03_蛋白质化学3

nullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnull1. The structure of the amino acid isoleucine isHow many chiral centers does it have? (b) How many optical isomers? (c) Draw perspective formulas透视图 for all the optical isomers of isoleucine光学异构体.null2. A peptide has the sequence: Glu-His-Trp-Ser-Gly-Leu-Arg-pro-Gly (a) What is the net charge of the molecule at pH 3, 8, and 11? (Use pK values for side chains and terminal amino and carboxyl groups as given in text. (b) Estimate the pI for this peptide.null3. One method for separating polypeptides makes use of their differential solubilities.The solubility of large polypeptides in water depends upon the relative polarity of their R groups, particularly on the number of ionized groups: the more ionized groups there are, the more soluble the polypeptide. Which of each pair of the polypeptides that follow is more soluble at the indicated pH?(a) (Gly)20 or (Glu)20 at pH 7.0 (b) (Lys-Ala)3 or (Phe-Met)3 at pH 7.0 (c) (Ala-Ser-Gly)5 or (Asn-Ser-His)5 at pH 6.0 (d) (Ala-Asp-Gly)5 or (Asn-Ser-His)5 at pH 3.0