Avance neo

All NMR spectra were recorded at 25 °C on 500 MHz Agilent DirectDrive 2 and 700 MHz Bruker Neo Avance spectrometers equipped with room temperature triple-resonance probes. A two-dimensional ¹H-¹⁵N-HSQC and three-dimensional HNCACB, CBCA(CO)NH, HN(CA)CO and HNCO experiments were used to obtain the backbone resonance assignments. Additionally, a two-dimensional ¹H-¹³C-HSQC and three-dimensional (H)CC(CO)NH-TOCSY, H(CCO)NH-TOCSY, ¹H-¹⁵N-TOCSY-HSQC, ¹H-¹⁵N-NOESY-HSQC and ¹H-¹³C-NOESY-HSQC experiments were used for side-chain assignments. Aromatic side-chains of phenylalanines and tyrosines were assigned with aromatic ¹H-¹³C-HSQC experiments. Data was processed with NMRPipe (Delaglio et al. 1995 (link)) and the CcpNMR software package was used for resonance assignment (Vranken, 2005 (link)).

⁷³Ge NMR experiments were performed on a 20 T Bruker Neo
Avance spectrometer equipped with a low-gamma 4 mm HX probe tuned
to X = ⁷³Ge at ν₀ = 29.66 MHz. One-dimensional
NMR spectra were acquired under static conditions using the WURST-QCPMG
and Double Frequency Sweeps (DFS) DFS spin echo pulse sequences,^{42 (link)−44 (link)} and the experimental parameters were varied in an attempt to detect
signal. The unfavorable NMR properties of ⁷³Ge (see Table S1) precluded the observation of ⁷³Ge resonances.

All reagents were procured from Alfa, Aldrich (Miami, FL, USA), and further utilized without purification. In a 10 mL closed brown vial attached with a septum, a solution of H₂O/MeCN (2 mL/2 mL) within l-histidine (2.0 g) was purged with N₂ for 15 min, and then 4 equivalents of NH₄Cl was mixed. Afterward, the solution was stirred at room temperature for about 2 h. The resultant suspension was dried in vacuum, and the powder was purified by water and Et₂O subsequently. Then, the residue was dried in vacuum to obtain the purified product. The structure was analyzed by Fourier transform infrared spectroscopy (FTIR; Thermo Scientific Nicolet; Waltham, MA, USA), nuclear magnetic resonance spectroscopy (NMR; Bruker AVANCE NEO; Karlsruhe, Baden-Württemberg, Germany), and high-resolution mass spectra (HRMS; Finnigan LTQ-FT instrument; Waltham, MA, USA), respectively.

The details of the method for analyzing l-pantolactone, d-pantolactone, and ketopantolactone by gas chromatography (Agilent 7890A, Agilent Technologies, Inc., Santa Clara, CA, USA) are as follows: detector: FID; chiral capillary column: BGB-174 (30 m × 250 µm × 0.25 µm; BGB Analytik, Böckten, Switzerland); carrier gas: N₂; flow rate: 30.0 mL/min; split ratio: 30:1; injection volume: 1 µL; injector and detector temperature: 250 °C [8 (link)]. The column temperature was kept constant at 175 °C for 10 min. The retention times for d-pantolactone, l-pantolactone, and ketopantolactone were 6.19, 6.51, and 6.78 min, respectively (Figure S9).
Upon completion of the catalytic reaction, the reaction mixture was treated with ethyl acetate; then, the product in the resulting organic phase was collected through evaporation of the solvent. The resulting crude product was validated by GC-MS (Agilent 7890A/5975C, Agilent Technologies Inc., Santa Clara, CA, USA) using previously reported parameters (Figure S10) [8 (link)]. The crude product was dissolved in CDCl₃ for NMR analysis (Avance NEO, Bruker, Switzerland) to further verify the remaining product, d-pantolactone. The NMR spectroscopy was operated at 600 MHz for ¹H and 151 MHz for ¹³C detection (Figure S11).