The CAR for long-chain bisquaternary tetrahydroisoquinolines like atracurium, cisatracurium, mivacurium, and doxacurium is hard to determine because of their bulky onium heads and large number of rotatable bonds and groups. These agents must follow the same receptive topology as others, which means that they do not fit between the receptive sites without bending. Mivacurium for example has a molecular length of 3.6 nm when stretched out, far from the 2 to 2.1 nm optimum. Mivacurium, atracurium, and doxacurium have greater N-N distance and molecular length than d-tubocurarine even when bent. To make them fit, they have flexible connections that give their onium heads a chance to position themselves beneficially. This bent N-N scenario probably does not apply to laudexium and decamethylene bisatropium, which prefer a straight conformation.