● 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 CO H CH, HN+H=—CO2= 一CO CH SH HSCH The order of decreasing sequence rule pre 一>HSCH 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 Han CO2 Clockwise: therefore R The reason why L-cysteine has the R configu while all the other L-amino acids have the S configuration lies in the fact that the sH substituent is the only sic outranks-CO, according to the sequence rule. Remember, rank order is determined by 752 Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website752 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 H3N HSCH2   CO2  H L-Cysteine CO2 H CH2SH H3N NH3 CO2 HSCH2 H C H3N CO2 HSCH2 H C Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website