Keele Research Repository

The interactions in solution and the solid state oT the antibiotic drugs ethidium bromide, adriamvcin and daunomycin with deoxyribonucleic acid have been investigated with the aim of understanding at a molecular level the basis of their chemotherapeutic activity.
Both ethidium bromide and the two related anthracycline drugs adriamycin and daunomycin possess planar aromatic chromophores by virtue of which they can intercalate into the deoxyribonucleic acid helix. A study of the spectroscopic properties o^ the drugs in solution as a function of concentration, solvent and ionic strength gave an insight into the properties of the drug chromonhores, Which helped in understanding the spectral changes which take place when complexes form between the drugs and polynucleotides or polyelectrolytes.
A microspectrophotoneter was specially constructed to measure the linear dichroism (in the visible range) and the birefringence of oriented fibres and fibre sections of drug-deoxyribonucleic acid complexes and an X-ray diffraction study of the same specimens was carried out in parallel.
It was found that a change in the degree of hydration of the nucleic acid helices results in a redistribution of the drug molecules between the intercalated and non-intercalated states. The intercalations driven by hydrophobic forces (additional stabilization being gained from specific electrostatic and hydrogen bonding interactions) and is opposed by the hydrophilic nature of some features of the drugs. The helix pitch increases with increasing relative humidity up to a maximum value which may represent the situation in which no nett increase in the fraction of intercalated drug is possible because these two effects counterbalance each other. An estimate of the lower limit to the values of the unending angle of the deoxyribonucleic acid helix at the site of intercalation was made on the basis of the maximim pitch values. This angle (12° - 3°) fitted the data obtained from both ethidium bromide and the anthracyclines.
Ethidium bromide dissociates completely from the intercalated state at low humidities, whereas a large fraction of the anthracvcline molecules remain intercalated even at very low relative humidities.
The enhanced chemotherapeutic activity of adriamycin, as compared to daunomycin, was associated with the observed increased stability of the intercalated state of adriamycin under conditions of low hydration and could be justified in terns of an additional hydrogen bond formed by adriamycin rath the ITM backbone.
While the X-ray diffraction technique provides information mainly on the drug soecies which is intercalated, the linear dichroism and birefringence reflect the state of both intercalated and non- intercalated soecies present in the fibre. A quantitative analysis of the combined X-ray diffraction and optical data, coupled rath computer simulations of the fibre dichroism and birefringence, produced on one hand, a picture of the degree of internal alignment of the helices in the fibre and, on the other hand, an estimate of the orientation of the non-intercalated species. It was concluded that in the case of ethidium bromide, and possibly, in the case of the anthracyclines, the non-intercalated drug is externally bound, essentially perpendicular to the deoxyribonucleic acid helix axis. Limits to the possible tilt angles and orientational freedom of the externally bound species were derived.