Through-space Electron Transfer Research Articles

Photoinduced Electron Transfer in Multicomponent Truxene-Quinoxaline Metal-Organic Frameworks.

Metal-organic frameworks (MOFs) can respond to light in a number of interesting ways. Photochromism is observed when a structural change to the framework is induced by the absorption of light, which results in a color change. In this work, we show that introducing quinoxaline ligands to MUF-7 and MUF-77 (MUF = Massey University Framework) produces photochromic MOFs that change color from yellow to red upon the absorption of 405 nm light. This photochromism is observed only when the quinoxaline units are incorporated into the framework and not for the standalone ligands in the solid state. Electron paramagnetic resonance (EPR) spectroscopy shows that organic radicals form upon irradiation of the MOFs. The EPR signal intensities and longevity depend on the precise structural details of the ligand and framework. The photogenerated radicals are stable for long periods in the dark but can be switched back to the diamagnetic state by exposure to visible light. Single-crystal X-ray diffraction analysis reveals bond length changes upon irradiation that are consistent with electron transfer. The multicomponent nature of these frameworks allows the photochromism to emerge by allowing through-space electron transfer, precisely positioning the framework building blocks, and tolerating functional group modifications to the ligands.

Controllable Electron Transfer in Mixed-Valence Bridged Norbornylogous Compounds: Ab Initio Calculation Combined with a Parametric Model and Through-Bond and Through-Space Interpretation.

In the context of a computationally guided approach to the controllable electron transfer in mixed-valence (MV) systems, in this article, we study the electron transfer (ET) in the series of oxidized norbornadiene C7H8 (I) and its polycyclic derivatives, C12H12 (II), C17H16, (III), C27H24 (IV), and C32H28 (V), with variable lengths of the bridge connecting redox sites. The work combines an ab initio CASSCF evaluation of the electronic structure of systems I-V with the parametric description in the framework of the biorbital two-mode vibronic model. The model involves coupling with the "breathing" mode and intercenter vibration modulating the distances between the redox fragments. The ab initio calculations were performed for two types of optimized structures of I-V: (a) charge-localized global minimum (Cs) and (b) symmetric configuration (C2v) with the delocalized charge. This allows one to estimate the potential barrier separating charge-localized configurations as well as vibronic coupling parameters and the electron transfer integral. Along with the adiabatic approach, the quantum-mechanical analysis of the vibronic levels has been applied to precisely estimate the quantum effect of tunneling splitting. We estimate the "through-space" and "through-bond" contributions to the parameters interrelated with the charge transfer (CT). The through-space effect proves to be a major factor of ET at a short distance between the redox centers, whereas the through-bond contribution is dominant at a long distance. Vibronic coupling under the condition of through-space ET leads to the localization of the positive charge on the π-chromophore, while the through-bond component of ET results in compensating σ-shifts and subsequent charge delocalization over the bridge. The limitations of the parametric approach were discussed in the context of the two components contributing to the ET. Particularly, the bridge polarization in the course of through-bond ET proves to be beyond the basis of the employed parametric model.

Mechanistic examination of Cα -Cβ tyrosyl bond cleavage: Spectroscopic investigation of the generation of α-glycyl radical cations from tyrosyl (glycyl/alanyl)tryptophan.

In this study, dissociative one-electron transfer dissociation of [CuII (dien)Y(G/A)W]•2+ [dien = diethylenetriamine; Y(G/A)W = tyrosyl (glycyl/alanyl)tryptophan] was used to generate the tripeptide radical cations [Y(G/A)W]•+ ; subsequent loss of the Tyr side chain formed [Gα • (G/A)W]+ . The π-centered species [YGWπ • ]+ generated the α-centered species [Gα • GW]+ through Cα -Cβ bond cleavage, as revealed using infrared multiple photon dissociation (IRMPD) measurements and density functional theory (DFT) calculations. Comparisons of experimental and theoretical IR spectra confirmed that both the charge and spin densities of [Y(G/A)Wπ • ]+ were delocalized initially at the tryptophan indolyl ring; subsequent formation of the final [Gα • (G/A)W]+ structure gave the highest spin density at the α-carbon atom of the N-terminal glycine residue, with a proton solvated by the first amide oxygen atom. The IRMPD mass spectra and action spectra of the [Gα • (G/A)W]+ species were all distinctly different from those of their isomeric [G(G/A)Wπ • ]+ species. The mechanism of formation of the captodative [Gα • (G/A)W]+ species-with the charge site separated from the radical site-from [Y(G/A)Wπ • ]+ has been elucidated. DFT calculations suggested that the Cα -Cβ bond cleavage of the tyrosine residue in the radical cationic [Y(G/A)Wπ • ]+ precursor involves (a) through-space electron transfer between the indolyl and phenolic groups; (b) formation of proton-bound dimers through Cα -Cβ cleavage of the tyrosine residue; and (c) a concerted proton rearrangement from the phenolic OH group to the carboxyl group and formation of the α-carbon-centered product [Gα • (G/A)W]+ through hydrogen bond cleavage. The barriers for the electron transfer (a), the Cα -Cβ cleavage (b), and the protonation rearrangement (c) were 12.8, 26.5, and 10.3 kcal mol-1 , respectively.

