Protonated adenine: Tautomers,solvated clusters,and dissociation mechanisms

Ab initio and density functional theory calculations at the B3-MP2 and CCSD(T)/6-311 + G(3df,2p) levels of theory are reported that address the protonation of adenine in the gas phase, water clusters, and bulk aqueous solution. The calculations point to N-1-protonated adenine (1+) as the thermodynamically most stable cationic tautomer in the gas phase, water clusters, and bulk solution. This strongly indicates that electrospray ionization of adenine solutions produces tautomer 1+ with a specificity calculated as 97-90% in the 298-473 K temperature range. The mechanisms for elimination of hydrogen atoms and ammonia from 1+ have also been studied computationally. Ion 1+ is calculated to undergo fast migrations of protons among positions N-1, C-2, N-3, N-10, N-7, and C-8 that result in an exchange of five hydrogens before loss of a hydrogen atom forming adenine cation radical at 415 kJ mol(-1) dissociation threshold energy. The elimination of ammonia is found to be substantially endothermic requiring 376-380 kJ mol(-1) at the dissociation threshold and depending on the dissociation pathway. The overall dissociation is slowed by the involvement of ion-molecule complexes along the dissociation pathways. The competing isomerization of 1+ proceeds by a sequence of ring opening, internal rotations, imine flipping, ring closures, and proton migrations to effectively exchange the N-1 and N-10 atoms in 1+, so that either can be eliminated as ammonia. This mechanism explains the previous N-1/N-10 exchange upon collision-induced dissociation of protonated adenine.     

金属离子与腺嘌呤异构体复合物的稳定性

在B3LYP/6-311++G**//6-31+G*基组水平上结合PCM模型系统的优化了气相和液相环境中14种腺嘌呤异构体与四种金属离子(Na+,K+,Mg2+,Ca2+)形成的132个稳定复合物。通过能量对比,得到了所有复合物异构体在气液两相中的稳定性顺序及不同金属离子与同一异构体形成的复合物的能量排序,首次给出了液相中这些不同金属离子复合物的最稳定结构。结果发现溶剂效应导致了液相中的复合物稳定性顺序与气相中的相比发生了很大变化;同一异构体与不同金属离子形成的复合物其稳定性在排序中也变化很大。对于这些变化,本文分别从金属离子与腺嘌呤的复合物在气相中的结合能(EBE)及在液相中的溶质溶剂相互作用能(Epol)能等方面进行了系统的阐述     

Acid-base and metal-ion-binding properties of 9-[2-(2-phosphonoethoxy)ethyl]adenine (PEEA), a relative of the antiviral nucleotide analogue 9-[2-(phosphonomethoxy)ethyl]adenine (PMEA). An exercise on the quantification of isomeric complex equilibria in solution

The acidity constants of 3-fold protonated 9-[2-(2-phosphonoethoxy)ethyl]adenine, H3(PEEA)+, and of 2-fold protonated (2-phosphonoethoxy)ethane, H2(PEE), and the stability constants of the M(H;PEEA)+, M(PEEA), and M(PEE) complexes with M2+ = Mg2+, Ca2+, Sr2+, Ba2+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+, or Cd2+ have been determined (potentiometric pH titrations; aqueous solution; 25 degrees C; I = 0.1 M, NaNO3). It is concluded that in the M(H;PEEA)+ species, the proton is at the phosphonate group and the metal ion at the adenine residue. The application of previously determined straight-line plots of log K(M(R-PO3))M versus pK(H(R-PO3))H for simple phosph(on)ate ligands, R-PO3(2-), where R represents a residue that does not affect metal-ion binding, proves that the M(PEEA) complexes of Co2+, Ni2+, Cu2+, Zn2+, and Cd2+ as well as the M(PEE) complexes of Co2+, Cu2+, and Zn2+ have larger stabilities than is expected for a sole phosphonate coordination of M2+. For the M2+ complexes without an enhanced stability (e.g., Mg2+ or Mn2+), it is concluded that M2+ binds in a monodentate fashion to the phosphonate group of the two ligands. Combination of all of the results allows the following conclusions: (i) The increased stability of the Co(PEE), Cu(PEE), Zn(PEE), and Co(PEEA) complexes is due to the formation of six-membered chelates involving the ether-oxygen atom of the aliphatic residue (-CH2-O-CH2CH2-PO3(2-)) of the ligands with formation degrees of about 15-30%. (ii) Cd(PEEA) forms a macrochelate with N7 of the adenine residue (formation degree about 30%); Ni(PEEA) has similar properties. (iii) With Zn(PEEA), both mentioned types of chelates are observed, that is, Zn(PEEA)(cl/O) and Zn(PEEA)(cl/N7), with formation degrees of about 13 and 41%, respectively; the remaining 46% is due to the "open" isomer Zn(PEEA)(op) in which the metal ion binds only to the PO3(2-) group. (iv) Most remarkable is Cu(PEEA) because a fourth isomer, Cu(PEEA)(cl/O/N3), is formed that contains a six-membered ring involving the ether oxygen next to the phosphonate group and also a seven-membered ring involving N3 of the adenine residue with a very significant formation degree of about 50%. Hence, PEEA(2-) is a truly ambivalent ligand, its properties being strongly dependent on the kind of metal ion involved. Comparisons with M2+ complexes formed by the dianions of 9-[2-(phosphonomethoxy)ethyl]adenine (PMEA) and related ligands reveal that five-membered chelates involving an ether-oxygen atom are considerably more stable than the corresponding six-membered ones. This observation offers an explanation of why PMEA is a nucleotide analogue with excellent antiviral properties and PEEA is not.     

