Model Studies of the (6−4) Photoproduct Photolyase Enzyme: Laser Flash Photolysis Experiments Confirm Radical Ion Intermediates in the Sensitized Repair of Thymine Oxetane Adducts

Photolytic splitting of homodimeric quinone-derived oxetanes studied by ultrafast transient absorption spectroscopy and quantum chemistry

The photoinduced cycloreversion of oxetane derivatives is of considerable biological interest since these compounds are involved in the photochemical formation and repair of the highly mutagenic pyrimidine (6-4) pyrimidone DNA photoproducts ((6-4)PPs). Previous reports have dealt with the photoreactivity of heterodimeric oxetanes composed mainly of benzophenone (BP) and thymine (Thy) or uracil (Ura) derivatives. However, these models are far from the non-isolable Thy〈º〉Thy dimers, which are the real precursors of (6-4)PPs. Thus, we have synthesized two chemically stable homodimeric oxetanes through the Paternò-Büchi reaction between two identical enone units, i.e. 1,4-benzoquinone (BQ) and 1,4-naphthoquinone (NQ), that led to formation of BQ-Ox and NQ-Ox, respectively. Their photoreactivity has been studied by means of steady-state photolysis and transient absorption spectroscopy from the femtosecond to the microsecond time scale. Thus, photolysis of BQ-Ox and NQ-Ox led to formation of the monomeric BQ or NQ, respectively, through ring opening in a "non-adiabatic" process. Accordingly, the transient absorption spectra of the triplet excited quinones (3BQ* and 3NQ*) were not observed as a result of direct photolysis of the quinone-derived oxetanes. In the case of NQ-Ox, a minor signal corresponding to 3NQ* was detected; its formation was ascribed to minor photodegradation of the oxetane during acquisitions of the spectra during the laser experiments. These results are supported by computational analyses based on density functional theory and multiconfigurational quantum chemistry (CASSCF/CASPT2); here, an accessible conical intersection between the ground and excited singlet states has been characterized as the main structure leading to deactivation of excited BQ-Ox or NQ-Ox. This behavior contrasts with those previously observed for heterodimeric thymine-derived oxetanes, where a certain degree of ring opening into the excited triplet state is observed.

Regioselectivity in the adiabatic photocleavage of DNA-based oxetanes

Direct absorption of UVB light by DNA may induce formation of cyclobutane pyrimidine dimers and pyrimidine-pyrimidone (6-4) photoproducts. The latter arise from the rearrangement of unstable oxetane intermediates, which have also been proposed to be the electron acceptor species in the photoenzymatic repair of this type of DNA damage. In the present work, direct photolysis of oxetanes composed of substituted uracil (Ura) or thymine (Thy) derivatives and benzophenone (BP) have been investigated by means of transient absorption spectroscopy from the femtosecond to the microsecond time-scales. The results showed that photoinduced oxetane cleavage takes place through an adiabatic process leading to the triplet excited BP and the ground state nucleobase. This process was markedly affected by the oxetane regiochemistry (head-to-head, HH, vs. head-to-tail, HT) and by the nucleobase substitution; it was nearly quantitative for all investigated HH-oxetanes while it became strongly influenced by the substitution at positions 1 and 5 for the HT-isomers. The obtained results clearly confirm the generality of the adiabatic photoinduced cleavage of BP/Ura or Thy oxetanes, as well as its dependence on the regiochemistry, supporting the involvement of triplet exciplexes. As a matter of fact, when formation of this species was favored by keeping together the Thy and BP units after splitting by means of a linear linker, a transient absorption at ∼400 nm, ascribed to the exciplex, was detected.

