Book of Abstracts: Albany 2009
June 16-20 2009
© Adenine Press (2008)
Hoechst 33258 Dimers Bind Mainly to dsDNA GC Pairs
It is known that Hoechst 33258 can form not only monomeric, but also dimeric complexes on dsDNA (1). A specific feature of the formation of dimeric complexes is the appearance of excitonic circular dichroism (CD) spectra. The excitonic spectrum of Hoechst 33258 dimeric complex with poly(dA-dT)·poly(dA-dT) is characterized the positive long wavelength band and the negative short wavelength band (2). Both dimeric and monomeric complexes of Hoechst 33258 with poly(dA-dT)·poly(dA-dT) have positive linear dichroism values at 360 nm (LD360) (3). On the contrary, in dimeric complexes of Hoechst 33258 with poly(dG-dC)·poly(dG-dC), long-wavelength bands of excitonic CD have negative values and short wavelength bands, positive ones (4, 5), whereas both dimeric (4) and monomeric (3) Hoechst 33258-poly(dG-dC)·poly(dG-dC) complexes are described by negative LD360 values. Thereby spectral measurements provide identification of Hoechst 33258 complexes of four types: monomeric or dimeric with binding on AT or GC sites. For example, the lack of a CD excitonic spectrum of the monomeric complex of Hoechst 33258 with calf thymus dsDNA and positive LD360 (6) indicate binding at AT-enriched dsDNA regions. With an increase in dsDNA filling with Hoechst 33258 molecules the complex acquires a CD excitonic spectrum (dimerization), whose long wavelength band is negative (6), and the LD360 value varies from positive to negative magnitudes (1, 6). Hence, with an increase of the number of Hoechst 33258 molecules on dsDNA the binding of its monomers to AT pairs is displaced by its dimers binding to GC pairs. Thus, we experimentally conformed the assumption made as early as in 1996 (7) on the change of binding specificity from AT to CG as the ligand occupies more and more space on dsDNA.
References and Footnotes
S. A. Streltsov
Engelhardt Institute of Molecular Biology