Engineering the structure and magnetic properties of crystalline solids via the metal-directed self-assembly of a versatile molecular building unit

We report the supramolecular chemistry of several metal complexes of N-(4-pyridyl)benzamide (NPBA) with the general formula [Ma(NPBA)2AbSc], where M = Co2+, Ni2+, Zn2+, Mn2+, Cu2+, Ag+; A = NO3-, OAc-; S = MeOH, H2O; a = 0, 1, 2; b = 0, 1, 2, 4; and c = 0, 2. NPBA contains structural features that can engage in three modes of intermolecular interactions: (1) metal-ligand coordination, (2) hydrogen bonding, and (3) pi-pi stacking. NPBA forms one-dimensional (1-D) chains governed by hydrogen bonding, but when reacted with metal ions, it generates a wide variety of supramolecular scaffolds that control the arrangement of periodic nanostructures and form 1- (2-4), 2- (5), or 3-D (6-10) solid-state networks of hydrogen bonding and pi-pi stacking interactions in the crystal. Isostructural 7-9 exhibit a 2-D hydrogen bonding network that promotes topotaxial growth of single crystals of their isostructural family and generates crystal composites with two (11) and three (12) different components. Furthermore, 7-9 can also form crystalline solid solutions (M,M')(NPBA)2(NO3)2(MeOH)2 (M, M' = Co2+, Ni2+, or Zn2+, 13-16), where mixtures of Co2+, Ni2+, and Zn2+ share the same crystal lattice in different proportions to allow the formation of materials with modulated magnetic moments. Finally, we report the effects that multidimensional noncovalent networks exert on the magnetic moments between 2 and 300 K of 1-D (4), 2-D (5), and 3-D (7, 8, 10, and 13-16) paramagnetic networks.     

Hydrogen abstraction from deoxyribose by a neighbouring uracil-5-yl radical

Hydrogen abstraction from the C1' and C2' positions of deoxyadenosine by a neighbouring uracil-5-yl radical in the 5'-AU*-3' DNA sequence is explored using DFT. This hydrogen abstraction is the first step in a sequence leading to single or double strand break in DNA. The uracil-5-yl radical can be the result of photolysis or low-energy electron (LEE) attachment. If the radical is produced by photolysis the neighbouring adenine will become a cation radical and if it is produced by LEE the adenine will remain neutral. The hydrogen abstraction reactions for both cases were investigated. It is concluded that it is possible for the uracil-5-yl to abstract hydrogen from C1' and C2'. When adenine is neutral there is a preference for the C1' site and when the adenine is a radical cation the C2' site is the preferred. If adenine is positively charged, the rate-limiting step when abstracting hydrogen from C1' is the formation of an intermediate crosslink between uracil and adenine. This crosslink might be avoided in dsDNA, making C1' the preferred site for abstraction.     

