13C NMR Relaxation Study of Molecular Motion in the DNA-Binding Ligand Hoechst 33258

Abstract

I3C spin-lattice relaxation times (TI) and nuclear Overhauser enhancement (NOE) factors have been measured at three magnetic field strengths (corresponding to resonance frequencies of 22.5, 75, and 100 MHz) for the DNA-binding ligand Hoechst 33258. This molecule comprises two benzimidazole moieties, an Nmethylpiperazine ring and a phenol ring, and adopts an anisotropic curved shape that assists in its binding to the minor groove of DNA. The I3C relaxation data were interpreted in terms of various models for molecular motion. It was determined that in solution the molecule tumbles anisotropically and that there is a significant degree of internal motion associated with both the phenol and piperazine moieties. Models which involve the rapid jumping of C-H moieties between conformational states provided a good fit to the relaxation data, as did the model-free approach of Lipari and Szabo (J. Am. Chem. SOC. 1982, 104, 4546).