Amino acid salts
Except for two commercially available sodium salts, monosodium L-glutamate and L-proline sodium salt, amino acid salts were prepared following the previous methods (Dai, et al., 2019; Jiang, 1969). In this study, a crystallization step was added to obtain crystalline solids (steps 3-6 in the following procedure). A general procedure is: 1) Alanine (3.0 g, 33.7 mmol) was weighed in a 250 ml round bottom flask and 25% (wt.) NaOH solution (5.39 g, 33.7 mmol) was added. 2) Water (2 g) was added, and alanine was completely dissolved in the solution. 3) When ethanol (100 ml) was added, precipitates were formed. 4) The volume of the solution was reduced to about 10 ml using a rotary evaporator at 30 °C, and ethanol (100 ml) was added. This step was repeated twice more. 5) At the end of the step 4, 30 ml of ethanol was added in the flask instead of 100 ml, which was then placed in a freezer (-20 °C) for 1 h. 6) Solid was filtered and dried under vacuum overnight. Hereafter we use the acronym, amino acid Na or K for sodium or potassium salt of an amino acid. For example, “alanine Na” refers to the sodium salt of alanine. Alanine Na (2.20 g, 58.8% yield) was obtained as white powder. Arginine Na (2.69 g, 79.6%), asparagine Na (2.29 g, 65.4%), glutamic acid di-Na (2.12, 54.1%), phenylalanine Na (2.20 g, 64.7%), and tyrosine Na (2.83 g, 84.1%) were obtained by this method. It should be noted that this study did not attempt to optimize the yields, but focused more on producing fine powder. Glutamine Na did not form crystalline solids. To obtain powder of Glutamine Na, after step 6, all the solvent was evaporated using the rotary evaporator followed by further drying using a vacuum pump overnight, and the solid was scraped off the round bottom flask.
Amino acid potassium salts were prepared following the general procedure used for sodium salts, but with an equimolar amount of 25% (wt.) KOH solution. Alanine K (2.82 g, 65.8%), glutamine K (1.89 g, 50.0%), phenylalanine K (2.10 g, 56.9%), and proline K (2.35 g, 58.9%) were obtained. Other amino acid potassium salts such as arginine K, asparagine K, glutamic acid K, glutamic acid di-K, and tyrosine K were also prepared, but they were hygroscopic, did not make powder, and could not be dispersed in oil. The purpose of this study is to compare the antioxidant activity of amino acid salts with the corresponding amino acids, which were dispersed in oil. However, amino acid salts that cannot be dispersed in oil as power cannot be compared with the corresponding amino acids under the same conditions. Therefore, these amino acids were not included in this study.