Abstract
It is generally acknowledged that G-quadruplexes (G4s) acquire peroxidase activity upon interaction with hemin. Hemin has been demonstrated to bind selectively to the 3'-terminal G-tetrad of parallel G4s via end-stacking; however, the relationships between different terminal G-tetrads and the catalytic functions of G4/hemin DNAzymes are not fully understood. Herein, the oligonucleotide d(AGGGGA) and its three analogues, d(AGBrGBrGGA), d(AGBrGGGBrA) and d(AGBrGGBrGA) (GBr indicates 8-bromo-2'-deoxyguanosine), were designed. These oligonucleotides form three parallel G4s and one antiparallel G4 without loop regions. The scaffolds had terminal G-tetrads that were either anti-deoxyguanosines (anti-dGs) or syn-deoxyguanosines (syn-dGs) at different proportions. The results showed that the parallel G4 DNAzymes exhibited 2 to 5-fold higher peroxidase activities than the antiparallel G4 DNAzyme, which is due to the absence of the 3'-terminal G-tetrad in the antiparallel G4. Furthermore, the 3'-terminal G-tetrad consisting of four anti-dGs in parallel G4s was more energetically favorable and thus more preferable for hemin stacking compared with that consisting of four syn-dGs. We further investigated the influence of 3' and 5' deoxyadenosine (dA) caps on the enzymatic performance by adding 3'-3' or 5'-5' phosphodiester bonds to AG4A. Our data demonstrated that 3' dA caps are versatile residues in promoting the interaction of G4s with hemin. Thus, by increasing the number of 3' dA caps, the DNAzyme of 3'A5'-5'GG3'-3'GG5'-5'A3' with two 5'-terminal G-tetrads can exhibit significantly high catalytic activity, which is comparable to that of 5'A3'-3'GG5'-5'GG3'-3'A5' with two 3'-terminal G-tetrads. This study may provide insights into the catalytic mechanism of G4-based DNAzymes and strategies for promoting their catalytic activities.