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494 Chapter 26 AMINO ACIDS, PEPTIDES, PROTEINS, AND NUCLEIC ACIDS: NITROGEN-CONTAINING POLYMERS IN NATURE Treatment of this product with hot aqueous acid gives cysteine directly. Otherwise. 1. NaOH 1. H₂O₂ 2. N Alkylate on sulfur C(CO₂CH₂CH₃)₂ CH₂=CHCH₂SCH₂ 41. The alloisoleucines are diastereomers of the isoleucines. so simple recrystallization will separate them. Crystals 1. Dissolve in (+)- and (-)-isoleucine hot 80% ethanol 2. Cool to 0° Mixture Solutions (+)- and (-)-alloisoleucine Continue with each mixture of enantiomers separately, making use of brucine as a resolving agent. 1. Brucine 0°C salt of (+)-acid RCNH 2. Separate (crystallization) (+)/(-) salt of (-)-acid Finally, treatment releases each pure amino acid enantiomer in turn. 42. (a) tripeptide; (b) dipeptide; (c) tetrapeptide; (d) pentapeptide. The peptide bonds are simply the amide linkages, For in tripeptide (a): (CH₃)₂CH 43. By the short notation format always begins with the end of the peptide chain with the amino group (the "N-terminal" or "amino-terminal" end). (a) (b) Ser-Asp: (c) His-Thr-Pro-Lys: (d) Tyr-Gly-Gly-Phe-Leu