B型DNA中鳥嘌呤-胞嘧啶堿基對(duì)內(nèi)雙質(zhì)子轉(zhuǎn)移反應(yīng)

林月霞 王紅艷 高思敏 吳穎曦 李汝虎

(西南交通大學(xué)物理科學(xué)與技術(shù)學(xué)院,成都610031)

1 引言

電離輻射能夠破壞DNA結(jié)構(gòu),進(jìn)而誘發(fā)基因突變,因此電離輻射導(dǎo)致的DNA結(jié)構(gòu)變化受到越來越多科學(xué)家的關(guān)注.兩條DNA鏈依靠彼此堿基之間的氫鍵而結(jié)合成雙螺旋結(jié)構(gòu),氫鍵中的質(zhì)子在受到輻射時(shí)容易發(fā)生轉(zhuǎn)移,形成新的異構(gòu)體,導(dǎo)致DNA在復(fù)制、轉(zhuǎn)錄過程中出錯(cuò).因此研究堿基對(duì)氫鍵間質(zhì)子轉(zhuǎn)移反應(yīng)具有重要意義.其中一些自由基異構(gòu)體已在實(shí)驗(yàn)中檢測(cè)到,1-4而中性DNA堿基對(duì)結(jié)構(gòu)因?yàn)槠湎鄬?duì)穩(wěn)定,相對(duì)于離子結(jié)構(gòu)較難發(fā)生質(zhì)子轉(zhuǎn)移反應(yīng),但是理論和實(shí)驗(yàn)研究都表明中性DNA分子的質(zhì)子轉(zhuǎn)移反應(yīng)也是導(dǎo)致基因變異的主要因素之一,4-6因此研究中性DNA分子的質(zhì)子轉(zhuǎn)移機(jī)理是十分必要的.

早期科學(xué)家的研究主要集中在堿基、堿基對(duì)性質(zhì)7-11以及堿基對(duì)內(nèi)質(zhì)子轉(zhuǎn)移反應(yīng)上.12-16堿基對(duì)內(nèi)質(zhì)子轉(zhuǎn)移反應(yīng)受到多種環(huán)境因素的影響,水溶劑、堿基序列、堆積效應(yīng)和各種離子等都會(huì)影響堿基對(duì)內(nèi)質(zhì)子轉(zhuǎn)移.因此科學(xué)家開始關(guān)注這些環(huán)境因素的作用,17-32目前的研究結(jié)果表明,水分子有助于堿基對(duì)陰離子發(fā)生單質(zhì)子轉(zhuǎn)移反應(yīng),17,18不同堿基排序?qū)A基對(duì)陰離子的單質(zhì)子轉(zhuǎn)移分別起到促進(jìn)和阻礙作用,19堆積效應(yīng)在質(zhì)子轉(zhuǎn)移過程中改變氫鍵鍵長(zhǎng)并保持堿基對(duì)的平面性,20一些金屬離子也有助于單質(zhì)子轉(zhuǎn)移反應(yīng)的發(fā)生.32,33因此各種環(huán)境因素對(duì)于堿基對(duì)氫鍵間質(zhì)子轉(zhuǎn)移反應(yīng)發(fā)揮著重要作用,不容忽視.

雖然已經(jīng)有DNA三聚體陰離子單質(zhì)子轉(zhuǎn)移反應(yīng)的一些報(bào)道,但是對(duì)于DNA三聚體的雙質(zhì)子轉(zhuǎn)移反應(yīng)則鮮見文獻(xiàn)報(bào)道.堿基對(duì)氫鍵間雙質(zhì)子轉(zhuǎn)移反應(yīng)較單質(zhì)子轉(zhuǎn)移反應(yīng)更為復(fù)雜,因此我們選取單個(gè)GC堿基對(duì)和含GC堿基對(duì)的四種排序的DNA三聚體為研究對(duì)象,分析其在氣相和水溶劑中雙質(zhì)子轉(zhuǎn)移途徑以及轉(zhuǎn)移過程中能量和結(jié)構(gòu)特性,探索堿基排序和水溶劑對(duì)堿基對(duì)氫鍵間質(zhì)子轉(zhuǎn)移反應(yīng)的影響,進(jìn)一步了解雙質(zhì)子轉(zhuǎn)移機(jī)理.