Influence of the Linking Bridge on the Photoreactivity of Benzophenone-Thymine Conjugates

Benzophenone (BP) is present in a variety of bioactive molecules. This chromophore is able to photosensitize DNA damage, where one of the most relevant BP/DNA interactions occurs with thymine (Thy). In view of the complex photoreactivity previously observed for dyads containing BP covalently linked to thymidine, the aim of this work is to investigate whether appropriate changes in the nature of the spacer could modulate the intramolecular BP/Thy photoreactivity, resulting in an enhanced selectivity. Accordingly, the photobehavior of a series of dyads derived from BP and Thy, separated by linear linkers of different length, has been investigated by steady-state photolysis, as well as femtosecond and nanosecond transient absorption spectroscopy. Irradiation of the dyads led to photoproducts arising from formal hydrogen abstraction or Paterno-Büchi (PB) photoreaction, with a chemoselectivity that was clearly dependent on the nature of the linking bridge; moreover, the PB process occurred with complete regio- and stereoselectivity. The overall photoreactivity increased with the length of the spacer and correlated well with the rate constants estimated from the BP triplet lifetimes. A reaction mechanism explaining these results is proposed, where the key features are the strain associated with the reactive conformations and the participation of triplet exciplexes.

Regiochemical memory in the adiabatic photolysis of thymine-derived oxetanes. A combined ultrafast spectroscopic and CASSCF/CASPT2 computational study

The photoinduced cycloreversion of oxetanes has been thoroughly investigated in connection with the photorepair of the well-known DNA (6-4) photoproducts. In the present work, the direct photolysis of the two regioisomers arising from the irradiation of benzophenone (BP) and 1,3-dimethylthymine (DMT), namely the head-to-head (HH-1) and head-to-tail (HT-1) oxetane adducts, has been investigated by combining ultrafast spectroscopy and theoretical multiconfigurational quantum chemistry analysis. Both the experimental and computational results agree with the involvement of an excited triplet exciplex 3[BPDMT]* for the photoinduced oxetane cleavage to generate 3BP* and DMT through an adiabatic photochemical reaction. The experimental signature of 3[BPDMT]* is the appearance of an absorption band at ca. 400 nm, detected by femtosecond transient absorption spectroscopy. Its formation is markedly regioselective, as it is more efficient and proceeds faster for HH-1 (∼2.8 ps) than for HT-1 (∼6.3 ps). This is in line with the theoretical analysis, which predicts an energy barrier to reach the triplet exciplex for HT-1, in contrast with a less hindered profile for HH-1. Finally, the more favorable adiabatic cycloreversion of HH-1 compared to that of HT-1 is explained by its lower probability to reach the intersystem crossing with the ground state, which would induce a radiationless deactivation process leading either to a starting adduct or to a dissociated BP and DMT.

Generation of the Thymine Triplet State by Through-Bond Energy Transfer

Benzophenone (BP) and drugs containing the BP chromophore, such as the non-steroidal anti-inflammatory drug ketoprofen, have been widely reported as DNA photosensitizers through triplet-triplet energy transfer (TTET). In the present work, a direct spectroscopic fingerprint for the formation of the thymine triplet (³ Thy*) by through-bond (TB) TTET from ³ BP* has been uncovered. This has been achieved in two new systems that have been designed and synthesized with one BP and one thymine (Thy) covalently linked to the two ends of the rigid skeleton of the natural bile acids cholic and lithocholic acid. The results shown here prove that it is possible to achieve triplet energy transfer to a Thy unit even when the photosensitizer is at a long (nonbonding) distance.

The Paternò–Büchi reaction – a comprehensive review

This review represents the most complete description of the scientific results obtained on a photochemical reaction described 110 years ago by an Italian scientist.

Photosensitized Thymine Dimerization via Delocalized Triplet Excited States

A new mechanism of photosensitized formation of thymine (Thy) dimers is proposed, which involves generation of a delocalized triplet excited state as the key step. This is supported by chemical evidence obtained by combining one benzophenone and two Thy units with different degrees of freedom, whereby the photoreactivity is switched from a clean Paternò-Büchi reaction to a fully chemo-, regio-, and stereoselective [2+2] cycloaddition.

A quantum chemical perspective on (6-4) photolesion repair by photolyases

(6-4)-Photolyases are fascinating enzymes which repair (6-4)-DNA photolesions utilizing light themselves. It is well known that upon initial photo-excitation of an antenna pigment an electron is transferred from an adjacent FADH(-) cofactor to the photolesion initiating repair, i.e. restoration of the original undamaged DNA bases. Concerning the molecular details of this amazing repair mechanism, the early steps of energy transfer and catalytic electron generation are well understood, the terminal repair mechanism, however, is still a matter of ongoing debate. In this perspective article, recent results of quantum chemical investigations are presented, and their meaning for the repair mechanism under natural conditions is outlined. Consequences of natural light conditions, temperature and thermal equilibration are highlighted when issues like the initial protonation state of the relevant histidines and the lesion, or the direction of electron transfer are discussed.

