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Chapter 28: Problem 63

Eighteen of the nineteen 1 -amino acids have the \(S\) configuration at the \(\alpha\) carbon (the first carbon after the carboxyl carbon). Cysteine is the only 1 -amino acid that has an \(R\) configuration. Explain.

Short Answer

Expert verified
Cysteine is the only 1 -amino acid with an R configuration because of its molecular structure. The alpha carbon of Cysteine is bound to a -NH2, a -SH group, a -COOH group, and a hydrogen atom. According to the Cahn-Ingold-Prelog priority rules for deciding R/S configuration, sulfur (S) in the -SH group gets the highest priority due to its highest atomic number. Hence, when the lowest priority - hydrogen is oriented away from the observer, the rest of the groups follow a clockwise path (R configuration)

Step by step solution

01

Understand the structure of Cysteine

The molecular structure of Cysteine is H2N-CH(COOH)-SH. From this, it can be seen that the alpha carbon (the carbon next to the carboxyl group) is attached to a -NH2 group, a -SH group, a -COOH group, and a Hydrogen atom.
02

Apply the Cahn-Ingold-Prelog rules

Cahn-Ingold-Prelog rules are used to assign priorities to the groups attached to the chiral carbon. The atom with the highest atomic number gets the highest priority. If there is a tie, then we move to the next atom in each group. In Cysteine, sulphur (S) has the highest atomic number, followed by oxygen (O) in the carboxyl group, then nitrogen (N) in the amine group, and at last hydrogen (H) which has the lowest atomic number. Therefore, the -SH group has the highest priority, followed by the -COOH group, then the -NH2 group and then the hydrogen atom.
03

Determine the configuration

Using the right-hand rule or left-hand rule, depending on priorities and orientation of groups, one can determine the configuration. Here, the lowest priority group - Hydrogen is away from the observer. The rest of the groups follow in the order of clock (from -SH to -COOH to -NH2), indicating an R configuration. Hence, unlike other 1 amino acids, Cysteine has an R configuration due to the presence of sulphur atom in the place of usual carbon, which gives it the highest priority.

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Most popular questions from this chapter

The systematic (IUPAC) name for the amino acid threonine is \((2 S, 3 R)-2\) -amino- 3 -hydroxybutanoic acid. The systematic name indicates that threonine has two stereocenters, at positions 2 and \(3,\) with \(S\) and \(R\) configurations, respectively. Draw the Fischer projection of threonine. How many other possible stereoisomers of threonine are there?

If one strand of a DNA molecule has the base sequence TCT, what must be the sequence on the opposite strand? Draw a structure of this portion of the double helix, showing all hydrogen bonds.

Write the formulas of the species expected if the amino acid phenylalanine is maintained in (a) \(1.0 \mathrm{M}\) HCl; (b) \(1.0 \mathrm{M} \mathrm{NaOH} ;\) (c) a buffer solution with \(\mathrm{pH}=5.7\).

Draw the dashed-wedged line structure for ( \(S\) )-alanine and (S)-phenylalanine.

Describe what is meant by each of the following terms, using specific examples where appropriate: (a) \(\alpha\) -amino acid; (b) zwitterion; (c) isoelectric point; (d) peptide bond; (e) tertiary structure.
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