Adenine radicals in the gas phase: an experimental and computational study of hydrogen atom adducts to adenine

The elusive hydrogen atom adduct to the N-1 position in adenine, which is thought to be the initial intermediate of chemical damage, was specifically generated in the gas phase and characterized by neutralization-reionization mass spectrometry. The N-1 adduct, 1,2-dihydroaden-2-yl radical (1), was generated by femtosecond electron transfer to N-1-protonated adenine that was selectively produced by electrospray ionization of adenine in aqueous-methanol solution. Radical 1 is an intrinsically stable species in the gas phase that undergoes specific loss of the N-1-hydrogen atom to form adenine, but does not isomerize to the more stable C-2 adduct, 1,2-dihydroaden-1-yl radical (5). Radicals 1 that are formed in the fifth and higher electronically excited states of DeltaE > or = 2.5 eV can also undergo ring-cleavage dissociations resulting in expulsion of HCN. The relative stabilities, dissociation, and transition state energies for several hydrogen atom adducts to adenine have been established computationally at highly correlated levels of theory. Transition state theory calculations of 298 K rate constants in the gas phase, including quantum tunnel corrections, indicate the branching ratios for H-atom additions to C-8, C-2, N-3, N-1, and N-7 positions in adenine as 0.68, 0.20, 0.08, 0.03, and 0.01, respectively. The relative free energies of adenine radicals in aqueous solution point to the C-8 adduct as the most stable tautomer, which is predicted to be the predominating (>99.9%) product at thermal equilibrium in solution at 298 K.     

On the formation of radical dications of protonated amino acids in a "microsolution" of water or acetonitrile and their reactivity towards the solvent

In high-energy collisions (50 keV) between O2 and protonated amino acids AH+, radical dications AH2+* are formed for A = Phe, His, Met, Tyr, and Trp. When solvated by water or acetonitrile (S), AH2+*(S)1,2 are formed for A = Arg, His, Met, Tyr, and Trp. The stability of the hydrogen-deficient AH2+* in the "microsolution" depends on the energetics of the electron transfer reaction AH2+* +S --> AH++S+*, the hydrogen abstraction reaction AH2+*+S --> AH2(2+)+[S-H]*, and the proton transfer reaction AH2+* + S --> A+*+SH+. Using B3LYP/ 6-311+G(2d,p)//B3LYP/6-31+G(d) model chemistry, we describe these three reactions in detail for A=Tyr and find that the first two reactions are unfavorable whereas the third one is favorable. However, energy is required for the formation of Tyr+* and SH+ from TyrH2+*(S) to overcome the Coulomb barrier, which renders the complex observable with a life-time larger than 5 micros. The ionization energy, IE, of TyrH+ is calculated to be 11.1 eV in agreement with an experimental measurement of 10.1+/-2.1 eV ([IE(CH3CN)+IE(Tyr)]/ 2); hydration further lowers the IE by 0.3 eV [IE(TyrH+(H2O) = 10.8 eV, calculated]. We estimate the ionization energies of TrpH+, HisH+, and MetH+ to be 10.1+/-2.1 eV, 12.4+/-0.2 eV, and 12.4+/-0.2 eV, and that of PheH+ to be larger than 12.6 eV.     

取代基对腺嘌呤与胸腺嘧啶氢键复合物体系结合能的影响简

优化得到了17个取代胸腺嘧啶与腺嘌呤形成的氢键复合物的结构,并计算了这些复合物的结合能,探讨了胸腺嘧啶上不同取代基对结合能的影响. 结果表明,CF3取代的胸腺嘧啶与腺嘌呤间的结合能大于胸腺嘧啶与腺嘌呤间的结合能,这可能是屈氟尿苷具有阻止病毒及肿瘤扩散功能的原因所在. SO3H,CN和NO2取代的胸腺嘧啶与腺嘌呤间具有更大的结合能,表明这3个基团取代的胸腺嘧啶也可能具有潜在的抗肿瘤作用. 分子中原子理论与自然键轨道分析表明,在所有体系中,氢键N—H…N最强,N—H…O=C次之,C—H…O=C最弱,轨道作用在氢键作用中占有重要地位.     

