Sodium hydroxide anhydrous pellets

L-Lysine (> 98%, FG), hydrogen chloride (HCl, 37%) and sodium deuteroxide (NaOD) solutions, were purchased from Sigma Aldrich, and sodium hydroxide anhydrous pellets were purchased from Carlo Erba Reagent, ITA. Deuterium chloride (DCl, 20%) was purchased from Cambridge Isotope Laboratories, Inc. Distilled water was used as a solvent.
l-Lysine nanopolymers were obtained by hydrothermal treatment. 0.128 g of l-lysine were solubilized in 10 mL of distilled H₂O (0.87 M). The solutions were placed into a Teflon-lined stainless steel autoclave^{7 (link)} with a capacity of 50 mL and then heated at 130 or 200 °C for 15 h. After the hydrothermal treatment (HT), the obtained products were cooled down to 20 °C and then used for the characterizations. The samples hydrothermally treated at 130 and 200 °C are indicated in the text as HT-130 °C and HT-200 °C.
The pH of the precursor aqueous solutions of l-lysine was measured with a pH meter (pH 80 + DHS, Xs instrument). Four different pH values were used for the experiments: 2.5. 7.3 (obtained by adding 1 M HCl dropwise), 9.7 and 13 (obtained by adding droplets of 1 M NaOH).

Citric acid monohydrate (purity 99.9%, Honeywell Fluka^TM, Monza, Italy), urea for electrophoresis (purity 98%, Merck Sigma-Aldrich, Milan, Italy), sodium hydroxide anhydrous (pellets, Carlo Erba, Milan, Italy), sulfuric acid (96% w/w, Carlo Erba, Milan, Italy) and water (milli-Q) were used as received without further purification. Whatman GD/X syringe filters (Fisher Scientific, Rodano, Italy) made of a porous polyvinylidene difluoride (PVDF) membrane having a pore size of 0.20 μm were used for filtration.

The following chemical compounds were purchased from commercial suppliers and used without further purification: ureidosuccinic acid (Sigma-Aldrich Co., cat. n°69037-500MG), biuret (Sigma-Aldrich Co., cat. n°15270-25G), L-aspartic hemimagnesium salt dihydrate (Sigma-Aldrich Co., cat. n°11260-100G), L-aspartic acid (Sigma-Aldrich Co., cat. n°A8949-100G), magnesium carbonate basic (Sigma-Aldrich Co., cat. n°13118-1 KG) with a Brunauer–Emmett–Teller surface area of 31.5 m²/g, sodium cyanate (Sigma-Aldrich Co., cat. n°185086-100G), sodium hydroxide anhydrous pellets (Carlo Erba reagents, cat. n°480507), deuterium oxide 99.90% D (Eurisotop, cat. n°D214FE), linear dimer H-Asp-Asp-OH (Bachem, cat. n°4010210.0250), ammonium carbamate (Sigma-Aldrich Co., cat. n°292834-100G), ammonia 28% analaR Normapur (VWR Chemicals, cat n°21190.292), trisodium trimetaphosphate (Sigma-Aldrich Co., cat. n°T5508-500G), urea ACS reagent (Sigma-Aldrich Co., cat. n°U5128-100G), ammonium carbonate ACS reagent (Aldrich chemical company, Inc., cat. n°20786-1), carbamoyl phosphate disodium salt (Sigma-Aldrich Co., cat. n°C4135-1G), fumed silica Aerosil 380 (Evonik Industries), with a Brunauer–Emmett–Teller surface area of 380 m²/g.