Final answer:
Proline has a ring-like structure that limits the flexibility of the polypeptide chain, causing a bend in the protein, and is unique among the 20 standard amino acids.
Step-by-step explanation:
The amino acid that has an unusual R-group which prevents rotation and causes the protein region containing it to bend is proline. Proline is unique among the 20 standard amino acids because it has a side chain that is linked to the amino group, forming a ring-like structure. This structural peculiarity limits the flexibility of the polypeptide chain and often induces a bend in the protein structure. This characteristic is essential when considering the overall three-dimensional shape of proteins, and its presence can affect protein folding and function significantly.