Control of the distance between porphyrin sensitizers and the TiO2 surface in solar cells by designed anchoring groups

Unsymmetrical porphyrins were rationally-designed and synthesized to investigate the relation between their structure, properties and adsorption geometries, and their relative performance as dyes in dye-sensitized solar cells. Photophysics, electrochemical and TiO2 anchoring properties of the new unsymmetrical N-glycolic acid amino phenyl porphyrins were evaluated. Most dyes showed good energy matching between excited state energies and the TiO2 conduction band. Depending on the porphyrins, anchoring to TiO2 occurred with only one carboxyl anchor group or with two N-glycolic acid amino phenyl connected to opposite and adjacent phenyl groups. It was found that cell efficiencies normalized for surface coverage are strongly affected by the adsorption geometry and spacer linker flexibility. The effective distance between the porphyrin core and the TiO2 surface has key importance in cell efficiencies. The data is consistent with a through-space electron transfer and anchoring viaN-glycolic acid substituents located in adjacent phenyl groups results in higher surface coverage normalized cell efficiencies.

Cα–Cβ bond cleavage occurs at the side chain of the N-terminal phenylalanine residue during CID of tyrosine-containing peptide radical cations

Molecular radical cationic peptides (M+) display rich and diverse gas-phase ion chemistry that can differ substantially from that of their protonated counterparts. Dissociation of M+ radical cations can provide relevant structural information for protein sequencing, often complementary to that obtained using traditional approaches. In this study, low-energy collision-induced dissociation (CID) of a series of tyrosine-containing peptide molecular radical cations having an N-terminal phenylalanine residue led to new products resulting from radical-mediated Cα–Cβ bond cleavage at the side chain of the phenylalanine residue. The chemical identity of the species formed through side chain loss (91 Da) occurring via N-terminal Cα–Cβ bond cleavage was examined using a combination of low-energy CID experiments, isotopic labeling mass spectrometric experiments, and density functional theory calculations. This unusual dissociation process occurs with fascinating unimolecular gas phase ion chemistry and requires a π-radical cationic tyrosine residue with significant spin density delocalized into the N-terminal amino group. The radical character at the resulting –NH2+ moiety then induces β-scission at the side chain Cα–Cβ bond of the phenylalanine residue. This discovery also demonstrates the role of the NH2 group in facilitating Cα–Cβ bond cleavage of the phenylalanine residue by the proximal tyrosine π-radical cation, probably involving a through-space electron transfer process.

Through-Space Intervalence Charge Transfer as a Mechanism for Charge Delocalization in Metal-Organic Frameworks.

Understanding the nature of charge transfer mechanisms in 3-dimensional metal-organic frameworks (MOFs) is an important goal owing to the possibility of harnessing this knowledge to design electroactive and conductive frameworks. These materials have been proposed as the basis for the next generation of technological devices for applications in energy storage and conversion, including electrochromic devices, electrocatalysts, and battery materials. After nearly two decades of intense research into MOFs, the mechanisms of charge transfer remain relatively poorly understood, and new strategies to achieve charge mobility remain elusive and challenging to experimentally explore, validate, and model. We now demonstrate that aromatic stacking interactions in Zn(II) frameworks containing cofacial thiazolo[5,4- d]thiazole (TzTz) units lead to a mixed-valence state upon electrochemical or chemical reduction. This through-space intervalence charge transfer (IVCT) phenomenon represents a new mechanism for charge transfer in MOFs. Computational modeling of the optical data combined with application of Marcus-Hush theory to the IVCT bands for the mixed-valence framework has enabled quantification of the degree of charge transfer using both in situ and ex situ electro- and spectro-electrochemical methods. A distance dependence for the through-space electron transfer has also been identified on the basis of experimental studies and computational calculations. This work provides a new window into electron transfer phenomena in 3-dimensional coordination space, of relevance to electroactive MOFs where new mechanisms for charge transfer are highly sought after, and to understanding biological light-harvesting systems where through-space mixed-valence interactions are operative.