Rebek imides and their adenine complexes: preferences for Hoogsteen binding in the solid state and in solution

Rebek imides (3), formed from Kemp's triacid, were developed in the mid-1980's as model receptors for adenine derivatives. We report here the first account of their hydrogen-bonding preferences upon binding 9-ethyladenine (1a) in the solid state. Structural analysis begins with simple imides 3a-e that form discrete dimers, while bis-imide 4 forms ribbon-like structures in the crystalline phase. The hydrogen-bonding interface within each of the representative assemblies features short intermolecular N(3)imide...O(8*)imide* distances (ca. 2.95 A), indicative of two-point hydrogen bonding. Imides 3f-h could be co-crystallized with 1a; single-crystal X-ray analysis of the resulting complexes reveals hydrogen-bonding geometries nearly identical to those observed in nucleobase complexes of adenine and pyrimidine derivatives. Imides 3f and 3g form 2:1 ternary assemblies with 1a; the complex of the former, (3f)2 x 1a, displays both Watson-Crick- and Hoogsteen-type hydrogen bonding, whereas the complex of the latter, (3g)2 x 1a, shows the Hoogsteen motif and imide hydrogen bonding to N(3) of the purine base (N(3)adenine...N(3')imide = 3.07(1) A). Imide 3h forms a 1:1 complex with 1a (3h x 1a x CHCl3) and displays Hoogsteen binding exclusively. All of the 3 x 1a assemblies show C(adenine)...O(imide) distances (3.38-3.75 A) that are consistent with C-H...O hydrogen bonding. Base-pairing preferences for the Rebek imides are further explored in solution by 1H NMR one-dimensional NOE experiments and by computational means; in all cases the Hoogsteen motif is modestly favored relative to its Watson-Crick counterpart.     

取代基对腺嘌呤与胸腺嘧啶氢键复合物体系结合能的影响

优化得到了17个取代胸腺嘧啶与腺嘌呤形成的氢键复合物的结构, 并计算了这些复合物的结合能, 探讨了胸腺嘧啶上不同取代基对结合能的影响. 结果表明, CF₃取代的胸腺嘧啶与腺嘌呤间的结合能大于胸腺嘧啶与腺嘌呤间的结合能, 这可能是屈氟尿苷具有阻止病毒及肿瘤扩散功能的原因所在. SO₃H, CN和NO₂取代的胸腺嘧啶与腺嘌呤间具有更大的结合能, 表明这3个基团取代的胸腺嘧啶也可能具有潜在的抗肿瘤作用. 分子中原子理论与自然键轨道分析表明, 在所有体系中, 氢键N—H…N最强, N—H…O=C次之, C—H…O=C最弱, 轨道作用在氢键作用中占有重要地位.     

Spectrofluorometric determination of hydrogen peroxide based on oxidative catalytic reactions of p-hydroxyphenyl derivatives with metal complexes of thiacalix[4]arenetetrasulfonate on a modified anion-exchanger.

The peroxidase-like catalytic activity of metal complexes of thiacalix[4]arenetetrasulfonate (TCAS[4]) on a modified anion-exchanger (Me(n+)-TCAS[4]A-500; Me(n+) = H2, Fe3+, Fe2+, Mn3+, Co3+, Co2+, Cu2+, Zn2+, Ni2+) for the oxidation of p-hydroxyphenyl derivatives to produce fluorescent substances in the presence of hydrogen peroxide has been investigated. Among the Me(n+)-TCAS[4]A-500 tested, Fe(3+)-TCAS[4]A-500 exhibited the highest level of catalytic activity for the oxidation of p-acetoamidophenol in a carbonate buffer solution of pH 10. The catalytic activity of Fe(3+)-TCAS[4]A-500 was then used for the spectrofluorometric determination of hydrogen peroxide. The calibration curve for the Fe(3+)-TCAS[4]A-500 method was linear over a range spanning from 0.1 to 5.0 microg of hydrogen peroxide in a 1.0 ml sample solution.     

锥形硼烷B5H10X的结构和成键性质

采用从头算分子轨道法对锥型硼烷Ｂ５Ｈ１０Ｘ（Ｘ＝Ｂｅ＾－,Ｂ,Ｃ＾＋,ＢｅＨ,ＣＨ＾２＋,Ｂ＾２－,Ｃ＾－,Ｎ,Ｏ＾＋,ＢＨ＾－,ＣＨ,ＮＨ＾＋和ＯＨ＾２＋）进行了研究,结果表明,端氢和桥氢原子与帽基原子相对位置的变化是由帽基原子和环原子轨道的弥散程度及环的尺寸效应共同决定的,端氢原子对桥氢原子的空间位置有着制约作用。     

Mechanistic Study on the Monofunctional Binding of Hydrolysis Products of Non-planar Heterocyclic Complexes Trans-[PtCl_2NH_3（piperidine）] and Trans-[PtCl_2（piperidine）_2] to DNA Purine Bases

