CHEMPHYSCHEM 2003, 4, 1341 ± 1344 DOI: 10.1002/cphc.200300885 ¹ 2003 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim 1341

Electron Attachment to ™Naked∫ andMicrosolvated Nucleotide Anions:Detection of Long-Lived Dianions

Bo Liu, Shigeo Tomita, Jimmy Rangama,Preben Hvelplund,* and Steen Br˘ndsted Nielsen*[a]

When UV light ionises water, hydrated electrons are produced.[1]

Radiation damage of DNA and RNA is thought to originate fromattachment of these electrons to nucleobases.[2] The formedradical anions participate in chemical reactions that lead toalterations in their original structure and to loss of geneticinformation. Hence, the interaction between electrons and DNAand RNA is a field of intense research with the goal ofunderstanding the biological damage by the ™free∫ electrons.DNA and RNA are composed of nucleotides which consist of anucleobase, a furanose sugar, and a phosphate group. Areductionist approach has been taken in the study of electronattachment to isolated nucleobases in vacuum to elucidate theelectronic structure of the anion and to show howWatson ±Crickbase-pairing is affected, if at all.[3±15] It is well-documented thatelectron affinities (EAs) are positive due to the existence ofdipole-bound states.[6, 7] The dipole moment of adenine justsuffices to support such a state whereas occupation of the LUMO(lowest unoccupied molecular orbital) to form a covalent anionis energetically unfavourable.[5, 6, 9] Here, we move one stepfurther and describe electron binding to nucleotides, the intactbuilding blocks, based on a combination of experiments andtheoretical calculations.

A nucleotide carries a negative charge located on thephosphate group. Direct attachment of low kinetic energyelectrons (�0 eV) to anions in vacuum is hindered by a largeCoulomb barrier and is only possible through tunnelling. Tocircumvent this hindrance we have collided nucleotide mono-anions which have high translational kinetic energy (50 keV) withgaseous sodium and looked for electron capture. The idea wasthat an electron jump might happen at close approach beyondthe Coulomb barrier, an electron might ™sneak in∫ when attachedto a sodium atom. In this way we try to mimic the actualsituation in solution phase where the Coulomb barrier is lessimportant.

The mass-analyzed ion kinetic energy (MIKE) spectrumobtained for the collision between the AMP anion (adenosine5�-monophosphate, m/z 346, Scheme 1) and sodium is shown inFigure 1B. A spectrum for collisions with neon is included forcomparison (Figure 1A). The geometrical cross section of neon issimilar to that of sodium but neon's much larger ionizationenergy prevents electron transfer. Peaks corresponding tofragment ions are seen in both spectra. Interestingly, however,a peak at half the m/z value of the parent ion appears when

Scheme 1. Numbering scheme for the AMP anion.

Figure 1. Spectra obtained after collisions between AMP anions and neon (A)and sodium (B) and between dAMP anions and sodium (C). The inset in (C) showsthe region around half the m/z of the dAMP anion for neon (blue curve) andsodium (red curve) as collision gases.

sodium is used as the collision gas but is absent when neon isused. In dissociation processes kinetic energy is released withthe result of broad peaks in the MIKE spectra. The peak width ofthe ion at half the m/z is narrower than that of other peaks,which implies that this ion is not a fragment ion but instead adianion. The accuracy of the calibration is not adequate todetermine whether the doubly charged ion is the AMP dianionor the AMP dianion minus a hydrogen atom, but B3LYP6± 311��G(2d,p)//PM3 calculations[16, 17] indicate that hydrogen atomloss requires 1.4 ± 3 eV. The abundance of the dianion is about3% of that of the total abundance of anionic product ions.Hence, in the collision the anion becomes vibrationally excitedand/or an electron is transferred from the sodium (donor) to theanion (acceptor). Electron detachment results in either neutral orcationic products. The translational energy of the anions isimportant for the efficiency of the electron-transfer process togenerate the dianion. In the 30± 50 keV region, the abundance ofthe dianion is the same but it decreases at higher energies. Thus,

[a] Prof. P. Hvelplund, Dr. S. B. Nielsen, B. Liu, Dr. S. Tomita, Dr. J. RangamaDepartment of Physics and Astronomy, University of Aarhus, Ny Munkegade,DK-8000 Aarhus C, (Denmark)Fax: (�45) 86-12-0740E-mail : hvelplun@phys.au.dk, sbn@phys.au.dk

1342 ¹ 2003 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim www.chemphyschem.org CHEMPHYSCHEM 2003, 4, 1341 ± 1344

at 100 keV the abundance is roughly two thirds of that at 50 keV.When the collision energy becomes too high, the interactiontime may become too short. Assuming the anion interacts with asodium atom over a distance of 5 ± 15 ä, the interaction time is3 ± 9 fs (time of passage, 50 KeV translational energy), which isshorter than the time of any vibrational period. The electrontransfer is therefore assumed to be vertical. The lifetime of thedianion is at least in the order of microseconds as the flight timefrom the collision cell to the detector is 6 �s.

