外加電場下多價鹽溶液中高分子電解質之行為模擬

Unfolding of polyelectrolytes in tetravalent salt solutions in electric fields: A computer simulation study.

巫昆懋(Kun-Mao Wu)

指導教授 ： 蕭百沂

國立清華大學/原子科學院/工程與系統科學系/碩士(2007年)

https://doi.org/10.6843/NTHU.2007.00253

摘要

我們利用Langevin dynamics模擬方法探討外加電場對多價鹽溶液中高分子電解質性質的影響。高分子電解質的尺寸，包含旋轉半徑(radius of gyration)及末端距(end-to-end distance)，隨著電場變化而改變。在低電場下，旋轉半徑與末端距約為一定值，不會影響高分子電解質的行為；當電場加大超過某一臨界值，部分吸附在高分子上的反離子脫離高分子電解質束縛成為非吸附反離子，高分子電解質開始被拉伸；電場持續加大，聚離子鏈最後會被拉伸成棒狀。形狀因子Rg^2/Re^2與偏球率(Asphericity)的數值也反映出此一事實。臨界電場的值增加隨著鹽濃度增加而變大。當外加鹽濃度大約等於對等電荷濃度有最大臨界電場。在最緊密狀態高分子電解質球的拉伸臨界電場強度E*可由極化能與熱擾動能量相等時的電場求得，其結果顯示出與鏈長的相依性，E*~N^(-1/2) ，可以運用於電泳分離帶電且緊密摺疊的生物分子。非緊密摺疊的高分子電解質被拉伸的過渡行為較不明顯。在伸長狀態下高分子電解質的拉伸臨界電場與鏈長相依性的關係式約為E*~N^(-1) 。最後我們分析聚離子鏈以及吸附離子的動態行為，包含電泳遷移率，與吸附離子在聚離子鏈上分佈的情形。在低電場下，吸附反離子緊緊吸附在聚離子鏈上，減低了聚離子鏈的遷移率，吸附離子平均分佈在聚離子鏈上。在大電場下，電場對吸附反離子加速的力大於聚離子鏈的靜電力，吸附反離子在聚離子鏈上產生滑動現象，吸附反離子在聚離子上分佈產生不均勻現象，有較多吸附反離子分佈在順著電場方向的聚離子鏈一端。

關鍵字

高分子電解質 ； 電泳分離 ； 基因定序 ； 郎日凡動力學

並列摘要

By means of Langevin dynamics simultion,we investigated the properties of a flexible polyelectrolyte (PE) at different salt concentration in external electric fields. The radius of gyration (Rg) and end-to-end distance (Re) alter with the strength of electric field. At weak electric field, the values of Rg and Re are roughly constant. The field does not affect PE behavior. Further increasing electric field over a critical value, part of condensed counterions escape from the PE and become free counterions. The PE starts to be stretched and displays a rod like structure. The values of shape factor and asphericity of the chain show these behaviors too. Moreover, the higher the salt concentration, the higher the critical electric field will be. The critical electric field attains a maximum value when the salt concentration is around equivalence point. For the most compact PE globule, the unfolding critical field E* is determined by equaling the polarization energy and thermal energy(kT). The result gives the relation between the critical field E* and the chain length,E*~N^(-1/2) ,which can be used in electrophoretic separating of charged, globule biopolymers. The unfolding transition for noncollapsed PE is less pronounced. For elongated PE, we obtained a scaling law,E*~N^(-1). In the last section of study, we analyzed dynamic quantities including mobility of PE, and of condensed ions, and distribution of condensed ions on PE backbone. For small field, condensed counterions bind to PE and slow the mobility of PE. Condensed counterions distribute uniformly on PE backbone. For large field, the condensed counterions overcome the electrostatic interaction and can decondense or glide on the PE backbone. Condensed counterions thus distribute non-uniformly and prefer to stay on one side of PE backbone along the field direction.

並列關鍵字

polyelectrolyte ； electrophoresis ； DNA sequence ； Langevin dynamics simulations

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