Seventeen of the 20 naturally occurring amino acids (See the following table) exhibit exactly one chirality center. Of the remaining three amino acids, glycine has no chirality center, and the other two amino acids each have two chirality centers.

(a) Identify the amino acids with two chirality centers.

(b) Assign the configuration of each chirality center in these two amino acids.

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Abbreviation Name Structure Abbreviation Name Structure Amino acids with polar side chains Amino acids with nonpolar si de chalns н. HN. Glycine Gly G Asparagine Asn N он NH2 NH2 Gin Q Glutamine H2N Он H3C. Alanine Ala A Он NH2 NH2 Ser S Seine но он NH2 Valine Val v он он NH2 Threonine Thr T он NH3 он Leu L Leucine Tyr Y Туюaine он NH2 NH2 Cysteine Cys C HS он Isoleucine De I OH NH2 NH2 Amino acids with acidic side chains но. Aspartic acid Asp D он Methionine Met M Он NH2 NH2 Glưamic acid Giu E но он он NH2 Pro P Proline NH Amino acids with basic side chains Arg R Arginine он Phenylalanine он Phe F NH2 NH2 Histidine His H он NH2 NH он Trр W Tryptophan NH2 H2N. Lysine Lys K он NH2