P and MDescriptors

Some molecules are chiral even without having a stereogenic center. If two pairs of different substituents are coordinated along a chiral axis in perpendicular planes, but not attached to the same atom, we call this axial chirality. Stereodescriptors are aR, if the substituents' priority rotates in a clockwise manner, and aS for anti-clockwise rotation.

Helical chirality, a special case of axial chirality, is given when a molecular structure describes the form of a helix around an axis. If the molecule describes a helix in a clockwise manner away from the viewer, we call it a P-helix (P for plus), if it rotates in an anticlockwise manner away from the viewer, it is called a M-helix (M for minus). In DNA (P-helix), starch, and proteins (P-helix), helical chirality is of importance.

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