Base on: Vitamin B12 is well known as a compact and cobalt-coordinated base-on corrinoid, in which the corrin and nucleotide moieties are connected by their natural linker, (R)-isopropanolamine. In the synthetic isomer, isovitamin B12, an n-propanolamine linker produces a structural mismatch. X-ray crystallography revealed two distinct base-on conformers of isovitamin B12 (magenta; inward conformer, yellow: outward conformer).