1559r_ch26.454-46810/20/0515:39Page466 466 chapter 26 AMINO ACIDS,PEPTIDES,PROTEINS,AND NUCLEI ACIDS:NITROGEN-CONTAINING POLYMERS IN NATURE re for Sec above is negative.You need toreduce the pHto would resemble the Cys structure above,right.So the p/=(5.2+.03.6. (c)More cidic.buts moe. 53.(a) K+m1.H+,H0.△2.H0-,H,0.△ Mechanisms: 6:30cH,0 H2CHCH CHCOOCH CH.- 乙0：89cHcH NCH-CHCH CHCOOCH-CH,-(b) The pH 7 structure for Sec above is negative. You need to reduce the pH to below 5.2 (the pKa for the SeH group) to begin to see appreciable concentrations of the charge-neutral structure, which would resemble the Cys structure above, right. So the pI  (5.2 2.0)/2  3.6. (c) More acidic, but also more powerfully nucleophilic. 53. (a) (b) (i) Mechanisms: (i) NCH2 COOCH2CH3 O O O O NCH2CHCH2CHCOOCH2CH3 O O O O OCH2CH3 C NCH2CHCH2CHCOOCH2CH3 O O O OCH2CH3 C O NCH2CH O O CH2 CH(COOCH2CH3) O O N K O (ii) 1. H, H2O, ; 2. HO, H2O,  H N 2 H H COO OH 4R 2S 466 • Chapter 26 AMINO ACIDS, PEPTIDES, PROTEINS, AND NUCLEIC ACIDS: NITROGEN-CONTAINING POLYMERS IN NATURE 1559T_ch26_454-468 10/20/05 15:39 Page 466