Sodio C. N. Hsu

Proton Transfer in Guanine-Cytosine Radical Anion Embedded in B-Form DNA

The electron-attachment-induced proton transfer in the guanine-cytosine (G:C) base pair is thought to be relevant to the issues of charge transport and radiation damage in DNA. However, our understanding on the reaction mainly comes from the data of isolated bases and base pairs, and the behavior of the reaction in the DNA duplex is not clear. In the present study, the proton-transfer reaction in reduced G:C stacks is investigated by quantum mechanical calculations with the aim to clarify how each environmental factor affects the proton transfer in G:C(*-). The calculations show that while the proton transfer in isolated G:C(*-) is exothermic with a small energetic barrier, it becomes endothermic with a considerably enhanced energetic barrier in G:C stacks. The substantial effect of G:C stacking is proved to originate from the electrostatic interactions between the dipole moments of outer G:C base pairs and the middle G:C(*-) base-pair radical anion; the extent of charge delocalization is very small and plays little role in affecting the proton transfer in G:C(*-). On the basis of the electrostatic model, the sequence dependence of the proton transfer in the ionized G:C base pair is predicted. In addition, the water molecules in the first hydration shell around G:C(*-) display a pronounced effect that facilitates the proton-transfer reaction; further consideration of bulk hydration only slightly lowers the energetic barrier and reaction energy. We also notice that the water arrangement around an embedded G:C(*-) is different from that around an isolated G:C(*-), which could result in a very different solvent effect on the energetics of the proton transfer. In contrast to the important influences of base stacking and hydration, the effects of sugar-phosphate backbone and counterions are found to be minor. Our calculations also reveal that a G:C base pair embedded in DNA is capable of accommodating two excess electrons only in bulk hydration; the resultant G(N1-H)(-):C(N3+H)(-) dianion is stable and exists long enough to lead to DNA damage. The combination of the present results with the previous findings in literature suggests that the behaviors of charge transport and low-energy electron-induced damage in DNA are highly susceptible to the hydration level.

Characterization of A New Copper(I)-Nitrito Complex That Evolves Nitric Oxide

The complexes [Cu(kappa(2)-Ph(2)PC(6)H(4)(o-OMe))(2)(CH(3)CN)](BF(4)) (1) and [CuCl(Ph(2)PC(6)H(4)(o-OMe))(2)] (2) have been prepared by treating [Cu(CH(3)CN)(4)](BF(4)) or CuCl with two equivalents o-(diphenylphosphino)anisole (Ph(2)PC(6)H(4)(o-OMe)) at room temperature, respectively. The reaction of 1 and (PPN)(NO(2)) in acetonitrile solution affords a neutral compound [Cu(Ph(2)PC(6)H(4)(o-OMe))(2)(ONO)] (3). In contrast to the synthesis of 3, mixing NaNO(2) and 1 in MeOH yielded a unique dicopper(I) cationic species, [((Ph(2)PC(6)H(4)(o-OMe))(2)Cu)(2)(mu-NO(2))](+) (4) after ether/CH(2)Cl(2) crystallization. The molecular structures of 1-4 have been determined by an X-ray diffraction study. The copper(I)-nitrito adduct 3 containing phosphine-ether ligands forms nitric oxide gas from the reaction with acetic acid, suggesting the first example and model compound in the asymmetric O-bound copper(I) nitrite intermediate microenvironment of copper nitrite reductases (Cu-NIRs).

Copper(I) nitro complex with an anionic [HB(3,5-Me2Pz)3]− ligand: a synthetic model for the copper nitrite reductase active site.

The new copper(I) nitro complex [(Ph(3)P)(2)N][Cu(HB(3,5-Me(2)Pz)(3))(NO(2))] (2), containing the anionic hydrotris(3,5-dimethylpyrazolyl)borate ligand, was synthesized, and its structural features were probed using X-ray crystallography. Complex 2 was found to cocrystallize with a water molecule, and X-ray crystallographic analysis showed that the resulting molecule had the structure [(Ph(3)P)(2)N][Cu(HB(3,5-Me(2)Pz)(3))(NO(2))]·H(2)O (3), containing a water hydrogen bonded to an oxygen of the nitrite moiety. This complex represents the first example in the solid state of an analogue of the nitrous acid intermediate (CuNO(2)H). A comparison of the nitrite reduction reactivity of the electron-rich ligand containing the CuNO(2) complex 2 with that of the known neutral ligand containing the CuNO(2) complex [Cu(HC(3,5-Me(2)Pz)(3))(NO(2))] (1) shows that reactivity is significantly influenced by the electron density around the copper and nitrite centers. The detailed mechanisms of nitrite reduction reactions of 1 and 2 with acetic acid were explored by using density functional theory calculations. Overall, the results of this effort show that synthetic models, based on neutral HC(3,5-Me(2)Pz)(3) and anionic [HB(3,5-Me(2)Pz)(3)](-) ligands, mimic the electronic influence of (His)(3) ligands in the environment of the type II copper center of copper nitrite reductases (Cu-NIRs).