Effects of Molecular-crowding on Electron Transfer and Oxidative Damage in Pyrene-modified Oligonucleotides

We examined guanine damages caused by photoinduced electron transfer (ET) in biomimetic environments using pyrene-modified oligonucleotides and molecular crowders. As a result, guanine damage induced by ET in full-matched DNA was decreased under viscous crowding conditions. On the other hand, behaviors of mismatched DNA revealed that through-space ET between pyrene moiety and the consecutive guanine site was promoted under molecular-crowding conditions.

Temperature-dependent ESR and computational studies on antiferromagnetic electron transfer in the yeast NADH dehydrogenase Ndi1.

Ndi1 is a special type-II complex I nicotinamide-adenine-dinucleotide (NADH):ubiquinone (UQ) oxidoreductase in the yeast respiratory chain, with two bound UQs (UQI and UQII) mediating electron transfer from flavin cofactors to ubiquinone, in the absence of Fe-S chains. Here, we elucidate the underlying mechanism of electron transfer in Ndi1 through temperature-dependent Electron Spin Resonance (ESR) experiments in conjunction with quantum chemical calculations. It is revealed that electron transfer is mediated by antiferromagnetic (AFM) interactions between flavin-adenosine-dinucleotide (FAD) and UQI and between UQI and UQII. The π-stacking interactions among the aromatic complexes also enhance the through-space electron transfer. The FAD/UQI pair works as a rectifier converting double-electron co-transfer into sequential single-electron transfer events. The results not only expand our understanding on the observed AFM interactions among p-orbital aromatic mixed-stack in proteins, but also provide significant insights into the fabrication of materials with special magnetic properties using biological samples.

Electronic Coupling Calculations for Bridge-Mediated Charge Transfer Using Constrained Density Functional Theory (CDFT) and Effective Hamiltonian Approaches at the Density Functional Theory (DFT) and Fragment-Orbital Density Functional Tight Binding (FODFTB) Level.

In this article, four methods to calculate charge transfer integrals in the context of bridge-mediated electron transfer are tested. These methods are based on density functional theory (DFT). We consider two perturbative Green's function effective Hamiltonian methods (first, at the DFT level of theory, using localized molecular orbitals; second, applying a tight-binding DFT approach, using fragment orbitals) and two constrained DFT implementations with either plane-wave or local basis sets. To assess the performance of the methods for through-bond (TB)-dominated or through-space (TS)-dominated transfer, different sets of molecules are considered. For through-bond electron transfer (ET), several molecules that were originally synthesized by Paddon-Row and co-workers for the deduction of electronic coupling values from photoemission and electron transmission spectroscopies, are analyzed. The tested methodologies prove to be successful in reproducing experimental data, the exponential distance decay constant and the superbridge effects arising from interference among ET pathways. For through-space ET, dedicated π-stacked systems with heterocyclopentadiene molecules were created and analyzed on the basis of electronic coupling dependence on donor-acceptor distance, structure of the bridge, and ET barrier height. The inexpensive fragment-orbital density functional tight binding (FODFTB) method gives similar results to constrained density functional theory (CDFT) and both reproduce the expected exponential decay of the coupling with donor-acceptor distances and the number of bridging units. These four approaches appear to give reliable results for both TB and TS ET and present a good alternative to expensive ab initio methodologies for large systems involving long-range charge transfers.

Synthesis and Photoinduced Electron-Transfer Reactions in a La2 @Ih -C80 -Phenoxazine Conjugate.

A newly designed electron donor-acceptor conjugate consisting of an endohedral dimetallofullerene (La2 @Ih -C80 ) and phenoxazine (POZ) was successfully synthesized under Prato conditions. Our results document that the 1,3-dipolar cycloaddition took place across the [5,6] junction to afford exclusively the corresponding [5,6] cycloadduct. The structure of the conjugate was characterized by means of NMR spectroscopy, absorption spectroscopy, and electrochemical studies. Computational calculations suggest that the electron density of the highest occupied molecular orbital (HOMO) is distributed on the POZ moiety, whereas that of the lowest unoccupied molecular orbital (LUMO) is located at the endohedral La atoms, leading to efficient separation of the HOMO and LUMO in the conjugate. Time-resolved absorption spectroscopic investigations and spectroelectrochemical measurements corroborate the formation of the energetically low-lying (La2 @Ih -C80 ).- -(POZ).+ radical-ion-pair state by means of ultrafast through-space electron transfer.