In a similar experiment on the dAMP nucleotide anion(deoxyadenosine 5�-monophosphate, m/z 330) a peak wasobserved at half the m/z, again owing to the formation of adianion (Figure 1C), when sodium was the collision gas.

What is the electronic structure in the AMP and dAMPdianions? In the gaseous adenine anion the extra electron isdipole-bound by 12 meV,[6] but the dipole moment of adenine(2.6 Debye) is extremely close to the critical value for dipole-bound states.[18] Since the distance between the electron and theadenine molecule is 10 ä, the electron only slightly affects thestructure of adenine. Recent DFT calculations on isolatednucleobases indicate that Watson ±Crick base-pairing stabilisesa covalent anion structure[12±14] and, based on photoelectronspectroscopy experiments, Hendricks et al.[11] showed thathydration of the uracil base leads to a transformation from adipole-bound anion to a covalent anion. In the AMP dianion adipole-bound electron experiences the Coulomb repulsion ofthe negatively charged phosphate group, which raises theenergy by about 1 eV (14.4 eVä/R, R�10 ä being the distancebetween the two charges). Hence it is unlikely that a long-lived,purely dipole-bound state, which is repulsive, exists for the AMPdianion. Furthermore, because a dipole-bound electron is verysensitive to the molecular geometry which continuouslychanges because of atomic motion, autodetachment withinvibrational periods may occur.[19] Rotational motion of the dipolelimits the lifetime as well when the electron begins to slip behindfor high rotational quantum numbers.[19, 20] Most likely theelectron goes into the LUMO of the anion located on the adeninebase which is the lowest antibonding � orbital (Figure 2A).

Figure 2. A) The LUMO of the AMP anion. B) The SOMO of the AMP dianion witha molecular structure close to the monoanion. C) The SOMO of the AMP dianionin which the distance between the phosphate group and the adenine group ismaximised. D) The SOMO of the dianion formed after internal proton transferfrom C2�OH of the sugar to C2 of the adenine. E) The SOMO of the dianion formedafter internal proton transfer from C2�OH of the sugar to N3 of the adenine.

To elucidate the electronic structure of the covalent AMPdianion further, we performed theoretical calculations at theB3LYP6± 311��G(2d,p)//PM3 level of theory. We located aminimum of the dianion with a structure similar to that of themonoanion (Figure 2B) and found that this dianion is 2.3 eVhigher in energy than the monoanion. By starting out fromanother structure with maximum distance between the phos-phate group and the adenine group to reduce the Coulombrepulsion (Figure 2C), we located another dianion structure thatis 0.6 eV lower in energy than the other dianion, and hence theadiabatic EA of the monoanion is �1.7 eV. For comparison, wecalculated that adenine has an EA value of �0.4 eV (covalentanion), in good agreement with previous calculations.[5, 9] Thenegative charge of the phosphate group therefore makes theelectron more unbound by 1.3 eV. The SOMO (singly occupiedmolecular orbital) of the AMP dianion is similar to the LUMO ofthe monoanion though it has diffuse character at the NH2 group(Figure 2B, C). No antibonding character is introduced into theC�H or N�H bonds of the adenine.

Even though a dianion is unstable because of electronautodetachment to the continuum, the electron may be trappedbecause of the Coulomb barrier, a phenomenon previouslyobserved for multiply charged anions.[21a] A simple model oftenemployed is that of an electron in a square well potential at shortrange and a repulsive Coulomb potential at long range.[21b] Thelarge negative electron binding energy of the AMP dianion,however, requires a fast atomic rearrangement to occur in orderto stabilise the dianion against electron autodetachment.Otherwise attachment will only occur for monoanions in certaingeometries that support bound states, or for weakly unboundstates below the Coulomb barrier potential. In the following wediscuss the role of internal proton transfer (IPT) and internalhydrogen atom transfer (IHT).

