(9-Anthrylmethyl)ammonium chloride (AMAC, 1) binds to natural and synthetic DNA sequences with a high affinity, as deduced from the absorption and fluorescence spectral data. Scatchard plots constructed from these data gave binding constants in the range (2-8) x 10(4) M-1 of base pairs. Extensive hypochromism, broadening, and red shifts in the absorption spectra were observed when AMAC binds to various sequences of synthetic and natural DNA. Upon binding to DNA, the fluorescence from the anthryl chromophore was efficiently quenched by the DNA bases and the fluorescence spectra at high concentrations of CT DNA show significant broadening of the vibronic bands. Stern-Volmer quenching constants obtained from the linear quenching plots strongly depended on the DNA sequence. A high quenching constant of 1.4 x 10(4) M-1 for dI-dC sequences and a low value of 2.1 x 10(3) M-1 for homo AT sequences were estimated from this data. Time-resolved fluorescence measurements clearly show a biexponential decay behavior (lifetimes 8.2 and 30.6 ns) for AMAC bound to CT DNA. The fluorescence spectra obtained 50 ns after the excitation showed considerable red shift when compared to the spectra at early times. The red-shifted, long-lived emission spectra were consistent with the intercalative binding of 1. Triplet-triplet absorption spectra of AMAC in the presence of CT DNA show the complete quenching of the anthryl triplet by the DNA bases. Fluorescence polarization measurements with AMAC and various DNA sequences suggest that the bound chromophore is rigid on nanosecond time scales. The melting temperatures of CT DNA and poly(dA-dT) samples were increased by AMAC binding, from 78 and 56.6-degrees-C to 83 and 63-degrees-C, respectively. Excitation into the absorption bands of the DNA in the 260-300-nm region, where the anthryl absorption was negligible, resulted in an intense, red-shifted, and broad fluorescence spectrum from the anthryl chromophore. The sensitized fluorescence spectra were assigned to the anthryl chromophore on the basis of the excitation spectra as well as its resemblance to the emission spectrum of the long lived component detected in the time-resolved studies. Energy transfer from DNA bases depended on the temperature. For example, when the helix was melted, energy transfer could no longer be observed. Thus, the double-helical structure of the DNA polymer was essential for the energy transfer, consistent with the intercalative mode of binding of AMAC. It is noteworthy that binding of AMAC to DNA results in induced CD bands with distinct peaks that correspond to the absorption spectrum of the bound probe. Several pieces of strong evidence for the intercalative binding of the anthryl probe to the DNA helix are presented. The anthryl excited state serves as a sensitive probe to distinguish between homo and hetero AT sites as well as between AT and GC sites.
