2.2 NMR experiments
High-pressure NMR experiments were performed on an AVANCE3-950 (1H, 950.33 MHz) spectrometer (Bruker, Billerica, MA, USA) with a cryo-probe system using a pressure-resistant NMR cell (Daedalus Innovations, Aston, PA, USA). For temperature-variable experiments, the slot NMR tube for high-concentration electrolyte samples (SHIGEMI, Hachioji, Japan) was used. To measure PRE,1H/15N heteronuclear single-quantum correlation (HSQC) spectra of the MTSL-bound OspA and its variants were collected for the oxidized and reduced forms of MTSL at multiple pressures and temperatures. A 200 complex × 4096 real data matrix was acquired with spectral widths of 36.0 (15N,F 1) and 12.98 ppm (1H,F 2). NMR data were processed using Topspin version 3.2 (Bruker, Billerica, MA, USA), NMRPipe,26 and NMRViewJ.27 1H chemical shifts were referenced to the methyl signals of 2,2-dimethyl-2-silapentane-5-sulfonate (DSS), and 15N chemical shifts were indirectly referenced to DSS (0 ppm for1H).