2 理論方法

文中所選用的單個(gè)鳥嘌呤-胞嘧啶堿基對(duì)(GC)和四種不同排序的B型DNA三聚體(d(5?-AGA-3?)·d(3?-TCT-5?)、d(5?-GGG-3?)·d(3?-CCC-5?)、d(5?-TGT-3?)·d(3?-ACA-5?)、d(5?-CGC-3?)·d(3?-GCG-5?))通過 HyperChem軟件包34中的核酸數(shù)據(jù)庫構(gòu)建.在文中為了方便識(shí)別每一層的堿基對(duì),將其簡(jiǎn)記為dATGCAT、dGCGCGC、dTAGCTA、dCGGCCG.

單個(gè)GC堿基對(duì)直接采用M06-2X/6-31G*方法35-38優(yōu)化結(jié)構(gòu)和尋找過渡態(tài),為了節(jié)約機(jī)時(shí)并保證計(jì)算精度,四種DNA三聚體均采用ONIOM(M06-2X/6-31G*:PM3)分層計(jì)算方法,39-45如圖1所示,中間層GC堿基對(duì)采用M06-2X/6-31G*優(yōu)化,核糖-磷酸和上下相鄰堿基對(duì)采用PM3方法46并凍結(jié).M06-2X方法是由Zhao等35-38發(fā)展的適用于非鍵作用(如π-π堆積作用、氫鍵)體系的新密度泛函方法,已被應(yīng)用于多種研究體系,研究結(jié)果與實(shí)驗(yàn)數(shù)據(jù)和其它理論數(shù)據(jù)對(duì)比,證明此方法可以得到精確的化學(xué)反應(yīng)能壘.36水溶劑中采用PCM極化模型,47-49所有結(jié)構(gòu)優(yōu)化計(jì)算方法與氣相中相同.文中所有計(jì)算在Gaussian 09程序50中完成.

圖1 dATGCAT三聚體結(jié)構(gòu)及其中間GC堿基對(duì)的雙質(zhì)子轉(zhuǎn)移路徑和原子編號(hào)Fig.1 Structure of dATGCAT trimer as well as double-proton-transfer pathway and atomic numbers in the middle GC base pair

3 結(jié)果與討論

堿基對(duì)氫鍵間雙質(zhì)子轉(zhuǎn)移機(jī)理分為協(xié)同轉(zhuǎn)移和分步轉(zhuǎn)移兩種,協(xié)同轉(zhuǎn)移機(jī)理指兩個(gè)質(zhì)子H1和H4a同時(shí)轉(zhuǎn)移到對(duì)面堿基,得到雙質(zhì)子轉(zhuǎn)移產(chǎn)物(DPT);分步轉(zhuǎn)移機(jī)理指質(zhì)子H1先轉(zhuǎn)移到胞嘧啶C的N3位得到單質(zhì)子轉(zhuǎn)移產(chǎn)物(SPT),然后質(zhì)子H4a再轉(zhuǎn)移到鳥嘌呤的O6位得到雙質(zhì)子轉(zhuǎn)移產(chǎn)物(圖1).文中DTS、STS1和STS2分別表示雙質(zhì)子轉(zhuǎn)移過程中對(duì)應(yīng)的過渡態(tài)、第一個(gè)質(zhì)子H1轉(zhuǎn)移過程中形成的過渡態(tài)和第二個(gè)質(zhì)子H4a轉(zhuǎn)移過程中形成的過渡態(tài).

3.1 氣相中質(zhì)子轉(zhuǎn)移反應(yīng)

