1 (558)

Prévia do material em texto

Solutions to Problems 495 44. Determine the net charge on the amino acid or peptide at pH 7 and then recall that negative species migrate to the anode (A), positive to the cathode (C), and neutrals do not migrate at all (N). Amino acids (Problem 32): (a), (b), (d), (e) N, (f) A, (c), (g) C Peptides (Problem 42): (a) N, (b) A, (c) C, (d) N 45. The side chains are all small (-H, -CH₃, or and mostly nonpolar. In the illustrations, especially Figure 26-3, note that the sheet structure packs the R groups into small channels between layers of sheets, where only small groups will fit easily. The nonpolar nature of five of the six groups is also compatible with their location, a relatively nonpolar region with few hydrogen-bonding groups in the vicinity. 46. The α-helical stretches are fairly noticeable by their spiral shape (compare Figure 26-4). Myoglobin in fact contains eight significant α-helical stretches, which are labeled by the letters A-H: α-Helix Amino acid numbers α-Helix Amino acid numbers A 3-18 E 58-77 B 20-35 F 86-94 C 36-42 G 100-118 D 51-57 H 125-148 In the figure, all but α-helix D (which is viewed on-end from this perspective) are fairly easy to pick out. The four prolines are located at or near the ends of α-helices and coincide with "kinks" in the overall tertiary structure of the molecule, a result of the conformational characteristics of the five-membered ring. Pro 100 Pro 88 Pro 37 Pro 120 47. Except for the two histidines associated with the heme-bound iron atom, all the polar side chains are well positioned for hydrogen bonding with solvent molecules (water). In contrast, all the nonpolar side chains adopt interior positions, avoiding contact with polar solvent molecules. 48. (a) The sheet structure is favored by amino acids with small, nonpolar side chains and has very little ability to hydrogen bond to a polar solvent like water (Problem 45). (b) In globular proteins the polar side chains are exposed to the solvent. solubilizing the entire molecule (Problem 47). Similar effects are seen in micelles formed by soap in which polar groups are located on the facilitating water solubility, whereas nonpolar groups are buried in the inside (Chemical Highlight 19-1). (c) If the tertiary structure of a globular protein is disrupted. its nonpolar amino acid side chains become exposed to the polar solvent. greatly reducing the overall solubility of the protein molecule. 49. Purify and cleave the disulfide bridge (Section 9-10). (2) On a portion of the sample. degrade the entire chain by amide hydrolysis (6 N 110°C. 24 h) to determine amino acid composition by using an amino acid analyzer. (3) On another portion of this material. apply repetitive Edman degradation to determine the sequence of amino acids. Because only nine are present. the entire chain can be sequenced in this way.