Most amino acids? (AA's) have the configurations

Options

A. `R` and `L`
B. `R` and `D`
C. `S` and `L`
D. `S` and `D`

Answer

C

Explanation

Stereochemical studies of naturally occurring amino acids have shown that all have the same configuration about the carbon atom carrying the alpha - amino group, and that this configuration is the same as that in `L - (-) -` glyceraldehyde. Since the group `R` nearly always happens to have a lower [chains - Ingold - Prelog] priority than `COOH`, most of these have the `S` configuration
There are two amino acids that, although, `L`, like the other, happen to have the `R` configuration. They are `(-)` -cysteine and `(-) -` cystine. In all the amino acids. `H` has the lowest priority and `overset(+)NH_(3)` has the highest. In ost amino acids, `COO^(-)` has priority over `R` because of the `O` once removed from the chiral center. But in cysteine and cystine, `R` contains `S` once removed from the chiral cener and this gives `R` priority over `COO^(-)` (Because `S` has a greater atomic number than `O`, the `CH_(2) SH` Group of cysteine has priority over the `COO^(-)`). The reversal of priority of these groups ruesults in a change in specification from `S` and `R`.