Enhancement of DNA-mediated Energy Transfer from Ethidium to meso-Tetrakis(N-methylpyridinium-4-yl)porphyrin by Ca2+ Ion

摘要

The fluorescence intensity of DNA-intercalated ethidium with [ethidium]/[DNA base] being 0.005 was quenched upon the binding of another intercalating ligand, meso-tetrakis(N-methylpyridinium-4-yl)porphyrin (TMPyP). Addition of Ca2+ enhanced the quenching efficiency. The range of separations between donor and acceptor molecules, within which total quenching occurs, was calculated using a one-dimensional resonance energy transfer mechanism to be 9.5 base-pairs or 32.3 Å in the absence of Ca2+ ions. The distance increased to 18.7 base-pairs or about 63.6 Å in the presence 100 μM Ca2+. Considering that (1) Ca2+ had little effect on the binding modes of ethidium and TMPyP, which was investigated by reduced linear dichroism and (2) spectral overlap between the emission spectrum of ethidium and the absorption spectrum of TMPyP was maintained in the presence of Ca2+, contributions from orientation factor and spectral overlap to Ca2+-induced enhancement in DNA mediated energy transfer was limited. Although there is no direct evidence, electron transfer along the DNA stem may accompany the observed fluorescence quenching. In this respect, DNA bound Ca2+ act as a partially conducting medium.