The monofunctional substitution reactions between trans-[PtCl(H2O)(NH3)(pip)]+,trans-[Pt(H2O)2(NH3)(pip)]2+,trans-[PtCl(H2O)(pip)2]+,trans-[Pt(H2O)2(pip)2]2+ (pip = piperidine) and adenine/guanine nucleotides are explored by using B3LYP hybrid functional and IEF-PCM salvation models. For the trans-[Pt(H2O)2(NH3)(pip)]2+ and trans-[PtCl(H2O)(NH3)(pip)]+ complexes,the computed barrier heights in aqueous solution are 13.5/13.5 and 11.6/11.6 kcal/mol from trans-Pt-chloroaqua complex to trans/cis-monoadduct for adenine and guanine,and the corresponding values are 20.7/20.7 and 18.8/18.8 kcal/mol from trans-Pt-diaqua complex to trans/cis-monoadduct for adenine and guanine,respectively. For trans-[PtCl(H2O)(pip)2]+ and trans-[Pt(H2O)2(pip)2]2+,the corresponding values are 21.5/21.3 and 19.4/19.4 kcal/mol,and 26.0/26.0 and 20.7/20.8 kal/mol for adenine and guanine,respectively. Our calculations demonstrate that the barrier heights of chloroaqua are lower than the corresponding values of diaqua for adenine and guanine. In addition,the free energies of activation for guanine in aqueous solution are all smaller than that for adenine,which predicts a preference of 1.9 kcal/mol when trans-[PtCl(H2O)(NH3)(pip)]+ and trans-[Pt(H2O)2(NH3)(pip)]2+ are the active agents and ～1.9 and ～ 5.3 kcal/mol when trans-[PtCl(H2O)(pip)2]+ and trans-[Pt(H2O)2(pip)2]2+ are the active agents,respectively. For the reaction of trans-Pt-chloroaqua (or diaqua) to cis-monoadduct,we obtain the same transition-state structure as from the reaction of trans-Pt-chloroaqua (or diaqua) to trans-monoadduct,which seems that the trans-Pt-chloroaqua (or diaqua) complex can generate trans-or cis-monoadduct via the same transition-state.     

笼形聚氨肟树脂的研究：碱处理对吸附性能的影响

研究碱处理的笼形聚氨肟树脂（ＢＣＡＯ）对二价金属离子的吸附行为。吸附结果若生成２：１配的，得Ｍｇ＾２＋，Ｃａ＾２＋，Ｂａ＾２＋，Ｍｎ＾２＋，Ｃｏ＾２＋，Ｎｉ＾２＋，Ｚｎ＾２＋，Ｃｄ＾２＋，Ｐｂ＾２＋，Ｃｕ＾２＋离子的吸附效率分别是２７．１、２９．２、３０．４、２９．０、２７．１、３０．８、４５．０、３９．８、６０．３、６２．１％，Ｈｇ＾２＋离子的吸附效率高达１０５％，表明在吸附过程中同时生成２：１     

Switching binding sites: low-temperature NMR studies on adenosine-aspartic acid interactions

Low-temperature NMR experiments were performed on mixtures of adenine nucleosides and aspartic acid derivatives in a freonic solvent. By acquiring spectra at temperatures as low as 123 K, the regime of slow hydrogen bond exchange is reached and hydrogen-bonded complexes can be characterized in detail. With 2'-deoxyadenosine lacking a 2'-substituent, N-Boc-protected aspartic acid benzyl ester binds through its carboxylic acid side chain to the Watson-Crick site of the adenine base, forming a strong hydrogen bond with the proton located close to the center between the oxygen donor and adenine N1 nitrogen acceptor. However, in the case of 2'-O-silylated adenine ribofuranosides, noncovalent interactions of the 2'-substituent with protecting groups on the amino acid shift the binding mode toward a Hoogsteen geometry with only a moderately strong hydrogen bond involving adenine N7.     