Insights into an efficient light-driven hybrid P450 BM3 enzyme from crystallographic, spectroscopic and biochemical studies

BackgroundIn order to perform selective CH functionalization upon visible light irradiation, Ru(II)-diimine functionalized P450 heme enzymes have been developed. The sL407C-1 enzyme containing the Ru(bpy)2PhenA (bpy=2,2′-bipyridine and PhenA=5-acetamido-1,10-phenanthroline) photosensitizer (1) covalently attached to the non-native single cysteine L407C of the P450BM3 heme domain mutant, displays high photocatalytic activity in the selective CH bond hydroxylation of several substrates. MethodsA combination of X-ray crystallography, site-directed mutagenesis, transient absorption measurements and enzymatic assays was used to gain insights into its photocatalytic activity and electron transfer pathway. ResultsThe crystal structure of the sL407C-1 enzyme was solved in the open and closed conformations revealing a through-space electron transfer pathway involving highly conserved, F393 and Q403, residues. Several mutations of these residues (F393A, F393W or Q403W) were introduced to probe their roles in the overall reaction. Transient absorption measurements confirm rapid electron transfer as heme reduction is observed in all four hybrid enzymes. Compared to the parent sL407C-1, photocatalytic activity was negligible in the dF393A-1 enzyme while 60% increase in activity with total turnover numbers of 420 and 90% product conversion was observed with the dQ403W-1 mutant. ConclusionsIn the sL407C-1 enzyme, the photosensitizer is ideally located to rapidly deliver electrons, using the naturally occurring electron transfer pathway, to the heme center in order to activate molecular dioxygen and sustain photocatalytic activity. General significanceThe results shed light on the design of efficient light-driven biocatalysts and the approach can be generalized to other members of the P450 superfamily.

Strong electronic coupling and electron transfer in a Ce2@Ih-C80–H2P electron donor acceptor conjugate

A newly designed electron donor-acceptor conjugate, namely Ce2@Ih-C80-H2P consisting of an endohedral dimetallofullerene Ce2@Ih-C80 and a free-base prophyrin (H2P), has been synthesized and systematically investigated. Basic characterization by means of NMR spectroscopy, steady-state absorption spectroscopy, and electrochemistry points to a folded configuration with sizeable interactions between Ce2@Ih-C80 and H2P. Complementary DFT optimization also results in the same conclusions. Time-resolved absorption spectroscopic investigations corroborate the formation of the (Ce2)˙(-)@Ih-C80-(H2P)˙(+) radical ion pair state in non-polar as well as polar media. Overall, the modus operandi is an ultrafast through-space electron transfer enabled by the folded configuration in the ground and excited state.

Role of the bridge in photoinduced electron transfer in porphyrin-fullerene dyads.

The role of π-conjugated molecular bridges in through-space and through-bond electron transfer is studied by comparing two porphyrin-fullerene donor-acceptor (D-A) dyads. One dyad, ZnP-Ph-C60 (ZnP = zinc porphyrin), incorporates a phenyl bridge between D and A and behaves very similarly to analogous dyads studied previously. The second dyad, ZnP-EDOTV-C60, introduces an additional 3,4-ethylenedioxythienylvinylene (EDOTV) unit into the conjugated bridge, which increases the distance between D and A, but, at the same time, provides increased electronic communication between them. Two essential outcomes that result from the introduction of the EDOTV unit in the bridge are as follows: 1) faster charge recombination, which indicates enhanced electronic coupling between the charge-separated and ground electronic states; and 2) the disappearance of the intramolecular exciplex, which mediates photoinduced charge separation in the ZnP-Ph-C60 dyad. The latter can be interpreted as a gradual decrease in electronic coupling between locally excited singlet states of D and A when introducing the EDOTV unit into the D-A bridge.

Synthesis and through-space charge transfer of dioctyloxy diperfluorohexyl substituted [2.2]paracyclophane-1,9-diene

Paracyclophanedienes containing dioctyloxy and diperfluorohexyl substituents have been synthesized in three steps from substituted dithia[3.3]paracyclophanes and the structures were fully characterized by nuclear magnetic resonance spectroscopy. A mixture of pseudo-geminal and pseudo-ortho diastereomers is produced and the pure compounds can be simply obtained by flash column chromatography using hexane as an eluent. The pseudo-geminal isomer exhibited more efficient through space charge transfer between electron donating and withdrawing substituents than the pseudo-ortho isomer. The absorption and photoluminescence spectra of both pseudo-geminal and pseudo-ortho diastereomers in a more polar solvent show the solvatochromism of the through-space electron transfer between dioctyloxy substituted phenyl and diperfluorohexyl substituted phenyl rings.

