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instructor_supplement_26 - I nstructor Supplemental...

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Instructor Supplemental Solutions to Problems © 2010 Roberts and Company Publishers Chapter 26 Amino Acids, Peptides, and Proteins Solutions to In-Text Problems 26.1 The peptide is drawn in the form that exists at neutral pH. (b) The structure of Glu-Gln-Phe-Arg: 26.3 (b) L -Alloisoleucine has the 2 S configuration, that is, the same configuration at the a -carbon as L -leucine, but has a different configuration at the b -carbon. D -Alloisoleucine is the enantiomer of L -alloisoleucine. 26.4 (b) In terms of the sequence rules for determining configuration, the two carbons attached to the a -carbons of cysteine or serine can be represented as C(O,O,O) and C(X,H,H), where X = S or O, respectively. When X = S (cysteine), the latter carbon receives priority. When X = O (serine), the former receives priority. Sulfur receives priority over oxygen because it has higher atomic number, and the priority is decided at the first point of difference . (See Sec. 4.2B, text p. 134.) 26.5 (b) 26.6 (b) In the major neutral form of G-D-G-L-F (Gly-Asp-Gly-Leu-Phe), the side-chain carboxy group of Asp is un- ionized and the carboxy-terminal carboxy group is ionized because it has the lower p K a of the two carboxy
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INSTRUCTOR SUPPLEMENTAL SOLUTIONS TO PROBLEMS • CHAPTER 26 2 groups (Table 26.1, text p. 1268). This form would exist at a pH between the p K a values of the two carboxy groups. That is, the pH at which this neutral form predominates is relatively acidic. (See Sec. 26.3B.) Notice that the major neutral form of a peptide (such as this one) may not be the form that predominates at neutral pH . 26.7 (a) The amino group of tyrosine can be protonated, and both the carboxy group and the phenolic O—H group can be ionized. (b) At pH 6, the net charge on tyrosine is zero. A pH value of 6 is below the p K a of the conjugate acid of the amino group, and the amino group is therefore protonated; a pH value of 6 is above the p K a of the carboxy group, which is therefore ionized; and a pH value of 6 is below the p K a of the phenolic O—H group, which is therefore un-ionized. (c) The structure of tyrosine in aqueous solution at pH 6: 26.9 The general rule of thumb is that if the peptide contains more acidic than basic groups it is an acidic peptide; if it contains more basic than acidic groups it is a basic peptide; and if the number of acidic and basic groups are equal, the peptide is neutral. (An acidic group is a group that is in its conjugate-base form at neutral pH, such as a carboxy group; a basic group is a group that is in its conjugate-acid form at neutral pH, such as an amino group.) (b) This peptide contains three basic groups: the a -amino group of the amino-terminal residue Leu and the side- chain amino groups of the Lys residues. The peptide contains one acidic group: the carboxy group of the carboxy-terminal Lys residue. Because the peptide contains more basic residues than acidic residues; it is a basic peptide. Its net charge at pH 6 is +2.
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