Spectroscopically Orthogonal Labelling to Disentangle Site-Specific Nitroxide Label Distributions

Area of Science:

• Structural biology
• Biophysics
• Spectroscopy

Background:

• Pulse dipolar electron paramagnetic resonance spectroscopy (PDS) is a powerful tool for biomolecular structure determination.
• Site-directed spin labeling commonly uses nitroxides, but alternative labels like paramagnetic metal ions offer potential for orthogonal labeling.
• Analyzing PDS data can be complex, with rotamer information often lost and contributions to distance distribution width unclear.

Purpose of the Study:

• To investigate the utility of double-histidine (dHis) motifs labeled with Cu(II) chelate complexes as spin labels in PDS.
• To compare the contributions of Cu(II) and nitroxide labels to distance distribution widths.
• To assess the impact of different in silico modeling approaches on PDS data interpretation.

Main Methods:

• Utilized double-histidine (dHis) motifs for precise labeling with Cu(II) chelate complexes.
• Employed cysteine-specific nitroxide labeling for orthogonal spin labeling.
• Combined dHis Cu(II) and nitroxide labels on a model protein (GB1).
• Analyzed distance distributions and rotamer contributions using various in silico modeling techniques.

Main Results:

• Cu(II) chelate complexes attached to dHis motifs showed negligible contribution to distance distribution widths.
• The study successfully gathered insights into label rotamers at distinct sites by combining orthogonal labels.
• Discrepancies were observed between different in silico modeling approaches.

Conclusions:

• Cu(II) chelate complexes are precise and minimally broadening spin labels for PDS applications in structural biology.
• Combining orthogonal labels like dHis Cu(II) and nitroxides provides richer information on rotameric states.
• Careful consideration of in silico modeling discrepancies is crucial when selecting labeling sites for PDS studies.