pH induced DNA folding at interface.

DNA chains with cytidine-enriched sequences have been grafted on a gold surface at different pH values monitored by a quartz crystal microbalance with dissipation (QCM-D) in real time. DNA chains grafted under acidic conditions with i-motif structures have a lower density due to the larger chain coverage. In contrast, DNA chains grafted under basic conditions with extended conformations are more densely grafted. After the grafted DNA chains completely unfold at pH 8.5, we have investigated the DNA folding with lower and higher grafting densities by use of QCM-D and surface plasmon resonance (SPR). We find that DNA chains with low grafting density can fold into i-motif structures, whereas those with higher grafting density only partially fold because crowding provides limited space for their conformational changes.