Self-assembly and redox modulation of the cavity size of an unusual rectangular iron thiolate aryldiisocyanide metallocyclophane.

The decarbonylation reaction of ferric carbonyl dicationic [Cp(2)Fe(2)(μ-SEt)(2)(CO)(2)](BF(4))(2) [1(BF(4))(2)] carried out in refluxing acetonitrile affords a binuclear iron-sulfur core complex [Cp(2)Fe(2)(μ-SEt)(2)(CH(3)CN)(2)](BF(4))(2) [2(BF(4))(2)] containing two acetonitrile coordinated ligands. The treatment of 2(BF(4))(2) with 2 equiv of the 1,4-diisocyanobenzene (1,4-CNC(6)H(4)NC) results in the formation of the diisocyanide complex [Cp(2)Fe(2)(μ-SEt)(2)(1,4-CNC(6)H(4)NC)(2)](BF(4))(2) [3(BF(4))(2)]. The rectangular tetranuclear iron thiolate aryldiisocyanide metallocyclophane complex [Cp(4)Fe(4)(μ-SEt)(4)(μ-1,4-CNC(6)H(4)NC)(2)](BF(4))(4) [4(BF(4))(4)] has been synthesized by a self-assembly reaction between equimolar amounts of 2(BF(4))(2) and 1,4-diisocyanobenzene or by a stepwise route involving mixing of a 1:1 molar ratio of complexes 2(BF(4))(2) and 3(BF(4))(2). Chemical reduction of 4(BF(4))(4) by KC(8) was observed to produce the reduction product 4(BF(4))(2). The spectroscopic and electrochemical properties of the iron-sulfur core complexes 1(PF(6))(2), 3(BF(4))(2), 4(BF(4))(4), and 4(BF(4))(2) were determined. Finally, differences between the redox control cavities of rectangular tetranuclear iron thiolate aryldiisocyanide complexes are revealed by a comparison of the X-ray crystallographically determined structures of complexes 4(BF(4))(4) and 4(BF(4))(2).

SYNTHESES AND CHARACTERIZATION OF TRICARBONYL TUNGSTEN COMPLEXES CONTAINING 1,1'-BIS(DIPHENYLPHOSPHINO)FERROCENE LIGAND

Abstract Treatment of W(CO) 3 (NCCH 3 ) 3 with 1,1-bis(diphenylphosphino)ferrocene (dppf) affords W(CO) 3 ( NCCH 3 )(η 2 - dppf ) ( 1 ) in 85%) yield. Reaction of 1 with I 2 produces a seven-coordinated complex W(CO) 3 (I) 2 ( η 2 -dppf) ( 2 ), which is further oxidized by H 2 O 2 to give W(CO) 3 (I) 2 ( η 2 -dppf(O)) ( 3 ) in 80% yield. Compound 1 reacts with PMe 3 forming fac -W(CO) 3 ( η 2 -dppf)(PMe 3 ) ( 4 ) in 90% yield. However, reaction of 1 with PPh 2 Cl generates fac -W(CO) 3 ( η 2 -dppf)(PPh 2 H) ( 5 ) and fac -W(CO) 3 ( η 2 -dppf)(PPh 2 OH) ( 6 ) after separation of the reaction mixture by TLC. The structures of compounds 1 and 3 have been characterized by X-ray diffraction methods.

Theoretical study of the protonation of the one-electron-reduced guanine-cytosine base pair by water.

Prototropic equilibria in ionized DNA play an important role in charge transport and radiation damage of DNA and, therefore, continue to attract considerable attention. Although it is well-established that electron attachment will induce an interbase proton transfer from N1 of guanine (G) to N3 of cytosine (C), the question of whether the surrounding water in the major and minor grooves can protonate the one-electron-reduced G:C base pair still remains open. In this work, density functional theory (DFT) calculations were employed to investigate the energetics and mechanism for the protonation of the one-electron-reduced G:C base pair by water. Through the calculations of thermochemical cycles, the protonation free energies were estimated to be in the range of 11.6-14.2 kcal/mol. The calculations for the models of C(•-)(H(2)O)(8) and G(-H1)(-)(H(2)O)(16), which were used to simulate the detailed processes of protonation by water before and after the interbase proton transfer, respectively, revealed that the protonation proceeds through a concerted double proton transfer involving the water molecules in the first and second hydration shells. Comparing the present results with the rates of interbase proton transfer and charge transfer along DNA suggests that protonation on the C(•-) moiety is not competitive with interbase proton transfer, but the possibility of protonation on the G(-H1)(-) moiety after interbase proton transfer cannot be excluded. Electronic-excited-state calculations were also carried out by the time-dependent DFT approach. This information is valuable for experimental identification in the future.

Theoretical evidence of barrier-free proton transfer in 7-azaindole-water cluster anions.