First, what is the effect of the electron on the adenine base? Insolution, protonation on N3 or N7 of the adenine anion by anearby water molecule is likely to occur.[14] In the case of RNA,adenine protonation is serious as it may lead to loss of selectivityin base-pairing such that adenine can pair with both uracil andcytosine.[14] In the present experiment there is no water present.However, within the gaseous AMP dianion there are severalproton donors: the OH groups on the C2� and C3� atoms of thesugar ring, the OH group on the P atom, and the NH2 group ofadenine. Also, deprotonation of C1�H or C2�H to generatecarbanions are possibilities. We have calculated the energiesassociated with IPT and the electron-binding energies of thesedianions (Table 1). The dianion structure similar to the lowestenergy monoanion structure (Figure 2A) was chosen as startingpoint for the IPT reaction. We note that in some cases, the chargedensities indicate that a hydrogen atom is transferred and not aproton (Table 1). It is evident that the electron becomes lessunbound and for certain dianion geometries positively boundafter IPTor IHT. Proton transfer from C2�OH to either the C2 or N3atoms are the IPT reactions that require the least energy, 0.2 eVand 0.3 eV, respectively. The SOMO of the dianion formed byprotonation of the C2 atom is mainly located on the sugar unit(Figure 2D) whereas the SOMO of the dianion formed byprotonation of the N3 atom is mainly located on the adenine

CHEMPHYSCHEM 2003, 4, 1341 ± 1344 www.chemphyschem.org ¹ 2003 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim 1343

(Figure 2E). Both SOMOs have purely covalent character with nodiffuse character at the NH2 group. The investigated IHTreactions require significantly more energy than IPT (Table 1).

Another possibility is that attachment only occurs to mono-anion geometries where the proton is partly or fully transferredto the N3 atom, that is, at geometries where electronic bound orweakly unbound states exist. Since the AMP anion in thezwitterionic structure (sugar CO� ¥¥¥ �H�N3 adenine) is higher inenergy by 1.0 eV, only monoanions with enough internal energyhave a probability of exploring the part of the potential energysurface at which the geometry is close to that of the proton-transferred product. Electron attachment to a monoanion insuch a geometry leads to a dianion with low internal energyaccording to the Franck ±Condon principle. Hence, the ™cold∫dianion has a long lifetime since the equilibrium geometry is thatof the dianion and not the monoanion.

We note that the limitation in this theoretical study is the lowlevel used for geometry optimization (PM3 semiempiricalmethod) and future calculations at a higher level, such as DFT,are planned.

Finally, we generated AMP anions in small water clusters,AMP� ¥ (H2O)n (n�1 ±10), and collided them with sodium. Thespectrum obtained for AMP� ¥ (H2O)4 is shown in Figure 3. Themajor peaks are due to water loss from collisionally excited ions.However, AMP dianions with water attached are formed as well :AMP2� ¥ (H2O) and AMP2� ¥ (H2O)2 .

Figure 3. Spectrum obtained after collisions between AMP� ¥ (H2O)4 and sodium.

In conclusion, an isolated nucleotide (AMP and dAMP) with anextra electron was generated in collisions between nucleotideanions and sodium vapour and identified with a mass spec-trometer. This is the first time the initial product of electronattachment has been detected. The excess electron is necessarilyin a covalent orbital. The dianion is surprisingly long-lived(microseconds), which makes it a potential hazardous reagentfor chemical reactions. As a possible rationalisation for theobservation of the dianion we suggest that internal protontransfer from the sugar to the adenine anion occurs to stabilisethe dianion with respect to autodetachment. Furthermore, theelectron is trapped as crossing to the repulsive curve involvestunnelling through a Coulomb barrier. Further studies onmicrosolvated dianions are planned to investigate their fasci-nating chemistry.

Experimental Section

The experimental layout (Figure 4) has been described in detailelsewhere.[22] Briefly, the nucleotide anions were formed by electro-spray ionization, accelerated to an energy of 50 keV, mass-selected bya magnet and collided with sodium vapour in a collision cell at apressure of 4� 10�4 Torr (single-collision conditions). The productions were analyzed with a hemispherical electrostatic analyzer.

Figure 4. Diagram of the experimental layout.

Acknowledgements

This work was supported by the Danish Research Foundationthrough the Aarhus Center for Atomic Physics (ACAP). S.B.N.gratefully acknowledges a Steno grant from the Danish NaturalScience Research Council. We thank Profs. Jens Ulrik Andersen andJames S. Forster for many good discussions.

Keywords: DNA damage ¥ electron transfer ¥ massspectrometry ¥ molecular dianions ¥ nucleotides

Table 1. Calculational results at the B3LYP6 ± 311��G(2d,p)//PM3 level oftheory for internal proton transfer (IPT) or internal hydrogen transfer (IHT) inthe AMP dianion (see numbering in Scheme 1).

