In crystals of complexes of thermine and d(CGCGCG)(2) molecules grown at 4, 10, and 20 degrees C, the numbers of thermine molecules connected to the DNA molecule were dependent on the temperature of the crystallization. Two molecules of thermine and one Mg2+ ion were connected to DNA molecule when thermine and d(CGCGCG)(2) were co-crystallized at 4 and at 20 degrees C. When an increased concentration of magnesium and thermine molecules were co-crystallized with d(CGCGCG)(2) molecules at 10 degrees C, three Mg2+ ions and only one thermine molecule were bound with a d(CGCGCG)(2) molecule. The number of polyamines and of Mg2+ ions connected to DNA was dependent on the atomic values of the polyamine and of the metal ion. The binding of more Mg2+ ions occurred when the atomic value of Mg2+ exceeded that of the corresponding mono- or polyamine, and when the Mg2+ ion concentration was elevated. Furthermore, this study is the first documentation of a naturally occurring polyamine bound to the minor groove of DNA in a crystal structure. (c) 2008 Elsevier Inc. All rights reserved.