants by solution-state 77Se NMR. For four of
the GB1 variants we were also able to determine the chemical shift
anisotropy tensor of SeM by solid-state 77Se NMR. To enable
interpretation of the NMR data, the structures of five of the GB1
variants were solved by X-ray crystallography to a resolution of 1.2
Å, allowing us to unambiguously determine the conformation of
the selenomethionine. Finally, we combine our solution- and solid-state
NMR data with the structural information to arrive at general insights
regarding the execution and interpretation of 77Se NMR
experiments that exploit selenomethionine to probe proteins.
. 67, 2071 (1989).The total assignment of 'H NMR spectra of a pentacyclic triterpene from Loeselia mexicana was performed using selected 2D-NMR experiments (COSY, NOE). X-ray diffraction data were obtained from the parent compound as supplemental information to the NMR investigations. The data allowed for the unambiguous assignment of the structure and the stereochemistry of the title compound.
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