Spectroscopic studies of drug-DNA interactions

Abstract

Amsacrine has been studied by resonance Raman (Ar+ 457.9 to 501.6 nm), Raman (Kr+ 647.1 nm), FT-Raman (YAG 1064 nm) and UV-visible spectroscopy, while various analogues of amsacrine and the binding of these compounds to DNA has been examined primarily by resonance Raman spectroscopy. The acridine ring system was identified as the chromophore contributing to the resonance Raman spectrum by a comparison of the Raman spectra of acridine and amsacrine together with data from the crystal structure of amsacrine. Intensity changes observed in the spectra of amsacrine recorded under different pH conditions enabled the identification of bands attributable to the free base and protonated forms of amsacrine in solution. Normal coordinate analysis and MNDO calculations were performed on acridine (as a model of amsacrine) and the resulting potential energy distribution and normal mode description of the in-plane vibrations of acridine enabled a qualitative assignment of the resonance Raman spectrum of amsacrine in the range 1100 to 1800 cm-1. The shift of the absorption band centred at 434 nm to 442 nm observed in the UV-visible spectrum of amsacrine upon binding to DNA was consistent with previously reported data. Interpretation of the band intensity changes detected in the resonance Raman spectrum of DNA-bound amsacrine supported the inference that amsacrine binds to DNA by an intercalative mode. Such band intensity changes were suggested to result from the perturbation of the π-electrons of the acridine ring perpendicular to the plane of the chromophore, induced by an electrostatic interaction between the intercalated chromophore and the base-pairs of DNA. Calculation of the intrinsic association constants of amsacrine in the pH range 5 to 9 indicated that the protonated form of amsacrine binds more strongly to DNA. A model was proposed to explain the changes observed in the resonance Raman spectra of amsacrine upon binding to DNA under different pH conditions, in which amsacrine is suggested to form an external DNA complex prior to intercalation. The changes observed in the excitation profiles of amsacrine upon binding were consistent with those expected as the UV-visible absorption band shifted to a longer wavelength. The effect of the size of the substituent (methyl to tert-butyl) and the position of substitution of the acridine ring (positions 1- to 4-) on the binding of various analogues of amsacrine to DNA were examined by resonance Raman spectroscopy. The effect of the addition of various electron-donating and withdrawing groups to the 4- position of the acridine ring upon amsacrine-DNA binding and the shift of the most intense band in the resonance Raman spectra of amsacrine to lower wavenumbers was correlated with the electronic character of the substituent. An investigation of the simultaneous intercalation of amsacrine and 4-NO2 amsacrine was performed. The construction of DNA binding calibration curves for the individual compounds enabled the calculation of the percentage of each compound bound to DNA in mixed amsacrine/4-NO2 amsacrine solutions. A comparison of the calculated percentage bound of the mixed compounds with that determined by UV-visible spectroscopy for the individual compounds indicated a percentage binding error of ±.6%. Resonance Raman spectroscopy was also employed o study the binding of 2-styryl quinoline analogues of amsacrine to DNA. Based upon the interpretation of the spectral changes detected upon binding, an intercalative binding mode was suggested.