Photocontrol of end-grafted lambda-phage DNA

We study the response to light stimulation of end-grafted lambda-phage DNAs in a solution of AzoTAB, a photosensitive compacting agent for the DNA chains. The biotinylated double-stranded DNA molecules are end-attached to a streptavidinated substrate and incubated in a 120 mM tris-borate-EDTA buffer in the presence of the cationic surfactant AzoTAB. We find that the conformations and the dynamics of the tethered DNAs can be tuned by the AzoTAB concentration and strongly depend on illumination conditions. Under visible light or in the dark AzoTAB is in a trans configuration and it induces DNA adsorption onto the substrate for [AzoTAB] >= 0.3 mM. In contrast, under UV illumination, AzoTAB is in a cis configuration and it induces DNA adsorption for [AzoTAB] >= 0.4 mM. In the AzoTAB concentration range 0.3-0.4 mM, one can thus reversibly switch between the adsorbed and the non-adsorbed states of the end-grafted chains by exposing the sample respectively to visible light (> 400 nm) and to UV illumination (365 nm). These light-induced adsorption transitions occur for AzoTAB concentrations approximately half of those required to control the DNA conformation in bulk solution, which highlights the predominance of chain-surface interactions over monomer-monomer intra-chain interactions for DNA molecules in the close vicinity of a solid substrate.