Figure 1.

Low temperature solid-state NMR (SSNMR) ¹⁵N─¹³C correlation spectra of (a) ¹³C,¹⁵N-Ile labeled E. coli DHFR:TMP (I-DHFR) and (b) ¹³C,¹⁵N-Ile, ¹⁵N-Gly labeled E. coli DHFR:TMP (IG-DHFR) measured using DNP enhanced SSNMR at 105 K. Ile60-Ile61(sheet) is the only residue pair expected in I-DHFR DNP spectra because of a sparse isotopic enrichment scheme (Ile only is enriched). In IG-DHFR, 4 more residue pairs are observed (Ile2-Ser3 (sheet), Ile14-Gly15 (coil), Ile50-Gly51 (helix), Ile94-Gly95 (coil)). The 5 pairs are highlighted with stick rendering of the side chains in cartoon of the structure of E. coli DHFR:TMP (c): I2-S3 (green), I14-G15 (cyan), I50-G51 (blue), I94-G95 (orange), and I60-I61 (red). The predicted chemical shifts for conformations sampled from MD and predicted with QM/MM are overlaid: 100 snapshots were minimized and calculated for Ile60-Ile61 in (a) and 20 snapshots were used for residue pairs in (b). Both C and N chemical shifts from QM/MM calculation were calibrated with the offsets from average chemical shifts of snapshots and solution-state NMR chemical shifts. The lowest contour levels in SSNMR spectra are set to be 15 times (a) and 3.5 times (b) the RMS noise level, and the multiplier is 1.3 for both (a) and (b). The peak at ¹³Cα 53–55 ppm in (b) results from a spinning sideband and was not predicted. The cross markers in (b) indicate the solution-state NMR chemical shifts for the 5 residue pairs.