3.1.1 氣相中質(zhì)子轉(zhuǎn)移方式

單個(gè)中性GC堿基對(duì)和含GC堿基對(duì)的三種排序的DNA三聚體dATGCAT、dGCGCGC和dTAGCTA中發(fā)生雙質(zhì)子協(xié)同轉(zhuǎn)移,而dCGGCCG排序的DNA三聚體則發(fā)生了分步雙質(zhì)子轉(zhuǎn)移.分析表明質(zhì)子轉(zhuǎn)移方式主要由堿基對(duì)間的靜電相互作用和質(zhì)子接受位的質(zhì)子親和勢(shì)兩個(gè)因素決定.電荷分布表明DNA三聚體中間堿基對(duì)GC的兩個(gè)堿基單體在質(zhì)子轉(zhuǎn)移前后沒有產(chǎn)生明顯的電荷差,但是由于上下相鄰堿基對(duì)的偶極矩存在,不同堿基排序影響了中間堿基對(duì)質(zhì)子轉(zhuǎn)移.圖2所示的偶極矩示意圖中,AT堿基對(duì)偶極矩約為6.7×10-30C·m,GC堿基對(duì)偶極矩約為2.0×10-29C·m,方向從A/G指向T/C,中間堿基對(duì)質(zhì)子轉(zhuǎn)移是氫原子攜帶正電荷轉(zhuǎn)移,其轉(zhuǎn)移方向與上下堿基對(duì)偶極矩方向相反時(shí)恰形成靜電吸引作用.因此上下相鄰堿基對(duì)與中間堿基對(duì)的靜電相互作用使得dATGCAT和dGCGCGC的排序有助于中間堿基對(duì)GC中H4a質(zhì)子轉(zhuǎn)移(從胞嘧啶C轉(zhuǎn)移到鳥嘌呤G),dTAGCTA和dCGGCCG的排序有助于中間堿基對(duì)中H1質(zhì)子轉(zhuǎn)移(從鳥嘌呤G轉(zhuǎn)移到胞嘧啶C),且dGCGCGC和dCGGCCG中上下鄰近GC偶極矩大于dATGCAT和dTAGCTA中AT偶極矩,因此對(duì)中間堿基對(duì)的質(zhì)子轉(zhuǎn)移方式影響更大.同時(shí)胞嘧啶C的N3位質(zhì)子親和勢(shì)比鳥嘌呤G的O6位大21.3 kJ·mol-1,從質(zhì)子親和勢(shì)角度看,質(zhì)子H1轉(zhuǎn)移比質(zhì)子H4a轉(zhuǎn)移容易.

單個(gè)GC堿基對(duì)中兩質(zhì)子H4a和H1轉(zhuǎn)移速度相當(dāng),發(fā)生了協(xié)同雙質(zhì)子轉(zhuǎn)移,過渡態(tài)結(jié)構(gòu)如圖3(a)所示.dATGCAT三聚體中靜電相互作用有助于質(zhì)子H4a轉(zhuǎn)移,而質(zhì)子親和勢(shì)有助于質(zhì)子H1轉(zhuǎn)移.由于AT堿基對(duì)偶極矩較小,靜電相互作用微弱,因此質(zhì)子轉(zhuǎn)移方式與單個(gè)GC堿基對(duì)相同,發(fā)生協(xié)同雙質(zhì)子轉(zhuǎn)移(圖3(b)).dGCGCGC中靜電相互作用有助于H4a轉(zhuǎn)移,而質(zhì)子親和勢(shì)有助于H1轉(zhuǎn)移,兩種因素平衡結(jié)果使dGCGCGC中仍保持協(xié)同轉(zhuǎn)移機(jī)理.質(zhì)子轉(zhuǎn)移過程中由于GC堿基對(duì)間較大的靜電相互作用,過渡態(tài)結(jié)構(gòu)(圖3(c))中質(zhì)子H4a轉(zhuǎn)移比質(zhì)子H1更快,O6…H4a鍵長(zhǎng)僅為0.1058 nm.dTAGCTA中靜電相互作用和質(zhì)子親和勢(shì)作用都促進(jìn)質(zhì)子H1轉(zhuǎn)移,但因?yàn)锳T的偶極矩較小,上下相鄰堿基對(duì)對(duì)中間堿基對(duì)的作用很小,因此仍保持協(xié)同轉(zhuǎn)移方式.圖3(d)中過渡態(tài)結(jié)構(gòu)顯示出質(zhì)子H1轉(zhuǎn)移較快,H4a轉(zhuǎn)移較慢,H4a…N4鍵長(zhǎng)仍為0.1097 nm.dCGGCCG中兩個(gè)因素同時(shí)有助于質(zhì)子H1轉(zhuǎn)移,且GC的偶極矩較大,上下相鄰堿基對(duì)對(duì)中間堿基對(duì)的作用較強(qiáng),因此發(fā)生了分步雙質(zhì)子轉(zhuǎn)移方式.質(zhì)子H1先轉(zhuǎn)移到C上得到單質(zhì)子轉(zhuǎn)移產(chǎn)物,然后質(zhì)子H4a再轉(zhuǎn)移到G上,最后得到雙質(zhì)子轉(zhuǎn)移產(chǎn)物,過渡態(tài)結(jié)構(gòu)見圖3(e,f).因此在中性DNA三聚體結(jié)構(gòu)中,上下相鄰堿基對(duì)的靜電相互作用和質(zhì)子接受位的質(zhì)子親和勢(shì)共同影響質(zhì)子轉(zhuǎn)移速率.

