atomic number at thefrst point of difference, and—C—S outranks—C—O. In all theother amino acids—CO22outranks the substituent at the stereogenic center. The reversal inthe Cahn–Ingold–Prelog descriptor comes not from any change in the spatial arrangement ofsubstituents at the stereogenic center but rather from a reversal in the relative ranks of thecarboxylate group and the side chain.(c)The order of decreasing sequence rule precedence in L-methionine isSulfur is one atom further removed from the stereogenic center, and so C—O outranksC—C—S.The absolute confguration is S.27.2The amino acids in Table 27.1 that have more than one stereogenic center are isoleucine and threo-nine. The stereogenic centers are marked with an asterisk in the structural formulas shown.27.3(b)The zwitterionic form of tyrosine is the one shown in Table 27.1.(c)As base is added to the zwitterion, a proton is removed from either of two positions, the am-
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