AMINO ACIDS, PEPTIDES, AND PROTEINS. NUCLEIC ACIDS 753 atomic number at the first point of difference, and-C-S outranks-C-O. In all the other amino acids-CO, 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 che (c) The order of decreasing sequence rule precedence in L-methionine is H,N->-CO.>>-CHCHSCH >H- Sulfur is one atom further removed from the stereogenic center, and so C-O outranks CO, H3N CH,,SCH CHSCHCH The absolute configuration 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. CH3CH,CH--CHCO CH NH OH NH Isoleucine Threonine 27.3(b The zwitterionic form of tyrosine is the one shown in Table 27.1 HO CHCHCO NH3 (c) As base is added to the zwitterion, a proton is removed from either of two positions, the am- monium group or the phenolic hydroxyl. The acidities of the two sites are so close that it is not possible to predict with certainty which one is deprotonated preferentially. Thus two struc- tures are plausible for the monoanion: HO -CH CHCO CH, CHCO, NH NH In fact, the proton on nitrogen is slightly more acidic than the phenolic hydroxyl, as measured by the pka values of the following model compounds pk,9.75 CHCHCO CHO -CH,CHCO, N(CH3)3 Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Websiteatomic number at the first point of difference, and —C—S outranks —C—O. In all the other amino acids —CO2 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 configuration 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￾monium group or the phenolic hydroxyl. The acidities of the two sites are so close that it is not possible to predict with certainty which one is deprotonated preferentially. Thus two struc￾tures are plausible for the monoanion: In fact, the proton on nitrogen is slightly more acidic than the phenolic hydroxyl, as measured by the pKa values of the following model compounds: HO CH2CHCO2 N(CH3)3 CH3O CH2CHCO2 NH3 pKa 9.75 pKa 9.27 HO and CH2CHCO2 NH2 O CH2CHCO2 NH3 HO CH2CHCO2 NH3 Isoleucine CH3CH2CH CH3 CHCO2 * * NH3 Threonine CH3CH OH CHCO2 * * NH3 CH2CH2SCH3 H CO2 H3N CO2 NH3 H CH3SCH2CH2 C H3N H  CO2 CH2CH2SCH3   AMINO ACIDS, PEPTIDES, AND PROTEINS. NUCLEIC ACIDS 753 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website