Influence of Supramolecular Interactions on Electron-Transfer Photochromism of the Crystalline Adducts of 4,4′-Bipyridine and Carboxylic Acids

We have studied the electron-transfer photochromism of the crystalline adducts of 4,4′-bipyridine (Bpy) and carboxylic acids and revealed the key structural parameters that decide whether the photochromism can happen for the first time. Experimental and theoretical analyses on nine known examples showed that the hydrogen bonds, instead of π–π stacking interactions, are the defining factor to the photochromism. Only the presence of N–H···O hydrogen bonds can fulfill the electron transfer from the carboxylate group to the Bpy part, although both the N···O separations of O–H···N and N–H···O hydrogen bonds are suitable for the so-called through-space electron transfer. These results can not only help to screen out the photochromic species from the known hundreds of Bpy–carboxylic acid adducts deposited in the Cambridge Crystallographic Data Centre (CCDC) database but also guide the design and syntheses of new adducts using diverse N-heterocyclic aromatic molecules and carboxylic acids.

Bond overlap population analysis on through-space electron transfer in [3n]cyclophanes (n = 2 – 5)

Through-space electron transfer between the benzene rings of [3n]cyclophanes, n=2 – 5, was investigated using density functional theory calculations, CAM-B3LYP/6-311++G**. An external point charge, q, was placed on the axis perpendicular to a benzene ring of the cyclophane molecule. The dependence of the highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) gap and bond overlap population (BOP) of cyclophanes on the values of q and n are discussed. The HOMO–LUMO gap generally decreases as q decreases and n increases. It is found from the BOP analysis that electron transfer occurs mainly through LUMO+2 for n=2, LUMO for n=3 and n=4, and LUMO+4 for n=5. The result that the values of the BOP for n=2 and 5 increase as q decreases means that the through-space electron transfer is enhanced by the negative charge on the neighboring molecule in the solid.

Biometal binding-site mimicry with modular, hetero-bifunctionally modified architecture encompassing a Trp/His motif: Insights into spatiotemporal noncovalent interactions from a comparative spectroscopic study

Metal-site Trp/His interactions are crucial to diverse metalloprotein functions. This paper presents a study using metal-motif mimicry to capture and dissect the static and transient components of physicochemical properties underlying the Trp/His aromatic side-chain noncovalent interactions across the first- and second-coordination spheres of biometal ions. Modular biomimetic constructs, EDTA-(L-Trp, L-His) or EWH and DTPA-(L-Trp, L-His) or DWH, featuring a function-significant Trp/His pair, enabled extracting the putative hydrophobic/hydrophilic aromatic interactions surrounding metal centers. Fluorescence, circular dichroism (CD) spectroscopic titrations and ESI mass spectrometry demonstrated that both the constructs stoichiometrically bind to Ca(2+), Co(2+), Cu(2+), Ni(2+), Mn(2+), Zn(2+), Cd(2+), and Fe(2+), and such binding was strongly coupled to stereospecific side-chain structure reorientations of the Trp indole and His imidazole rings. A mechanistic dichotomy corresponding to the participation of the indole unit in the binding event was revealed by a scaffold-platform correlation of steady-state fluorescence-response landscape, illuminating that secondary-coordination-sphere ligand cation-π interactions were immediately followed by subsequent transient physicochemical processes including through-space energy transfer, charge transfer and/or electron transfer, depending on the type of metals. The fluorescence quenching of Trp side chain by 3d metal ions can be ascribed to through-space d-π interactions. While the fluorescence titration was capable of illuminating a two-component energetic model, clean isosbestic/isodichroic points in the CD titration spectra indicated that the metallo-constructs, such as Cu(2+)-EWH complex, fold thermodynamically by means of a two-state equilibrium. Further, the metal-ion dependence of Trp conformational variation in the modular architecture of metal-bound scaffolds was evidenced unambiguously by the CD spectra and supported by MMFF calculations; both were capable of distinguishing between the coordination geometry and the preference for metal binding mode. The study thus helps understand how aromatic rings around metal-sites have unique capabilities through the control of the spatiotemporal distribution of noncovalent interaction elements to achieve diverse chemical functionality.

