Exercício de Química Sintética e Organica Mista (757)

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CHAPTER 27
AMINO ACIDS, PEPTIDES, AND PROTEINS.
NUCLEIC ACIDS
SOLUTIONS TO TEXT PROBLEMS
27.1 (b) L-Cysteine is the only amino acid in Table 27.1 that has the R configuration at its stereogenic
center.
The order of decreasing sequence rule precedence is
When the molecule is oriented so that the lowest ranked substituent (H) is held away from us,
the order of decreasing precedence traces a clockwise path.
The reason why L-cysteine has the R configuration while all the other L-amino acids have
the S configuration lies in the fact that the —CH2SH substituent is the only side chain that
outranks —CO2
� according to the sequence rule. Remember, rank order is determined by
CH2SH
H3N CO2
�
�
Clockwise; therefore R
HSCH2H3N
�
� � �CO2
� H
� �
L-Cysteine
CO2
�
H
CH2SH
H3N
�
NH3
�
CO2
�
HSCH2
H
C
H3N
�
CO2
�
HSCH2 H
C
	CHAPTER 27 AMINO ACIDS, PEPTIDES, AND PROTEINS. NUCLEIC ACIDS