A pretty couple: Two copper–salen metal–base pairs were incorporated into a DNA double helix in neighboring positions (see picture). The two paramagnetic CuII ions are antiferromagnetically coupled, and the exchange coupling constant is −2J=22.4 cm−1. The dipolar coupling constant yielded a Cu⋅⋅⋅Cu distance of 3.7 Å, which is comparable to the base-pair distance in natural B-type DNA.