Our main research interests have focused on cation-DNA and cation-protein interactions in terms of their roles in higher order chromosomal structure. Our previous study directly analyzed the cation composition of cryo-preserved mammalian interphase and mitotic cells, as well as fractionated untreated and cation-depleted chromosomes at a resolution of 50 nm using the University of Chicago high resolution scanning ion microprobe (UC-SIM). Quantitative direct UC-SIMS signals and subsequent imaging of cryo-preserved cells demonstrated that Na+ and K+ were associated with chromatin throughout the cell cycle, whereas in contrast Ca2+ and Mg2+ exhibited a localization change during interphase and mitosis. Interphase Ca2+ and Mg2+ were found mainly throughout the cytoplasm, but at mitosis associated with chromatin. Our findings of chromatin association of Na+ and K+ support a role of these cations in both interphase and mitotic chromatin compaction, whereas Ca2+ and Mg2+ binding points to an essential function in mitotic chromatin compaction. In our present investigation using SIMS images obtained from both human and Indian muntjac metaphase chromosomes, we reveal specific binding of Mg2+ to chromosomal "p" and "q" arm heterochromatic regions correlating with conventional Giemsa (G-) bands. In addition, detailed Mg2+ image density peaks along the chromosomes indicate that Mg2+ peaks correspond to the location of G-bands. Our results support a direct role for Mg2+ in promoting and maintaining the higher order chromatin structure of heterochromatic regions on chromosome arms represented by G-bands, possibly due to both Mg2+-DNA and Mg2+-protein interactions. (C) 2004 Elsevier B.V. All rights reserved.