Rotamere

Rotamere, more commonly spelled rotamer, refers to a conformational isomer of a molecule that interconverts by rotation about a single bond, typically a carbon–carbon, carbon–nitrogen, or carbon–oxygen bond. The different rotamers arise from distinct dihedral angles around that bond and can correspond to local energy minima in the molecule’s conformational landscape. Rotamers are a subset of conformers and are especially relevant when rotation around a bond is slow enough to give rise to observable distinct species or populations.

In practice, many single-bond rotations are rapid at room temperature, so the molecule rapidly samples many conformations and no single rotamer is isolable. In other cases, hindered rotation or strong steric/electronic effects lead to persistent rotamer populations. Classic examples include butane, which exhibits different rotamers around the central C–C bond (anti and gauche forms), and side-chain conformations in larger organic molecules or biomolecules, where specific dihedral angles influence stability and reactivity.

Rotamer analysis is central to fields such as structural biology and computational chemistry. For proteins, side-chain rotamers around chi angles are cataloged in rotamer libraries to help model protein structure and dynamics. Detection and quantification of rotamers can be achieved by NMR spectroscopy, crystallography, or computational sampling, depending on the system and timescale of interconversion. The term rotamere is an occasional variant spelling found in older or non-English literature; today the more common term is rotamer.