D histidine

The chemical structure of the PBI derivative (D-PBI) is reported in Figure 1. It was obtained according to the procedure reported in the literature (Gershberg et al., 2015 (link)).
Milli-Q grade water was used to dissolve the PBI derivative and all the organic compounds (D-phenylalanine, L-phenylalanine, D-serine, D-histidine), used in this contribution, were purchased from Sigma-Aldrich and used as received.

All L-amino acids, D-amino acids (except D-histidine), purines, Valine analogs, Terbium chloride (212903), Maltose (M5895), Sodium phosphate dibasic (S7907), o-phthaldialdehyde (P0657), N-acetyl cysteine (A7250), Sodium tetraborate decahydrate (B3545), HPLC grade methanol (34860), 1-Propanol (34871), Trifluoroacetic acid (302031) and Mutanolysin enzyme (M9901) were purchased from Sigma. 2-Propanol (29113-95) was obtained from Nacalai Tesque. D-histidine (sc-255057) was purchased from SantaCruz Biotechnology. Acetonitrile (1.00317) and HCl (1.00029) were purchased from Merck. Percoll (17089109) was purchased from GE Healthcare. Oxyrase for broth was purchased from Oxyrase Inc and Sigma (SAE0013). BBL polypeptone peptone (211910), Dehydrated cooked meat media (226730), Brain heart infusion broth (BHI, 237500) and Reinforced Clostridial medium (RCM, 218081) were purchased from BD. Fetal bovine serum (S1810) was obtained from iDNA Biotechnology.
C. novyi-NT was a gift from the Kinzler-Vogelstein lab in John Hopkins University, USA.

D-methionine, D-tryptophan, D-leucine, D- or L-cysteine, D- or L-aspartic acid, D- or L-glutamic acid, D-alanine, D-threonine, D-valine, D-isoleucine, D-glutamine, D-phenylalanine, D-histidine, D-proline, D-lysine, D-arginine, and glycine (Sigma-Aldrich, St. Louis, MO, USA) were all prepared at a concentration of 40 mM, and D-tyrosine was prepared at a concentration of 0.8 mM due to its low solubility. Nisin powder (2.5% purity, Sigma-Aldrich, St. Louis, MO, USA) was dissolved in a solution at pH 2 to obtain a stock solution with a concentration of 1 g/L.

All reagents were of analytical grade. FMOC-Cl, β-CD, pentane, sodium tetraborate, sodium hydroxide, glycine, d-glutamic acid, d-histidine, d-threonine, l-alanine, l-arginine, l-asparagine, l-aspartic acid, l-cysteine, l-glutamic acid, l-glutamine, l-histidine, l-isoleucine, l-leucine, l-lysine, l-methionine, l-proline, l-serine, l-threonine, l-tryptophan, l-tyrosine and l-valine, dl-alanine, dl-arginine, dl-asparagine, dl-aspartic acid, dl-cysteine, dl-glutamic acid, dl-histidine, dl-isoleucine, dl-leucine, dl-lysine, dl-phenylalanine, dl-proline, dl-serine, dl-tryptophan, and dl-valine were from Sigma-Aldrich (Steinheim, Germany). Isopropanol, dl-methionine, dl-tyrosine, sodium dodecyl sulfate, and acetonitrile were supplied by Fluka (Steinheim, Germany). Water was deionized and purified with a Milli-Q purification system (Millipore, Belford, NJ, USA).
The optimal BGE was 40 mM sodium tetraborate (adjusted to pH 9.5 with 1 M sodium hydroxide) containing 15% (v/v) isopropanol, 30 mM sodium dodecyl sulfate (SDS), and 30 mM β-CD. The BGE was filtered prior to use through 0.45-μm pore size disposable nylon filters from VWR (Amsterdam, The Netherlands). Stock solutions (3 mM) of AAs were prepared in 0.2 M sodium tetraborate (pH 9.5).