圖2 腺嘌呤-腺嘧啶(AT)和GC堿基對(duì)的偶極矩示意圖Fig.2 Diagrams for dipole moment of the adenine-thymine(AT)and GC base pairs

圖3 GC、dATGCAT、dGCGCGC、dTAGCTA和dCGGCCG五種模型的過渡態(tài)結(jié)構(gòu)Fig.3 Transition state structures for five models of GC,dATGCAT,dGCGCGC,dTAGCTA,and dCGGCCG

3.1.2 氣相中質(zhì)子轉(zhuǎn)移能線圖

圖4(a)描繪出了氣相中五種選定的中性體系的雙質(zhì)子轉(zhuǎn)移反應(yīng)能線圖.dATGCAT和dTAGCTA三聚體中,雙質(zhì)子轉(zhuǎn)移反應(yīng)的能壘與單個(gè)GC堿基對(duì)的能壘65.3 kJ·mol-1僅相差1.7和0.4 kJ·mol-1.主要是因?yàn)锳T/TA堿基對(duì)的偶極矩約為6.7×10-30C·m(圖2),上下相鄰堿基對(duì)為AT/TA時(shí),對(duì)中間堿基對(duì)的靜電相互作用不明顯.在dGCGCGC三聚體中,反應(yīng)能壘比單個(gè)GC堿基對(duì)中的能壘小3.8 kJ·mol-1,主要是因?yàn)镚C/CG的偶極矩約為2.0×10-29C·m(圖2),上下相鄰堿基對(duì)為GC/CG時(shí)對(duì)中間堿基對(duì)的靜電相互作用顯著.dCGGCCG三聚體中,分步轉(zhuǎn)移過程中相對(duì)能明顯地相差較小,第一、二個(gè)質(zhì)子轉(zhuǎn)移能壘分別為58.2和54.0 kJ·mol-1.

在五種模型中,雙質(zhì)子轉(zhuǎn)移反應(yīng)都是吸熱反應(yīng).dATGCAT和dGCGCGC三聚體發(fā)生雙質(zhì)子轉(zhuǎn)移反應(yīng)的反應(yīng)能分別為34.3和33.5 kJ·mol-1,與其它體系相比較小,這主要是因?yàn)锳T/GC堿基對(duì)中偶極矩方向是從A/G指向T/C(圖2),dATGCAT和dGCGCGC三聚體中三個(gè)堿基對(duì)的偶極矩方向相同,由于堿基對(duì)間靜電排斥作用,結(jié)構(gòu)不穩(wěn)定,容易發(fā)生質(zhì)子轉(zhuǎn)移反應(yīng).相反,dTAGCTA和dCGGCCG三聚體由于靜電吸引作用,結(jié)構(gòu)相對(duì)穩(wěn)定,較難發(fā)生質(zhì)子轉(zhuǎn)移反應(yīng),其對(duì)應(yīng)的反應(yīng)能較大,分別為39.3和41.8 kJ·mol-1.雖然五個(gè)模型中質(zhì)子轉(zhuǎn)移反應(yīng)都為吸熱反應(yīng),但其反應(yīng)能壘(53.1-65.3 kJ·mol-1)在合理的范圍(12.6-117.2 kJ·mol-1)內(nèi),且其反應(yīng)能(33.5-41.8 kJ·mol-1)小于～54.4 kJ·mol-1,滿足經(jīng)典的過渡態(tài)理論和異構(gòu)體存在的能差,12因此當(dāng)外部條件滿足時(shí),雙質(zhì)子轉(zhuǎn)移反應(yīng)就可能發(fā)生.

圖4 氣相(a)和水溶液(b)中五種模型雙質(zhì)子轉(zhuǎn)移能線圖,包含協(xié)同轉(zhuǎn)移和分步轉(zhuǎn)移Fig.4 Energy profiles for double-proton transfer of the five models in gas phase(a)and in aqueous solution(b),including the concerted and stepwise processes

