R is for Arginine, named from the Greek árgyros word meaning silver by German chemist Ernst Schulze, who first isolated it in 1886 as arginine nitrate crystals with a silver-white appearance. Arginine can be synthesized in the human body from Glutamine. Arginine is important in the regulation of blood pressure due to it being involved in the synthesis of nitric oxide, a signaling molecule that regulates vasodilation. Arginine is also important for T-cell function, and therefore the immune system.
Arginine is encoded by 6 codons: AGA, AGG, and the 4 codons starting with CG. Note that Glutamine, from which Arginine can be synthesized, is encoded by the nearby codons CAA and CAG. Arginine is often substituted by Lysine, which encoded by nearby codons AAA and AAG. Arginine is also often substituted by Histidine, encoded by nearby codons CAC and CAU.
Most people take in 4-6 grams of Arginine per day through their diet. Blood pressure studies used doses of 6-30 grams of Arginine per day. Erectile dysfunction research showed positive results with 1.5-5 grams of Arginine per day.
Research has shown a positive impact on hospitalized covid-19 patients (https://doi.org/10.1016/j.eclinm.2021.101125).