Dolbier HW Solutions 659

Dolbier HW Solutions 659 - 753 AMINO ACIDS PEPTIDES AND...

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atomic number at the f rst point of difference, and C S outranks C O. In all the other amino acids CO 2 2 outranks the substituent at the stereogenic center. The reversal in the Cahn Ingold Prelog descriptor comes not from any change in the spatial arrangement of substituents at the stereogenic center but rather from a reversal in the relative ranks of the carboxylate group and the side chain. ( c ) The order of decreasing sequence rule precedence in L -methionine is Sulfur is one atom further removed from the stereogenic center, and so C O outranks C C S. The absolute con f guration is S . 27.2 The 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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