Hetero-cycloreversions mediated by photoinduced electron transfer

Discovered more than eight decades ago, the Diels-Alder (DA) cycloaddition (CA) remains one of the most versatile tools in synthetic organic chemistry. Hetero-DA processes are powerful methods for the synthesis of densely functionalized six-membered heterocycles, ubiquitous substructures found in natural products and bioactive compounds. These reactions frequently employ azadienes and oxadienes, but only a few groups have reported DA processes with thiadienes. The electron transfer (ET) version of the DA reaction, though less investigated, has emerged as a subject of increasing interest. In the last two decades, researchers have paid closer attention to radical ionic hetero-cycloreversions, mainly in connection with their possible involvement in the repair of pyrimidine(6-4)pyrimidone photolesions in DNA by photolyases. In biological systems, these reactions likely occur through a reductive photosensitization mechanism. In addition, photooxidation can lead to cycloreversion (CR) reactions, and researchers can exploit this strategy for DNA repair therapies. In this Account, we discuss electron-transfer (ET) mediated hetero-CR reactions. We focus on the oxidative and reductive ET splitting of oxetanes, azetidines, and thietanes. Photoinduced electron transfer facilitates the splitting of a variety of four-membered heterocycles. In this context, researchers have commonly examined oxetanes, both experimentally and theoretically. Although a few studies have reported the cycloreversion of azetidines and thietanes carried out under electron transfer conditions, the number of examples remains limited. In general, the cleavage of the ionized four-membered rings appears to occur via a nonconcerted two-step mechanism. The trapping of the intermediate 1,4-radical ions and transient absorption spectroscopy data support this hypothesis, and it explains the observed loss of stereochemistry in the products. In the initial step, either C-C or C-X bond breaking may occur, and the preferred route depends on the substitution pattern of the ring, the type of heteroatom, and various experimental conditions. To better accommodate spin and charge, C-X cleavage happens more frequently, especially in the radical anionic version of the reaction. The addition or withdrawal of a single electron provides a new complementary synthetic strategy to activate hetero-cycloreversions. Despite its potential, this strategy remains largely unexplored. However, it offers a useful method to achieve C═X/olefin metathesis or, upon ring expansion, to construct six-membered heterocyclic rings.

Oxetane synthesis through the Paternò-Büchi reaction

The Paternò-Büchi reaction is a photochemical reaction between a carbonyl compound and an alkene to give the corresponding oxetane. In this review the mechanism of the reaction is discussed. On this basis the described use in the reaction with electron rich alkenes (enolethers, enol esters, enol silyl ethers, enanines, heterocyclic compounds has been reported. The stereochemical behavior of the reaction is particularly stressed. We pointed out the reported applications of this reaction to the synthesis of naturally occuring compounds.

The Protonation States of the Active-Site Histidines in (6-4) Photolyase

The active sites of the (6-4) photolyases contain two conserved histidine residues, which, in the Drosophila melanogaster enzyme, correspond to His365 and His369. While there are nine combinations in which the three possible protonation states of the two histidines (with protons on Nδ (HID), Nε (HIE), or both Nδ and Nε (HIP)) can be paired, there is presently no consensus as to which of these states is present, let alone mechanistically relevant. EPR hyperfine couplings for selected protons of the FADH(•) radical have previously been used to address this issue. Our QM/MM calculations show, however, that the experimental couplings are equally well reproduced by each of the nine combinations. Since the EPR results seemingly cannot be used to unequivocally assign the protonation states, the pKa values of the two histidines were calculated using the popular PROPKA, H++, and APBS approaches, in various environments and for several lesions. These techniques consistently indicate that, at pH = 7, both His365 and His369 should be neutral, although His369 is found to be more prone to becoming protonated. In a comparative approach, a series of molecular dynamics simulations was performed with all nine combinations, employing various reference crystal structures and different oxidation states of the FAD cofactor. The overall result of this approach is in agreement with our pKa results. Consequently, although the introduction of the reduced cofactor results in an increased stability for selected protonated states, particularly the His365═HID and His369═HIP combination, the neutral combination His365═HID and His365═HIE stands out as the most relevant state for the activity of the enzyme.