Water clusters of 7-azaindole (7AI) and its radical anion with up to three water molecules have been investigated by B3LYP and MP2 methods. While the adiabatic electron affinities (AEAs) of 7AI(H(2)O)(n=0,1) and the most stable configuration of 7AI(H(2)O)(2) were calculated to be negative, the AEAs of 7AI(H(2)O)(3) were found to be positive, consistent with the experimental observation that the cluster anions of 7AI(-)(H(2)O)(n) start to appear continuously in mass spectra when n > or = 3. However, some high-energy configurations of 7AI(H(2)O)(2) were found to have potential for capturing excess electrons and forming stable anions. The B3LYP approach was shown to systematically overestimate the AEA due to its insufficient description of buckling of conjugated ring induced by electron attachment. The computational results show that the activation energy of proton transfer in 7AI(-)(H(2)O)(n) decreases as the number of water molecule increases. For n = 3, electron attachment was found to induce a barrier-free proton transfer from water to 7AI(-), resulting in the formation of a neutral radical of protonated 7AI solvated by a water cluster of hydroxyl anion, OH(-)(H(2)O)(2). The protonated structures were found to be lower in energy than the fully tautomerized structures where the tautomeric 7AI radical anion is solvated by a neutral water cluster. In addition, the tautomeric structures were found to be kinetically unstable with respect to the reverse transformation to the protonated structures. These results indicate that the protonated configuration of 7AI(-)(H(2)O)(3) is the major species detected in molecular beam experiments. This conclusion was further confirmed by the calculations of vertical detachment energies of cluster anions. The van der Waals structures of 7AI(-)(H(2)O)(3), in which the water molecules locate over the 7AI conjugated ring and point their O-H bonds toward the pi-electron cloud, were explored as well. Comparison of the protonation energies for DNA base anions and 7AI anion suggests that analogous proton-transfer reactions might occur in the water clusters of DNA base anions with only few water molecules.

Comparing L-lactide and ε-caprolactone polymerization by using aluminum complexes bearing ketiminate ligands: steric, electronic, and chelating effects

Our previous studies on the ring-opening polymerization of e-caprolactone using aluminum complexes bearing ketimine ligands as pre-catalysts with benzyl alcohol as an initiator clearly showed how the steric, electronic and chelating effects influenced the polymerization rate. Herein, the L-lactide polymerization rate of a series of Al complexes bearing ketimine ligands was also investigated, and the polymerization characteristics between L-lactide and e-caprolactone were compared. The kinetic results revealed complexes with more steric hindrance ligands that demonstrated greater propagation activity of the CL polymerization; however, an opposite trend was obtained in the L-lactide polymerization because of the larger size of L-lactide hindering its coordination with Al atoms in the crowded surroundings. The electron-withdrawing group of ligands, or less chelating ligands, demonstrated greater propagation activity both in L-lactide and e-caprolactone polymerization.

Synthesis, characterization, and catalytic activity of sodium ketminiate complexes toward the ring-opening polymerization of l -lactide

Studies of the ring-opening polymerization of L-lactide (LA) using Na complexes with Schiff base ligands as catalysts have revealed high catalytic activity but poor controllability of the polymer molecular weight. In this study, Na complexes bearing ketiminate ligands instead of Schiff base ligands were synthesized and their application in LA polymerization was tested. The polymerization results revealed that the catalytic activity of Na complexes bearing ketiminate ligands was higher than that of Na complexes bearing Schiff base ligands, but the poor controllability of polymer molecular weight was still a drawback. However, the poor controllability could be improved by means of using a high concentration of initiators in the polymerization system at 0 °C for 1 min. LPy–Na revealed the excellent controllability of polymer molecular weight depending on the ratio of [LA]/[initiator] (the molar averages of the number from 4500 to 35 000) with narrow polydispersity indexes, ranging from 1.29 to 1.35.

Improving the ring-opening polymerization of ε-caprolactone and l-lactide using stannous octanoate

A series of ligands were tested to meliorate the catalytic activity of ε-caprolactone (CL) and l-lactide (LA) polymerization by Sn(Oct)2 and BnOH. There are seven ligands (methyl 2-(dibenzylamino)acetate (MDBAA), di(1H-pyrazol-1-yl)methane, benzanide, 8-aminoquinoline, 2-bromo-1-phenylethanone, 2,6-di-tert-butyl-4-methylphenol, and dipyridine) were found to be useful in increasing the polymerization rate for CL polymerization. For LA polymerization, there are only two ligands (8-aminoquinoline and p-thiocresol) effectively raised the polymerization rate. Moreover, the kinetic study reveals a first-order dependency on [CL] and second-order dependency on [LA], indicating different polymerization mechanisms in Sn(Oct)2 catalytic system. The mechanistic study also explains that MDBAA could potentially modify the framework of the catalytic intermediate and produce the hydrogen, thereby increasing the rate of CL polymerization.Graphical Abstract