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SOLUTIONS FOR AMINO ACIDS, PEPTIDES, AND PROTEINS
From Chapter 24 of Solutions Manual for Organic Chemistry, Eighth Edition. Leroy G. Wade, Jr., Jan W. Simek. Copyright © 2013 by Pearson Education, 
Inc. All rights reserved.
C COO
CH(CH3)CH2CH3
H2N
H
H3N C
CH2CH2CH2NH
COO
H
C COO
CH2CH2COO
H3N
H
N COOHH
H
COOH
H2N H
CH2
NH
HH2N
COOH
COOH
H2N H
CH2SH
H2N
COOH
H
CH2SH
C NH2
NH2
COOH
H2N H
CH3
COOH
H2N H
COOH
H2N CH2OH
H
H2N
COOH
H
CH3
(d)(c)
(b)(a)
4 Here is a simple way of determining if a group will be protonated: at solution pH below the group's 
pKa value, the group will be protonated; at pH higher than the group's pKa value, it will not be protonated.
(d)(c)
(b)(a)
1
3 In their evolution, plants have needed to be more resourceful than animals in developing biochemical 
mechanisms for survival. Thus, plants make more of their own required compounds than animals do. The 
amino acid phenylalanine is produced by plants but required in the diet of mammals. To interfere with a 
plant's production of phenylalanine is fatal to the plant, but since humans do not produce phenylalanine, 
glyphosate is virtually nontoxic to us.
2
(a) The configurations around the asymmetric carbons of (R)-cysteine and (S)-alanine are the same. The 
designation of configuration changes because sulfur changes the priorities of the side chain and the COOH.
(S)-alanine with 
group priorities shown
4
1 3
2
4
1 2
3
(R)-Cysteine with group 
priorities shown; note that the 
COOH and the CH2SH priorities 
are reversed compared with (S)-
alanine.
(b) Fischer projections show that both (S)-alanine and (R)-cysteine are L-amino acids.
(S)-alanine
L-alanine
(R)-cysteine
L-cysteine
N
NH
pH 11
pH 2
pH 7 pH 7
Both groups are 
deprotonated.
Both groups are 
protonated.
Only the amine 
remains protonated.
Only the amines 
remain protonated.
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