人體端粒鳥嘌呤-四股結構去氧核醣核酸序列： 整體與單分子的研究

Abstract

真核細胞的端粒，對於染色體尾端穩定性是相當重要的。在有單價陽離子如鈉或鉀離子存在之下，端粒尾端富含鳥嘌呤單股的DNA序列，可以藉由Hoogsteen氫鍵形成一個二級結構稱之為鳥嘌呤-四股結構。為了驗證人類端粒是否具有鳥嘌呤-四股結構的存在，我們利用了雙光子激發螢光生命期顯微技術，來尋找人體鼻咽癌細胞，中期染色體之中鳥嘌呤-四股結構的位置所在。然而，富含鳥嘌呤序列，可以具有多樣性鳥嘌呤-四股結構，而且改變環境條件可能使其結構互相轉換。舉例來說，鈉鉀離子交換後，會產生一個快速的光譜變化。我們在此利用數種方法，來瞭解鈉鉀離子交換之中所引起的快速光譜變化，其中所隱含的機制。螢光共振能量傳遞與單分子栓球實驗的研究，暗示著這個因鉀離子所產生的快速光譜變化，有可能不是F1UFF2，須經由一個完全展開的中間態。此外，變換溫度的圓二色光譜研究顯示，F1與F2之間的能障幾乎可以忽略。因此，我們認為這個鈉鉀離子交換所產生的快速光譜變化，是由於F1到F2之間，經過了快速的鹼基位移與環的重組所造成的。另一方面，我們在脫水的環境中觀察到，由鈉離子溶液之中的反平行鳥嘌呤-四股結構，轉換為鉀離子溶液中的平行鳥嘌呤-四股結構。利用van’t Hoff的方法，在熱解旋曲線之中，來估計各個摺疊狀態到完全展開狀態之間其自由能的差別，以及利用Eyring的方法，以即時變溫圓二色光譜，來估計鳥嘌呤-四股結構變化所需的活化能，嘗試建立一個HT22在鈉離子溶液中加入脫水環境(40% (w/v) PEG 200)的反平行鳥嘌呤-四股結構，轉換到鉀離子溶液中平行鳥嘌呤-四股結構的熱力學能量圖。此外，由於Cu2+可以誘導鳥嘌呤-四股結構的崩解，再者，EDTA2-可以抑制Cu2+離子的作用，使鳥嘌呤-四股結構可以由展開狀態變回摺疊狀態，根據此方法，我們發現動力學產物在人體生理條件下比較容易生成。更進一步，利用Cu2+離子在室溫下誘導鳥嘌呤-四股結構展開，來作為篩選鳥嘌呤-四股結構配位基的一個新方法。因此，我們篩選出3,6,9三端取代的BMVC4分子，可以作為之後研究的重心。

Telomeres, the ends of eukaryotic chromosomes, are essential for the stability of chromosomes. In the presence of monovalent cations such as Na+ or K+, the G-rich single stranded DNA of telomere can form a secondary structure through Hoogsteen hydrogen bonds, termed G-quadruplex (G4). We have applied two-photon excitation fluorescence lifetime microscope (2PE-FLIM) to successfully verify and map the localizations of G4 structures in human nasopharyngeal carcinoma metaphase chromosomes. In addition, the G-rich sequences can adopt various G4 structures and possibly interconvert among these structures upon changing solvent and temperature conditions. For example, a fast spectral conversion occurs under Na/K cation exchange. We have developed a number of methods to elucidate the mechanisms of this spectral conversion. Ensemble-based fluorescence resonance energy transfer (FRET) and single molecule tethered particle motion (TPM) studies suggested that the fast spectral conversion is unlikely due to F1UFF2 via a totally unfolded intermediate induced by potassium cations. In addition, temperature-dependent circular dichroism (CD) studies suggested that the energy barrier from F1 to F2 is almost negligible. Thus, we consider that the fast spectral conversion during Na/K cation exchange is due to F1F2 via rapid base shift and loop rearrangement. On the other hand, the structural conversion from the antiparallel G4 structure in Na+ solution to the parallel G4 structure in K+ solution was observed in the presence of dehydrated reagents. Using thermodynamic and kinetic studies, a free energy diagram can be tentatively established for the structural conversion of HT22 from antiparallel form in Na+ solution to the parallel in K+ solution at 25℃ under 40 % (w/v) PEG 200 condition. It is known that the Cu2+ induces the unfolding of G4 structure while addition of the EDTA2- can chelate the Cu2+ to reverse the unfolded state to the folded state. Based on this and we found that the kinetic product is likely to play a major role in physiological condition. Furthermore, G4 stabilizers are screened by a novel method based on Cu2+ -induced G4 unfolding at room temperature. Thus, 3,6,9 tri-substitution of BMVC4 core molecules are ready to prepare in further study.