Rotamer

Rotamer is a specific conformer of a molecule that results from rotation about a single sigma bond, producing a distinct arrangement of atoms. The concept is widely used in organic chemistry and biochemistry. Rotamers arise around carbon–carbon, carbon–heteroatom, and other single bonds, and are described by dihedral angles. Energy differences between rotamers reflect steric, electrostatic, and hyperconjugative effects, so some conformations are more stable than others.

In many small molecules, rotation around a bond is rapid at typical temperatures, causing interconversion among rotamers and leading to averaged signals in spectroscopic measurements. In hindered or rigid systems, rotamers can be separable or kinetically trapped, revealing distinct forms and rates of interconversion.

In proteins, rotamer typically refers to a discrete side-chain conformation of an amino acid residue, described by one or more chi angles (such as chi1, chi2). Rotamer libraries collect observed conformations and their probabilities, and are used to model and refine protein structures, predict side-chain positions, and speed computational searches. Side-chain rotamer preferences depend on backbone conformation, neighboring residues, solvent exposure, and intra- or inter-molecular interactions. Experimental data from X-ray crystallography and NMR inform rotamer distributions, while software tools use them to reduce conformational search space.