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.