D threonine

d-Alanine, d-Arginine, d-Asparticacid, d-Asparagine, d-Glutamicacid, d-Histidine, d-Isoleucine, d-Leucine, d-Lysine, d-Methionine, d-Phenylalanine, d-Proline, d-Serine, d-Threonine, d-Tryptophan, d-Tyrosine, d-Valine, KBr (spectral grade), bovine serum albumin (BSA), glutathione and 3,3′,5,5′-tetramethylbenzidine (TMB) were purchased from Sigma-Aldrich Chemical Co. (St. Louis, MO, USA). d-Cystine and d-Glutarnine were purchased from Aladdin Reagent Co, Ltd. (Shanghai, China). All other chemicals and reagents were of analytical grade. All aqueous solutions were prepared with Milli-Q water.

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).