Probing the mechanism of electron capture and electron transfer dissociation using tags with variable electron affinity.

Electron capture dissociation (ECD) and electron transfer dissociation (ETD) of doubly protonated electron affinity (EA)-tuned peptides were studied to further illuminate the mechanism of these processes. The model peptide FQpSEEQQQTEDELQDK, containing a phosphoserine residue, was converted to EA-tuned peptides via beta-elimination and Michael addition of various thiol compounds. These include propanyl, benzyl, 4-cyanobenzyl, perfluorobenzyl, 3,5-dicyanobenzyl, 3-nitrobenzyl, and 3,5-dinitrobenzyl structural moieties, having a range of EA from -1.15 to +1.65 eV, excluding the propanyl group. Typical ECD or ETD backbone fragmentations are completely inhibited in peptides with substituent tags having EA over 1.00 eV, which are referred to as electron predators in this work. Nearly identical rates of electron capture by the dications substituted by the benzyl (EA = -1.15 eV) and 3-nitrobenzyl (EA = 1.00 eV) moieties are observed, which indicates the similarity of electron capture cross sections for the two derivatized peptides. This observation leads to the inference that electron capture kinetics are governed by the long-range electron-dication interaction and are not affected by side chain derivatives with positive EA. Once an electron is captured to high-n Rydberg states, however, through-space or through-bond electron transfer to the EA-tuning tags or low-n Rydberg states via potential curve crossing occurs in competition with transfer to the amide pi* orbital. The energetics of these processes are evaluated using time-dependent density functional theory with a series of reduced model systems. The intramolecular electron transfer process is modulated by structure-dependent hydrogen bonds and is heavily affected by the presence and type of electron-withdrawing groups in the EA-tuning tag. The anion radicals formed by electron predators have high proton affinities (approximately 1400 kJ/mol for the 3-nitrobenzyl anion radical) in comparison to other basic sites in the model peptide dication, facilitating exothermic proton transfer from one of the two sites of protonation. This interrupts the normal sequence of events in ECD or ETD, leading to backbone fragmentation by forming a stable radical intermediate. The implications which these results have for previously proposed ECD and ETD mechanisms are discussed.

Through-bond photoinduced electron transfer in a porphyrin-fullerene conjugate held by a Hamilton type hydrogen bonding motif

Control over the occurrence of through-bond electron transfer in self-assembled donor-acceptor conjugates is often difficult, since through-space electron transfer also competes due to the flexible nature of the spacer used to link the entities. In the present study, we have constructed a self-assembled donor-acceptor conjugate held solely by complementary hydrogen bonding and established through-bond electron transfer. The protocol used here is a Hamilton type hydrogen bonding motif involving self-assembly of a carboxylic acid functionalized porphyrin and 2-aminopyridine functionalized fullerene. Owing to the presence of two-point hydrogen bonds, the structure of the dyad is free from rotation with a donor-acceptor distance positioned appropriately to justify the through-bond electron transfer. Detailed spectral, computational and photochemical studies reveal efficient photoinduced charge separation and slow charge recombination in the studied conjugate, thus, bringing out the fundamental advantages of the directional hydrogen-bonding in the construction of donor-acceptor conjugates based on biomimetic principles and their functional role in governing electron transfer events.

Through-space and through-bond electron transfer within positively charged peptides in the gas phase

Processes by which an electron, initially attached to a protonated or fixed-charge side chain, can (i) undergo through-bond transfer to migrate to an S S σ* orbital and cleave a disulfide linkage, or (ii) undergo through-space transfer from that charged site to another positively charged side chain are examined using ab initio electronic structure methods. Earlier work from this group along these lines is extended in two directions: 1. The spacer units along which through-bond electron transfer occurs are extended to include olefinic as well as aliphatic units (because polypeptide backbones and side chains contain π as well as σ bonds). 2. The electron binding energies of positively charged side chains can vary substantially. In earlier work, protonated amine NH 3 + and fixed-charge N(CH 3) 3 + units were used as model systems. In this work, two positive units with more similar electron binding strengths are employed to see whether more facile electron transfer occurs when two such groups collide. The primary findings are 1. That through-bond electron transfer through combined olefinic–aliphatic linkages is not qualitatively different (in rate) than through aliphatic linkages. 2. That through-bond transfer can occur to S S bonds at experimentally relevant rates but only over ca. 7 intervening bonds. 3. That through-space transfer from one positive side chain to another can occur but only if the electron binding strengths of the two side chains’ positive sites are similar.