Correct option is B
Statement Analysis: Option 2 is correct (B and D )
Statement A: "All alpha-amino acids have the D stereochemical configuration."
Incorrect: Natural amino acids (found in proteins) are mostly L-amino acids, not D-amino acids. The L configuration is the one used in natural proteins. D-amino acids are rare in nature, typically found in certain bacterial cell walls or in some antibiotics, but not as the primary form in proteins. Therefore, Statement A is incorrect.
Statement B: "All L-amino acids have the S absolute configuration except cysteine, which has the R absolute configuration."
Correct: This statement is accurate. The L configuration of amino acids typically corresponds to the S absolute configuration (except for cysteine, which has the R configuration due to its thiol group). This is the standard configuration for amino acids in proteins.
Statement C: "All D-amino acids have the S absolute configuration except cysteine, which has the R stereochemical configuration."
Incorrect: This statement is reversed. The D-amino acids have the R absolute configuration (except cysteine, which still has the S configuration in D-amino acids). The S/R assignment is inversely related to the D/L designation, so this statement is incorrect.
Statement D: "In the absolute configuration system, L-threonine and L-isoleucine are (2S, 3R)-threonine and (2S, 3S)-isoleucine diastereomers, respectively."
Correct: This statement is true. In the absolute configuration system, L-threonine is identified as (2S, 3R)-threonine, and L-isoleucine is identified as (2S, 3S)-isoleucine. These are diastereomers because they differ in the configuration at the second and third chiral centers.