Photosensitized splitting of thymine dimer or oxetane unit by a covalently N-linked carbazole via electron transfer in different marcus regions

Although many similarities exist between the two classes of enzymes, cyclobutane photolyases and (6-4) photolyases have certain important differences. The most significant difference is in their repair quantum yields, cyclobutane photolyases with a uniformly high efficiency (0.7-0.98) and very low repair efficiency for (6-4) photolyases (0.05-0.1). To understand the significant difference, we prepared two classes of model compounds, covalently N-linked dimer- (1) or oxetane-carbazole (2) compounds with a dimethylene or trimethylene group as a linker. Under light irradiation, the dimer or oxetane unit of model compounds can be sensitized to split by the excited carbazole via an intramolecular electron transfer. The splitting reaction of dimer or oxetane unit in model compounds is strongly solvent dependent. In nonpolar solvents, such as cyclohexane or THF, no fluorescence quenching of the carbazole moiety of model compounds relative to a free carbazole, N-methylcarbazole, was observed and thus no splitting occurred. In polar solvents, two classes of model compounds reveal two reverse solvent effects on the splitting quantum yield. One is an inverse relation between the quantum yield and the polarity of the solvent for dimer-model systems, and another is a normal relation for oxetane-model systems. This phenomenon was also observed with another two classes of model compounds, covalently linked dimer- or oxetane-indole. Based on Marcus theory and thermodynamic data, it has been rationalized that the two reverse solvent effects derive from back electron transfer in the splitting process lying in the different Marcus regions. Back electron transfer lies in the Marcus inverted region for dimer-model systems and the normal region for oxetane-model systems. From repair solvent behavior of the two classes of model compounds, we gained some insights into the major difference in the repair efficiency for the two classes of photolyases.

Theoretical study on the repair mechanism of the (6-4) photolesion by the (6-4) photolyase

UV irradiation of DNA can lead to the formation of mutagenic (6-4) pyrimidine-pyrimidone photolesions. The (6-4) photolyases are the enzymes responsible for the photoinduced repair of such lesions. On the basis of the recently published crystal structure of the (6-4) photolyase bound to DNA [Maul et al. 2008] and employing quantum mechanics/molecular mechanics techniques, a repair mechanism is proposed, which involves two photoexcitations. The flavin chromophore, initially being in its reduced anionic form, is photoexcited and donates an electron to the (6-4) form of the photolesion. The photolesion is then protonated by the neighboring histidine residue and forms a radical intermediate. The latter undergoes a series of energy stabilizing hydrogen-bonding rearrangements before the electron back transfer to the flavin semiquinone. The resulting structure corresponds to the oxetane intermediate, long thought to be formed upon DNA-enzyme binding. A second photoexcitation of the flavin promotes another electron transfer to the oxetane. Proton donation from the same histidine residue allows for the splitting of the four-membered ring, hence opening an efficient pathway to the final repaired form. The repair of the lesion by a single photoexcitation was shown not to be feasible.

Dynamics and mechanism of repair of ultraviolet-induced (6-4) photoproduct by photolyase

One of the detrimental effects of UV radiation on DNA is the formation of the (6-4) photoproduct (6-4PP) between two adjacent pyrimidines1. This lesion interferes with replication and transcription and may result in mutation and cell death2. In many organisms a flavoenzyme called photolyase uses blue light energy to repair the 6-4PP3. The molecular mechanism of the repair reaction is poorly understood. Here, we use ultrafast spectroscopy to show that the key step in the repair photocycle is a cyclic proton transfer between the enzyme and the substrate. By femtosecond synchronization of the enzymatic dynamics with the repair function, we followed the function evolution and observed direct electron transfer from the excited flavin cofactor to the 6-4PP in 225 ps but surprisingly fast back electron transfer in 50 ps without repair. Strikingly, we found that the catalytic proton transfer between a histidine residue in the active site and the 6-4PP, induced by the initial photoinduced electron transfer from the excited flavin cofactor to 6-4PP, occurs in 425 ps and leads to 6-4PP repair in tens of nanoseconds. These key dynamics define the repair photocycle and explain the underlying molecular mechanism of the enzyme’s modest efficiency.