3.1.3 氣相中質(zhì)子轉(zhuǎn)移反應(yīng)的氫鍵變化

質(zhì)子轉(zhuǎn)移過程中氫鍵變化最明顯.在發(fā)生協(xié)同轉(zhuǎn)移的過渡態(tài)中,轉(zhuǎn)移質(zhì)子H4a和H1所在的氫鍵O6…H4a―N4和N1―H1…N3均明顯縮短(表1)有利于質(zhì)子轉(zhuǎn)移.在dATGCAT的過渡態(tài)中O6…H4a―N4氫鍵比轉(zhuǎn)移前縮短了0.0303 nm,N1―H1…N3氫鍵縮短了0.0279 nm.對(duì)于發(fā)生分步轉(zhuǎn)移的dCGGCCG三聚體,第一過渡態(tài)中N1―H1…N3氫鍵由0.2942 nm縮短到0.2695 nm,第二過渡態(tài)中O6…H4a―N4氫鍵由0.2841 nm縮短到0.2492 nm,極大地方便了質(zhì)子轉(zhuǎn)移.兩氫鍵在雙質(zhì)子轉(zhuǎn)移完成后又有所增長(zhǎng),但較質(zhì)子轉(zhuǎn)移前略短.沒有發(fā)生質(zhì)子轉(zhuǎn)移的氫鍵N2―H2a…O2鍵長(zhǎng)變化較小,且變化沒有規(guī)律性.

3.2 水溶劑中質(zhì)子轉(zhuǎn)移反應(yīng)

3.2.1 水溶劑中質(zhì)子轉(zhuǎn)移方式

考慮水溶劑作用后,雙質(zhì)子轉(zhuǎn)移中分步轉(zhuǎn)移機(jī)理占據(jù)了絕對(duì)優(yōu)勢(shì),沒有得到協(xié)同轉(zhuǎn)移的過渡態(tài),這是因?yàn)樗軇?duì)SPT結(jié)構(gòu)的穩(wěn)定作用.表2中列出了質(zhì)子轉(zhuǎn)移過程中各結(jié)構(gòu)的電荷分布,可以看出在原始反應(yīng)物和雙質(zhì)子轉(zhuǎn)移產(chǎn)物中中間堿基對(duì)的兩堿基間幾乎沒有電荷差,而其單質(zhì)子轉(zhuǎn)移產(chǎn)物中存在明顯的電荷差,正電荷主要分布在胞嘧啶C上而負(fù)電荷主要分布在鳥嘌呤G上,這種電荷分布不均衡的結(jié)構(gòu)在氣相中是極不穩(wěn)定的,而在水溶劑中,這樣的極化結(jié)構(gòu)卻被水溶劑大大地穩(wěn)定,因此分步轉(zhuǎn)移機(jī)理占優(yōu)勢(shì).

表1 氣相和水溶液(括號(hào)中數(shù)據(jù))中五種模型對(duì)應(yīng)的原始結(jié)構(gòu)(WC)、過渡態(tài)結(jié)構(gòu)(TS)和質(zhì)子轉(zhuǎn)移結(jié)構(gòu)(PT)中處于中間的堿基對(duì)結(jié)構(gòu)的三個(gè)氫鍵鍵長(zhǎng)Table 1 Three hydrogen-bond lengths for the middle GC base pair of the canonical Watson-Crick(WC),transition state(TS),and proton-transferred(PT)base pair structures in the five neutral models in gas phase and in aqueous solution(in brackets)

表2 水溶劑中五種模型的原始結(jié)構(gòu)、單質(zhì)子轉(zhuǎn)移產(chǎn)物和雙質(zhì)子轉(zhuǎn)移產(chǎn)物中處于中間的堿基對(duì)結(jié)構(gòu)的鳥嘌呤和胞嘧啶單體電荷分布Table 2 Charge distribution for the guanine and cytosine moieties of the middle GC base pairs of the canonical Watson-Crick,single-proton-transferred,and double-proton-transferred base pairs in the five neutral models in aqueous solution

