Amino Acids

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Chapter 4 . Amino Acids L-(-)-Glyceraldehyde L-Glyceric acid 2-Hydroxypropanoic acid CHO HO C H CH2OH COOH H C Br CH3 COOH HO C H CH2OH COOH H2N C H CH3 56 COOH HO C H CH3 COOH H2N C H CH2OH 2-Bromopropanoic acid L-(-)-Alanine L-(-)-Serine 12. Describe the stereochemical aspects of the structure of cystine, the structure that is a disulfide-linked pair of cysteines. Answer: Cystine (dicysteine) has two chiral carbons, the two a-carbons of the cysteine moieties. Since each chiral center can exist in two forms there are 4 stereoisomers of cystine. However, it is not possible to distinguish the difference between L-cysteine/D-cysteine and D-cysteine/Lcysteine dimers. So three distinct isomers are formed. H H H H -OOC +H 3N C CH 2 S S CH2 C COO- H NH 3+ L-cysteine- L-cysteine +H 3N +H 3N C CH 2 S S CH2 C COO- H COO- L-cysteine- D-cysteine -OOC -OOC C CH 2 S S CH2 C NH3+ H NH 3+ D-cysteine- L-cysteine +H 3N -OOC C CH 2 S S CH 2 C NH3+ H COO- D-cysteine- D-cysteine 13. Draw a simple mechanism for the reaction of a cysteine sulfhydryl group with iodoacetamide. Answer: +H 3N I CH2 C NH2 S- CH2 C COO- H O I - +H 3N NH2 C O CH 2 S CH2 C COO- 14. Describe the expected elution pattern for a mixture of aspartate, histidine, isoleucine, valine, and arginine on a column of Dowex-50. H
Chapter 4 . Amino Acids Answer: Dowex is an anion exchanger with a positive charge due to a quaternary amine. Thus, one expects amino acids to elute from Dowex in the following order: basic (positively-charged) amino acids; neutral (uncharged amino acids); acidic (negatively-charged amino acids). We expect arginine to elute first followed by histidine. Of these two basic amino acids, arginine will have a +1 charge at neutral pH whereas histidine’s charge will slightly less that +1. Next we expect isoleucine followed by valine. The charge on these amino acids will be close to (but not) zero; however, isoleucine will be slightly less negatively charged than valine. (The isoelectric point, the pH at which a molecule has a net charge of zero, is simply the average value of the pKas (for amino acids lacking ionizable R groups). Thus, isoleucine’s isoelectric point is (2.4 + 9.7)/2 = 6.05 whereas valine’s isoelectric point is (2.3 + 9.6)/2 = 5.95. At neutral pH we expect isoleucine to be slightly less negatively charged than valine.) Finally, we expect aspartate to elute last because it is an acidic, negatively-charged amino acid. 15. Assign (R,S) nomenclature to the threonine isomers of Figure 4.14. Answer: +H 3N H 57 COO- The groups about the α-carbon of L-threonine are assigned the following priority: NH3 + > COO- > Cβ> H. In Fischer projections, two groups joined to the α-carbon (i.e., NH3 + and H) are coming out of the plane while the other two groups are into the plane. By rotating the molecule such that the group with the lowest priority, namely H, is away from the reader, the NH3 + group will be located on the right side of the α-carbon, the carboxyl group above and to the left, and the remaining β-carbon to the left and below the α-carbon: 3 C C H CH 3 2 OH COO - H C C C α NH + β 3 Since moving from highest to lowest priority involves counterclockwise movement the α-carbon is in the S configuration. For the β-carbon, the priority is OH > Cα > CH3 > H. The orientation of the Cα-Cβ bond was already defined for the α-carbon as being into the plane therefore it must be out of the plane for the β-carbon, as must be the bond to -CH3. Thus, bonds to OH and H are into the plane. By rotating the molecule such that H is behind the β-carbon, we find the OH to the left, CH3 below and Cα above: HO 1 Cα C β Since moving from highest to lowest priority involves clockwise movement the α-carbon is in the R configuration. The β-carbon is in the R configuration. 2 1 H CH3 3
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Amino Acids

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