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Distribution of amino acids in 68 representative proteins is compared with their distribution among 61 codons of the genetic code. Average amounts of lysine, aspartic acid, glutamic acid, and alanine are above the levels anticipated from the genetic code, and arginine, serine, leucine, cysteine, proline, and histidine are below such levels. Arginine plus lysine account for 11.0 percent of codons and aspartic acid plus glutamic acid account for 11.3 percent; thus the average charge is roughly neutral.
Jukes et al. (1975) studied this question.