Chapter 4

Without consulting chapter figures, draw Fischer projection formulas for glycine, aspartate, leucine, isoleucine, methionine, and threonine.

Without reference to the text, give the one-letter and three-letter abbreviations for asparagine, arginine, cysteine, lysine, proline, tyrosine, and tryptophan.

Write equations for the ionic dissociations of alanine, glutamate, histidine, lysine, and phenylalanine.

How is the \(\mathrm{p} K_{\mathrm{a}}\) of the \(\alpha\) - \(\mathrm{NH}_{3}+\) group affected by the presence on an amino acid of the \(\alpha-\mathrm{COO}^{-}\)?

(Integrates with Chapter \(2 .\) ) Draw an appropriate titration curve for aspartic acid, labeling the axes and indicating the equivalence points and the \(\mathrm{p} K_{\mathrm{a}}\) values.

(Integrates with Chapter 2 .) Calculate the concentrations of all ionic species in a \(0.25 M\) solution of histidine at \(\mathrm{pH} 2, \mathrm{pH} 6.4,\) and \(\mathrm{pH} 9.3\).

Absolute configurations of the amino acids are referenced to \(\mathrm{D}\) - and t-glyceraldehyde on the basis of chemical transformations that can convert the molecule of interest to either of these reference isomeric structures. In such reactions, the stereochemical consequences for the asymmetric centers must be understood for each reaction step. Propose a sequence of reactions that would demonstrate that \(L(-)\) -serine is stereochemically related to \(L(-)\) -glyceraldehyde.

Describe the stereochemical aspects of the structure of cystine, the structure that is a disulfide-linked pair of cysteines.

Draw a simple mechanism for the reaction of a cysteine sulfhydryl group with iodoacetamide.

The artificial sweeteners Equal and Nutrasweet contain aspartame, which has the structure: What are the two amino acids that are components of aspartame? What kind of bond links these amino acids? What do you suppose might happen if a solution of aspartame was heated for several hours at a pH near neutrality? Suppose you wanted to make hot chocolate sweetened only with aspartame, and you stored it in a thermos for several hours before drinking it. What might it taste like?

Individuals with phenylketonuria must avoid dietary phenylalanine because they are unable to convert phenylalanine to tyrosine. Look up this condition and find out what happens if phenylalanine accumulates in the body. Would you advise a person with phenylketonuria to consume foods sweetened with aspartame? Why or why not?

Amino acids are frequently used as buffers. Describe the pH range of acceptable buffering behavior for the amino acids alanine, histidine, aspartic acid, and lysine.

Although the other common amino acids are used as buffers, cysteine is rarely used for this purpose. Why?

Draw all the possible isomers of threonine and assign \((R, S)\) nomenclature to each.