Covalent incorporation of methyl red dyes into double-stranded DNA for their ordered clustering

An ordered dye cluster of Methyl Reds was formed in doublestranded DNA by hybridizing two complementary DNA-dye conjugates, each involving a Methyl Red moiety on a threoninol linker and a 1,3-propanediot spacer arranged alternately in the middle of the DNA sequence. In the duplex, Methyl Reds from each strand were axially stacked antiparallel to each other, as determined from NMR analysis. This clustering of Methyl Reds induced distinct changes in both UV/Vis and CD spectra. Single-stranded DNA-Methyl Red conjugates on D-threoninol linkers and (1,3-propanediol) spacers exhibited broad absorption spectra with Am, at around 480 nm, and almost no CD was observed at around the absorption maximum of Methyl Red. However, as Methyl Reds were clustered by hybridization, Am., shifted towards shorter wavelengths with respect to its monomeric transition. This hypsochromic shift increased as the number of Methyl Red molecules increased. Furthermore, a positive couplet was also strongly induced here. These dye clusters are H-aggregates, in which molecular excitons are coupled. The positive couplet demonstrates that the clusters on D-threoninol form a right-handed helix. In contrast, the induced CD became much weaker with Methyl Red on L-threoninol, which intrinsically prefers counterclockwise winding. Thus, mutual orientation of the stacked dye molecules was controlled by the chirality of the linker.