3.2.2 水溶劑中質(zhì)子轉(zhuǎn)移能線圖

圖4顯示水溶劑中分步雙質(zhì)子轉(zhuǎn)移的反應(yīng)能壘較氣相中協(xié)同轉(zhuǎn)移能壘降低(分步轉(zhuǎn)移能壘約51.5 kJ·mol-1,協(xié)同轉(zhuǎn)移能壘約63.6 kJ·mol-1).上下相鄰堿基對(duì)對(duì)中間堿基對(duì)的靜電相互作用被減弱,不同排序DNA三聚體在質(zhì)子轉(zhuǎn)移過程中表現(xiàn)出相似的能量變化趨勢(shì).單個(gè)GC堿基對(duì)中質(zhì)子H1轉(zhuǎn)移能壘為42.7 kJ·mol-1,明顯低于四種DNA三聚體的質(zhì)子轉(zhuǎn)移能壘(約為51.7 kJ·mol-1),這主要是因?yàn)镾TS1_GC結(jié)構(gòu)中氫鍵N1―H1…N3較短的鍵長(zhǎng)(0.2624 nm)有利于H1轉(zhuǎn)移(DNA三聚體中N1―H1…N3氫鍵鍵長(zhǎng)為0.2682-0.2692 nm),質(zhì)子轉(zhuǎn)移過程中結(jié)構(gòu)的氫鍵鍵長(zhǎng)見表1.水溶劑中堿基對(duì)的雙質(zhì)子轉(zhuǎn)移仍為吸熱反應(yīng).

3.2.3 水溶劑中質(zhì)子轉(zhuǎn)移反應(yīng)的氫鍵變化

水溶劑作用后,質(zhì)子轉(zhuǎn)移過程中變化最明顯的部分仍為氫鍵.從表1中可以看出,第一個(gè)過渡態(tài)結(jié)構(gòu)中氫鍵N1―H1…N3明顯縮短,這極大地方便了質(zhì)子H1轉(zhuǎn)移,其次氫鍵O6…H4a―N4也有較明顯縮短,為質(zhì)子H4a轉(zhuǎn)移提供了便利.質(zhì)子H1轉(zhuǎn)移到對(duì)面胞嘧啶C的N3位后,H4a開始轉(zhuǎn)移,在第二個(gè)過渡態(tài)結(jié)構(gòu)中氫鍵O6…H4a―N4進(jìn)一步縮短到0.2480-0.2492 nm,這極大地方便了質(zhì)子H4a轉(zhuǎn)移到對(duì)面鳥嘌呤G的O6位.質(zhì)子轉(zhuǎn)移反應(yīng)完成之后,在雙質(zhì)子轉(zhuǎn)移產(chǎn)物結(jié)構(gòu)中氫鍵O6…H4a―N4和N1―H1…N3較反應(yīng)前都有縮短趨勢(shì),而未發(fā)生質(zhì)子轉(zhuǎn)移的氫鍵N2―H2a…O2則增長(zhǎng).

4 結(jié)論

計(jì)算結(jié)果表明,氣相中觀測(cè)到協(xié)同轉(zhuǎn)移和分步轉(zhuǎn)移兩種雙質(zhì)子轉(zhuǎn)移方式,主要是由上下相鄰堿基對(duì)的靜電相互作用和質(zhì)子接受位的質(zhì)子親和勢(shì)大小來決定.dATGCAT和dGCGCGC排序有助于質(zhì)子H4a轉(zhuǎn)移,而dTAGCTA和dCGGCCG排序有助于質(zhì)子H1轉(zhuǎn)移.胞嘧啶的N3質(zhì)子親和勢(shì)較鳥嘌呤的O6位大,使H1質(zhì)子較易轉(zhuǎn)移.溶劑化效應(yīng)使得靜電相互作用被減弱,也穩(wěn)定了分步雙質(zhì)子轉(zhuǎn)移過程中電荷分布不均衡的單質(zhì)子轉(zhuǎn)移產(chǎn)物,因此分步機(jī)理占據(jù)了絕對(duì)優(yōu)勢(shì).

氣相中,dATGCAT和dGCGCGC排序的DNA三聚體具有較小的雙質(zhì)子轉(zhuǎn)移反應(yīng)能,而dTAGCTA和dCGGCCG排序的DNA三聚體具有相對(duì)較大的反應(yīng)能,主要是因?yàn)樯舷孪噜弶A基對(duì)的靜電相互作用.溶劑化效應(yīng)使得五種模型雙質(zhì)子轉(zhuǎn)移過程中各結(jié)構(gòu)能量變化趨勢(shì)相似,水溶劑中雙質(zhì)子轉(zhuǎn)移的反應(yīng)能都比氣相中增大,從熱力學(xué)方面分析水溶劑雖然有助于單質(zhì)子轉(zhuǎn)移,卻不利于雙質(zhì)子轉(zhuǎn)移反應(yīng).所選五種模型的GC堿基對(duì)雙質(zhì)子轉(zhuǎn)移過程中氫鍵N1―H1…N3和O6…H4a―N4鍵長(zhǎng)有先縮短再增長(zhǎng)的變化趨勢(shì),有利于質(zhì)子轉(zhuǎn)移.

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