Preferential Binding of π-Ligand Porphyrin Targeting 5'-5' Stacking Interface of Human Telomeric RNA G-Quadruplex Dimer.

Human telomeric RNA (TERRA) containing thousands of G-rich repeats has the propensity to form parallel-stranded G-quadruplexes. The emerging crucial roles of TERRA G-quadruplexes in RNA biology fuel increasing attention for studying anticancer ligand binding with such structures, which, however, remains scarce. Here we utilized multiple steady-state and time-resolved spectroscopy analyses in conjunction with NMR methods and investigated thoroughly the binding behavior of TMPyP4 to a TERRA G-quadruplex dimer formed by the 10-nucleotide sequence r(GGGUUAGGGU). It is clearly identified that TMPyP4 intercalates into the 5'-5' stacking interface of two G-quadruplex blocks with a binding stoichiometry of 1:1 and binding constant of 1.92 × 106 M-1. This is consistent with the unique TERRA structural features of the enlarged π-π stacking plane of the A·(G·G·G·G)·A hexad at 5'-ends of each G-quadruplex block. The preferential binding of π-ligand porphyrin to the 5'-5' stacking interface of the native TERRA G-quadruplex dimer is first ascertained by the combination of dynamics and structural characterization.