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Problem 26-1
Aromatic: Phe, Tyr, Trp, His; sulfur-containing: Cys, Met; alcohols: Ser, Thr; hydrocarbon side chains: Ala, Ile, Leu, Val, Phe
Problem 26-2
The sulfur atom in the −CH2SH group of cysteine makes the side chain higher in ranking than the −CO2H group.
Problem 26-4
Net positive at pH = 5.3; net negative at pH = 7.3
Problem 26-5
Start with 3-phenylpropanoic acid: 1. Br2, PBr3; 2. NH3
Start with 3-methylbutanoic acid: 1. Br2, PBr3; 2. NH3
Problem 26-8
Val-Tyr-Gly (VYG), Tyr-Gly-Val (YGV), Gly-Val-Tyr (GVY), Val-Gly-Tyr (VGY), Tyr-Val-Gly (YVG), Gly-Tyr-Val (GYV)
Problem 26-12

Trypsin: Asp-Arg + Val-Tyr-Ile-His-Pro-Phe

Chymotrypsin: Asp-Arg-Val-Tyr + Ile-His-Pro-Phe

Problem 26-13
Problem 26-15
Val-Met-Trp-Asp-Val-Leu (VMWNVL)
Problem 26-16
This is a typical nucleophilic acyl substitution reaction, with the amine of the amino acid as the nucleophile and tert-butyl carbonate as the leaving group. The tert-butyl carbonate then loses CO2 and gives tert-butoxide, which is protonated.
Problem 26-17

(1) Protect the amino group of leucine.

(2) Protect the carboxylic acid group of alanine.

(3) Couple the protected amino acids with DCC.

(4) Remove the leucine protecting group.

(5) Remove the alanine protecting group.

Problem 26-18
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