A type of compound in which one or more chains of atoms in the molecule are looped in the configuration of a knot. The molecule may have only one closed chain forming the knot. If the knot is a trefoil knot the compound is chiral. Alternatively, molecular knots may have two or more separate loops tied together in a knot. In such cases there is no formal chemical bonding between the rings and they are held by mechanical bonding. Molecular knots can be produced synthetically and also occur naturally in certain proteins.