However, they do not all have the same configuration in the R,S system: L-cysteine is also R -cysteine, but all the other L-amino acids are S , but this just reflects the human decision to give a sulphur atom higher priority than a carbon atom, and does not reflect a real difference in configuration.
In prokaryotic cells, the first methionine in a protein is formylated.
It is useful to classify amino acids based on their R-groups, because it is these side chains that give each amino acid its characteristic properties.
Compare this to just 65536 different oligonucleotides of 8 monomeric units 8mer.