Absorption Spectroscopy to Probe Ligand Binding

Synonyms

DNA–ligand interactions; Ligand-binding interactions

Definition

When a ligand solution is added to a solution of biomacromolecules, if it does not bind to the macromolecules, then the UV–visible spectrum will simply be the sum of the DNA spectrum and the ligand spectrum. If the ligand does bind to the macromolecules, then the spectrum of the complex will be (at least slightly) different from the sum spectrum. One should note that the observed spectrum is probably a complicated mixture of the spectrum of bound and unbound ligands and free and complexed macromolecules.

Sometimes there are changes in the spectrum characteristic of the type of interaction. For example, when a planar aromatic molecule binds intercalatively to DNA (sandwiched between two base pairs), there is usually a characteristic decrease in the ligand absorbance signal (hypochromism) of up to 50% and a shift to longer wavelength (bathochromic shift) of between ∼2 and 20 nm as illustrated in Fig. 1. The DNA...