It is well reported that human telomeric DNA sequence［AGGG(TTAGGG)3］can form a welldefined quadruplex structure with the hydrogen bonds between guanine bases in the appearance of K+ and Na+. It was reported that the formation and stability of the structure was meaningful for both gene regulation and apoptosis. In this study

the replacement of guanine bases in this sequence with thymine

adenine and cytosine at different position was carried out respectively.Their ability to form G-quadruplex as well as the G-quadruplex stability has been studied by circular dichroism (CD) spectroscopy.All oligonucleotides were synthesized and purified by Saibaisheng company.The concentrations of the oligonucleotides

expressed in singlestrand molarity

were determined by UV absorption at 260 nm using a ‘nearestneighbor’ approximation. Circular dichroism experiments were performed on a JASCO model J810 circular dichroism spectrapolarimeter equipped with a Peltier temperature controller.All samples (10 μmol/L in 10 mmol/L trisHCl buffer containing 100 mmol/L NaCl at pH 7) were preheated to 95℃ for 5 min and slowly cooled to room temperature prior to measurements.A 1 mm path length cell was used for all measurements

and each spectrum was an average of five scans at 5℃.The Gquadruplex solutions were recorded in the 5℃ to 95℃ temperature range

using a scan rate of 0.5 K min-1 and following the variation of CD absorption at its peak around 295 nm.Analysis of the thermal curves was performed by using the software origin.Van′t Hoff thermodynamic analysis was carried out for the intramolecular G-quadruplex oligonucleotides.Their thermal reversibility (superimposable heating and cooling profiles) allowed the calculation of the value of the equilibrium constant (K) assuming a twostate model.Application of the Van′t Hoff equation (lnK=-ΔH/RT+ΔS/R) yielded to the determination of the enthalpy (ΔH) and entropy (ΔS) of the process

as described elsewhere.Results show that the replacement of guanine bases in the middle of the three stacking quartets brings the biggest influence on the stability of the formed G-quadruplex. Data also show that potassium ions bring more stability than sodium ions to the formed G-quadruplex in most cases. It is very interesting to find that

when guanine base is replaced by cytosine or adenine at specific sites

the formed structure has comparable or even higher stability as the original sequence.