L lysine hydrochloride

Chitosan, derived from crab shell with a minimum deacetylation degree of 85%, acetic acid (99%), anhydrous sodium acetate and tyrosinase from mushroom were purchased from Sigma-Aldrich. L-Lysine hydrochloride (BP386) was purchased from Fisher Bioreagents.
Chitosan molecular weight was found to be 900 kDa ± 5% determined by viscometric methods. The procedure has been described elsewhere.[36 (link)]
Tilapia fish skin gelatin (Type A, acidic extracted) was kindly donated by Rousselot (France). The average molecular weight was determined to be ca 36 kDa ± 12% using gel permeation chromatography (GPC; Smithers Rapra) with a melting temperature of ≈ 23 °C.
mTGase was purchased from N-Zyme BioTec GmbH (Darmstadt, Germany). mTGase has a specific activity of 1.6 Units mg⁻¹ solid, a molecular weight of 38 kDa and a purity superior to 80% according to sodium dodecylsulfonate-polyacrylamide gel electrophoresis (SDS-PAGE), as indicated by the supplier. All compounds were used as received. Deionised water was used throughout the experiments.

A pooled urine sample from healthy humans was obtained from (Innovative Research, Inc., Novi, MI). The following metabolites were obtained for the preparation of model mixtures: citric acid monohydrate (C₆H₈O₇•H₂O, 99%), N,N-dimethylglycine (C₄H₉O₂N, 99%), D-glucose (C₆H₁₂O₆, 99.5%), L-glutamic acid (C₅H₉NO_4, 98.5%), L-histidine (C₆H₉N₃O₂, 98.5%), sodium lactate (C₃H₅NaO₃, 98%), sodium phosphate dibasic (Na₂HPO₄, 99.0%), sodium phosphate monobasic (NaH₂PO₄, 99.0%), and 3-(Trimethylsilyl)-1-propanesulfonic acid sodium salt (DSS) were obtained from Sigma-Aldrich. L-alanine (C₃H₇NO₂, 98.5%), L-arginine (C₆H₁₄N₄O₂, 98.5%), L-asparagine monohydrate (C₄H₈N₂O₃•H2O, 98.5%), L-glutamine (C₅H₁₀N₂O₃, 98.5%), L-lysine hydrochloride (C₆H₁₄N₂O₂•HCl, 98.5%), and L-valine (C₅H₁₁NO₂, 98.5%) were purchased from Fisher BioReagents. Deuterium oxide (D₂O, 99.0%) was purchased from Cambridge Isotope Laboratory, Inc., Andover, MA. Amicon Ultra-15^® Centrifugal Filter Units (3k and 10k NMWL) was purchased from Merck Millipore Ltd., Darmstadt, Germany.