S -2 6Amino Acids, Peptides,and Proteinschapter 31. Absolute Configuration of Citrulline The citrulline isolated from watermelons has the structureshown below. Is it a D- or L-amino acid? Explain.Answer Rotating the structural representation 180Њ in the plane of the page puts the mosthighly oxidized group—the carboxyl (OCOOϪ ) group—at the top, in the same position as theOCHO group of glyceraldehyde in Figure 3–4. In this orientation, the a-amino group is on theleft, and thus the absolute configuration of the citrulline is L.2. Relationship between the Titration Curve and the Acid-Base Properties of Glycine A 100 mLsolution of 0.1 M glycine at pH 1.72 was titrated with 2 M NaOH solution. The pH was monitored and theresults were plotted as shown in the following graph. The key points in the titration are designated I to V.For each of the statements (a) to (o), identify the appropriate key point in the titration and justify yourchoice.Note: before considering statements (a) through (o), refer to Figure 3–10. The three species involvedin the titration of glycine can be considered in terms of a useful physical analogy. Each ionic species canbe viewed as a different floor of a building, each with a different net charge:122468011.300.5OHϪ (equivalents)pH1.01.52.0(V)9.60(IV)(III)2.34(I)(II)5.9710CCO)H (CHNH2NH22 2PHCNϩ H3COOϪ2608T_ch03sm_S26-S43 2/1/08 11:45AM Page 26 ntt 102:WHQY028:Solutions Manual:Ch-03:Chapter 3Amino Acids, Peptides, and ProteinsS-27ϩH3NOCH2OCOOHϩ1ϩH3NOCH2OCOOϪ0 (zwitterion)H2NOCH2OCOOϪϪ1The floors are connected by steep stairways, and each stairway has a landing halfway between thefloors. A titration curve traces the path one would follow between the different floors as the pHchanges in resp...