Proton/hydrogendonor

Proton/hydrogenacceptor

�E[a]

[eV]

EBE[b]

Adiabatic[eV]

EBE[b]

Vertical[eV]

Reaction

C1�H N3 1.6 � 1.0 �0.5 IPTC2�H C2 0.9 0.2 IHTC2�H N3 3.0 � 4.3 0.4 IHTC2�OH N3 0.3 � 1.6 0.2 IPTC2�OH C2 0.2 0.8 1.0 IPTC3�OH N3 0.5 � 0.4 0.3 IPTPOH N7 1.7 � 2.6 �1.0 IHTC6NH2 N7 0.8 � 2.2 �1.1 IHT

[a]�E : energy change of the proton-transfer or hydrogen-transfer reaction.[b] EBE: electron binding energy.

1344 ¹ 2003 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim CHEMPHYSCHEM 2003, 4, 1344 ± 1348

ATopological Analysis of the ElectronDensity in Anion ±� Interactions

Carolina Garau, Antonio Frontera,* David Quinƒonero,Pablo Ballester, Antoni Costa, and Pere M. Deya¡*[a]

The supramolecular chemistry of anions[1, 2] is a relativelyundeveloped field in comparison with that of cations, in partbecause concentrations of positive potential are less accessibleand manageable than concentrations of negative potential.[3]

Molecular recognition of anions is currently an expanding field ofresearch,[4] for instance, the design and synthesis of receptorscapable of binding anionic guests is of considerable interest inthe context of sensing and removal of environmental contam-inants such as nitrate anions[5] or the radioactive pertechnateproduced in the nuclear fuel cycle.[6] Synthetic receptors foranions are usually based on macrocyclic polyammonium/guanidinium cations,[7, 8] amides,[9] urea/thiourea,[10, 11] and func-tionalized calixarenes.[12, 13] The binding of anions to neutralreceptors is of special significance; first, to avoid the competingcounterion complexes present if cationic hosts are used, andsecond, the selectivity is highest in neutral receptors due to thedominance of directional interactions.[14] The neutral anionbinding receptors can be divided into two classes: receptorsthat bind anions by hydrogen bonding and receptors thatcoordinate anions at the Lewis acidic center of a neutralorganometallic ligand. The favorable interaction of anions withelectron-deficient aromatic rings can be applicable to the designof a new family of neutral anion receptors.

The cation ±� interaction is, in general, equally dominated byelectrostatic and cation-induced polarization.[15] The nature ofthe electrostatic component has been explained by puttingemphasis on the function of the permanent quadrupole mo-ment of benzene Qzz��8.48 B (Buckinghams, 1 B� 3.336�10�40 Cm2).[16] The benzene±hexafluorobenzene favorable inter-action has been studied, including the face-to-face stacking of itscrystal structure;[17, 18] this is explained through the permanentquadrupole moment of the two molecules, which are similar inmagnitude but of opposite sign Qzz(C6F6)��9.50 B[19] (seeFigure 1). The importance of quadrupole moment for under-

Figure 1. Representation of the quadrupole moment of benzene (left) andhexafluorobenzene (right).

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Bowen, J. Chem. Phys. 1996, 104, 7788.[8] K. Aflatooni, G. A. Gallup, P. D. Burrow, J. Phys. Chem. A 1998, 102, 6205.[9] S. S. Wesolowski, M. L. Leininger, P. N. Pentchev, H. F. Schaefer III, J. Am.

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the semiempirical PM3 method employing the Gaussian 98 programpackage.[17] The stationary points located are true minima on thepotential energy surfaces since real and positive frequency values wereobtained. The energies were calculated at a higher theoretical level,B3LYP6 ± 311��G(2d,p), and corrected for zero-point energy (ZPE).

[17] Gaussian 98, Revision A.9, , G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A.Robb, J. R. Cheeseman, V. G. Zakrzewski, J. A. Montgomery, R. E. Strat-mann, J. C. Burant, S. Dapprich, J. M. Millam, A. D. Daniels, K. N. Kudin,M. C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B.Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski, G. A. Petersson,P. Y. Ayala, Q. Cui, K. Morokuma, D. K. Malick, A. D. Rabuck, K. Raghava-chari, J. B. Foresman, J. Cioslowski, J. V. Ortiz, B. B. Stefanov, G. Liu, A.Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R. L. Martin, D. J. Fox, T.Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, C. Gonzalez, M.Challacombe, P. M. W. Gill, B. G. Johnson, W. Chen, M. W. Wong, J. L.Andres, M. Head-Gordon, E. S. Replogle, J. A. Pople, Gaussian, Inc. ,Pittsburgh, PA, 1998.

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Received: June 20, 2003 [Z885]

[a] Dr. A. Frontera, Prof. P. M. Deya¡, C. Garau, Dr. D. Quinƒonero, Prof. P. Ballester,Prof. A. CostaDepartament de QuÌmicaUniversitat de les Illes Balears07122 Palma de Mallorca (Spain)Fax: (�34) 971173426E-mail : tonif@soller.uib.es