Effects of Central Loop Length and Metal Ions on the Thermal Stability of G-Quadruplexes.

The central loop of G-quadruplex molecular beacons is a key element to sense target DNA or RNA sequences. In this study, circular dichroism spectroscopy (CD), thermal difference spectrum (TDS), non-denatured non-denaturing gel electrophoresis, and thermal stability analysis were used to investigate the effect of the central loop length on G-quadruplex features. Two series of G-quadruplexes, AG3TTAG3-(TTA)n-G3TTAG3T (n = 1-8) (named TTA series) and AG3TTTG3-(TTA)n-G3TTTG3T (n = 1-8) (named TTT series) were examined in K+ and Na+ solutions, respectively. CD and TDS spectral data indicated that TTA series adopted an antiparallel G-quadruplex structure in Na+ solution and a hybrid G-quadruplex structure in K+ solution respectively. TTT series exhibited a hybrid G-quadruplex structure in both Na+ and K+ solutions. UV melting curves indicated that the stability of G-quadruplex in both series was reduced by the elongation of central loop. Thermal stability analysis concluded that the G-quadruplex destabilization with long central loop is an entropy-driven process due to more flexible and longer central loops.

Human-specific features of the G-quadruplex in the androgen receptor gene promoter: A comparative structural and dynamics study.

The androgen receptor (AR) promoter contains guanine-rich regions that are able to fold into polymorphic G-quadruplex (GQ) structures, and whose deletion decreases AR gene transcription. Our attention was focused on this region because of the frequent termination of sequencing reactions during promoter methylation studies. UV and circular dichroism (CD) spectroscopy of synthetic oligonucleotides encompassing these guanine-rich regions suggested a parallel quadruplex topology with three guanine quartets and three side loops in the three cases. Melting curves revealed a lower thermostability of the human GQ compared to the rat/mouse QG structures, which is attributed to the presence of a longer central loop in the former. One molecular model is proposed for the highly similar sequences in the rat/mouse. Due to the polymorphism resulting from possible arrangements of the guanine tracts, two models were derived for the human GQ. Molecular dynamics (MD) simulations determined that both models for the human GQ had higher flexibility and lower stability than the rodent GQ models. These properties result from the presence of a longer central loop in the human GQ models, which contains 11 and 13 nucleotides, in comparison to the 2-nucleotide long loop in the rat/mouse GQ. Overall, the unveiled structural and dynamics features provide sufficient detail for the intelligent design of drugs targeting the human AR promoter.