Surface plasmon resonance (SPR)

Surface plasmon resonance (SPR) measurements were conducted by using a GLH sensor chip on a ProteOn XPR-36 Protein Interaction Array system (Bio-Rad Laboratories, Hercules, CA, USA). 5′-Biotin labeled DNAs were annealed in different buffers, and then immobilized (∼12 000 RU) in flow cells. In screening experiment, all ligands were diluted to different concentrations with running buffers (i-motif running buffer: 20 mM MES, 100 mM KCl, pH 5.5; G-quadruplex running buffer: 20 mM Tris–HCl, 100 mM KCl, pH 7.4). Ligands of different concentrations were injected at flow rate of 25 μl/min for 240 s of association phase, and 300 s of disassociation phase at 25°C. The GLH sensor chip was regenerated with short injection of 50 mM NaOH solution between consecutive measurements. The final graphs were obtained by subtracting blank sensorgram of corresponding i-motif or G-quadruplex. Kinetic and equilibrium measurements were analyzed with ProteOn manager software. Langmuir model was used to fit SPR kinetic sensorgrams. K_D values were obtained through equation K_D = k_d/k_a. For association process, d[AL]/d_t = k_a[A][L] – k_d[AL], and for disassociation process [A] = 0, d[AL]/d_t = -k_d[AL] (t means time, [A] is concentration of analyte, [L] is concentration of substance and [AL] is concentration of complex).