含氮杂冠醚和核酸碱基双亲聚合物的合成及性能

设计合成了含氮杂冠醚和胸腺嘧啶的双亲聚合物聚[N,N-二乙氧基-1,10-二氮杂-18冠-6-5-甲基-胸腺嘧啶-异酞酸酯](PCTSE). 用SEM观测到其在水溶液中自发聚集成直径为150~220 nm的纳米球; 用动态光散射测得PCTSE纳米球水溶液的粒径分布主要集中在130~240 nm, 用FTIR研究了PCTSE/腺嘌呤中胸腺嘧啶与底物腺嘌呤的分子识别作用, 结果表明, 聚合物中胸腺嘧啶环上C₄=O伸缩振动峰从1670 cm^-1位移至1664 cm^-1, 表明胸腺嘧啶与腺嘌呤间形成了氢键. 变温红外光谱表明, 该峰又随温度的升高逐渐向高波数位移, 最后位移到识别前的1670 cm^-1处, 表明所形成的氢键断裂.     

Intrinsic acidity and redox properties of the adenine radical cation

The intrinsic chemical properties of the gaseous adenine radical cation were examined by using dual cell Fourier transform ion cyclotron resonance mass spectrometry. The adiabatic recombination energy of the radical cation (ionization energy of neutral adenine) was found by bracketing experiments to be 8.55 ± 0.1 eV (at 298 K; earlier literature values range from 8.3 to 8.9 eV). Based on this value, the heat of formation (ΔH_f²⁹⁸) of the adenine radical cation is estimated to be 246 ± 3 kcal/mol. The acidity (ΔH_acid²⁹⁸) of the adenine radical cation was bracketed to be 221 ± 2 kcal/mol. These thermochemical values suggest that the adenine radical cation reacts with neutral guanine by electron abstraction or proton transfer, with neutral cytosine by proton transfer, and via neither pathway with neutral thymine, molecular water or a sugar moiety of DNA (modeled by tetrahydrofuran). Experimental examination of the gas-phase reactivity of the adenine radical cation revealed a slow deuterium atom abstraction from perdeuterated tetrahydrofuran. Hence, in the absence of a nearby guanine or cytosine, the adenine radical cation may be able to abstract a hydrogen atom from a sugar moiety of DNA.     

腺嘌呤与富马酸共晶体的太赫兹光谱分析

利用太赫兹时域光谱(THz-TDS)技术在室温下对腺嘌呤、富马酸及两者的共晶体进行测量, 实验结果显示腺嘌呤与富马酸共晶体在0.92、1.24、1.52 THz处有明显的吸收峰, 与腺嘌呤和富马酸不同, 表明共晶体物相结构不同于原料. 根据腺嘌呤分子氢键供体与受体的结构特点, 使用密度泛函理论(DFT)对腺嘌呤与富马酸三种可能的共晶体结构进行模拟. 结果显示其中一种可能的共晶体结构在0.89、1.16、1.41 THz处存在特征吸收峰, 与实验结果较好吻合. 由此判断腺嘌呤与富马酸共晶体氢键形成位置为腺嘌呤的氨基与富马酸其中一个羧酸的碳氧双键形成氢键, 而此羧酸的羟基与腺嘌呤六元环上的邻位氮原子形成第二处氢键. 本文还结合理论模拟的结果对腺嘌呤与富马酸共晶体的特征吸收峰对应的相关振动模式进行了归属.     

Effect of hydrogen bonding on the structure and vibrational spectra of the complementary pairs of nucleic acid bases. II. adenine-thymine

The vibrational spectra of an isolated complementary adenine-thymine pair are calculated in the B3LYP/6-311++G(d,p) approximation and analyzed. The effect of hydrogen bonds on the structure is shown along with the position of frequencies and intensities of normal vibrations of the pair in comparison with the spectra of isolated thymine and adenine molecules. A comparative analysis of the hydrogen bonding effect on the IR and Raman spectra of thymine and adenine is performed     

Electron attachment to the DNA bases adenine and guanine and dehydrogenation of their anionic derivatives: Density functional study

Density functional theory (DFT) calculations have been used to explore electron attachment to the purines adenine and guanine and their hydrogen atom loss. Calculations show that the dehydrogenation at the N9 site in the adenine and guanine transient anions is the lowest‐cost channel of hydrogen loss, and the N9? H bond scission has Gibbs free energies of dissociation ΔG° of 8.8 kcal mol^?1 for the anionic adenine and 13.9 kcal mol^?1 for the anionic guanine. The relatively high feasibility of low‐energy electron (LEE)‐induced N9? H bond cleavage in the purine nucleobases arises from high electron affinities of their H‐deleted counterparts. Unlike adenine, other N? H bond dissociations are competitive with the N9? H bond fission in the anionic guanine. The replacement of hydrogen in the ring of purine has a significant effect on the N9? H bond fragmentation. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007