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The special cases: glycine and proline.
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Amino Acids: Special Cases
Glycine
Has the simplest R group: a hydrogen atom.
Glycine’s fake R-group gives it many possible angles of rotation, which makes it unfavorable in some protein structures.
Proline
The Proline “R group” is a ring structure that includes the amino group and the alpha carbon, which is different from the other amino acids.
Proline uses a pentagon (an imino ring) to connect the central carbon to the nitrogen atom of the amino group.
Proline is unfavorable (like glycine) but for the opposite reason: its bulky imino ring constrains its flexibility.
Related Tutorials
Amino Acid Structures: Part I
Amino Acid Structures: Part II