Electronic structure of (6-4) DNA photoproduct repair involving a non-oxetane pathway

Mutagenic pyrimidine-pyrimidone (6-4) photoproducts are one of the main DNA lesions induced by solar UV radiation. These lesions can be photoreversed by (6-4) photolyases. The originally published repair mechanism involves rearrangement of the lesion into an oxetane intermediate upon binding to the (6-4) photolyase, followed by light-induced electron transfer from the reduced flavin cofactor. In a recent crystallographic study on a (6-4) photoproduct complexed with (6-4) photolyase from Drosophila melanogaster no oxetane was observed, raising the possibility of a non-oxetane repair mechanism. Using quantum-chemical calculations we find that in addition to repair via an oxetane, a direct transfer of the hydroxyl group results in reversal of the radical anion (6-4) photoproduct. In both mechanisms, the transition states have high energies and correspond to avoided crossings of the ground and excited electronic states. To study whether the repair can proceed via these state crossings, the excited-state potential energy curves were computed. The radical excitation energies and accessibility of the nonadiabatic repair path were found to depend on hydrogen bonds and the protonation state of the lesion. On the basis of the energy calculations, a nonadiabatic repair of the excited (6-4) lesion radical anion via hydroxyl transfer is probable. This repair mechanism is in line with the recent structural data on the (6-4) photolyase from D. melanogaster .

Evidence that the (6-4) photolyase mechanism can proceed through an oxetane intermediate

Using the analogue of TpT methylated at the 3'-end N3 position (Tpm3T), we demonstrate that when the oxetane/(6-4) pathway is precluded, water addition occurs at the 3'-end C6 position of the oxetane intermediate to provide its opening. Photoreversal of this (6-4) photoproduct C6 hydrate brings the first experimental evidence that the (6-4) photolyase repair can proceed through an oxetane intermediate.

A light-controlled reversible DNA photoligation via carbazole-tethered 5-carboxyvinyluracil

We describe a light-controlled template-directed reversible DNA photoligation via carbazole-tethered 5-carboxyvinyluracil. Carbazole-tethered 5-carboxyvinyl-2'-deoxyuridine-containing oligodeoxynucleotide (ODN) can be ligated by irradiation at 366 nm in the presence of template ODN, and the ligated ODN can be split by irradiation at 366 nm in the absence of template ODN.

Electron-Transfer Induced Repair of 6-4 Photoproducts in DNA: A Computational Study

The mechanism employed by DNA photolyase to repair 6-4 photoproducts in UV-damaged DNA is explored by means of quantum chemical calculations. Considering the repair of both oxetane and azetidine lesions, it is demonstrated that reduction as well as oxidation enables a reversion reaction by creating anionic or cationic radicals that readily fragment into monomeric pyrimidines. However, on the basis of calculated reaction energies indicating that electron transfer from the enzyme to the lesion is a much more favorable process than electron transfer in the opposite direction, it is suggested that the photoenzymic repair can only occur by way of an anionic mechanism. Furthermore, it is shown that reduction of the oxetane facilitates a mechanism involving cleavage of the C-O bond followed by cleavage of the C-C bond, whereas reductive fragmentation of the azetidine may proceed with either of the intermonomeric C-N and C-C bonds cleaved as the first step. From calculations on neutral azetidine radicals, a significant increase in the free-energy barrier for the initial fragmentation step upon protonation of the carbonylic oxygens is predicted. This effect can be attributed to protonation serving to stabilize reactant complexes more than transition structures.

Model Studies on a Carprofen Derivative as Dual Photosensitizer for Thymine Dimerization and (6–4) Photoproduct Repair

Cyclobutane pyrimidine dimers (CPD) and (6-4) photoproducts are among the main UV-induced DNA lesions. Both types of damage are mostly repaired in prokaryotes by photolyase enzymes. The repair mechanism of (6-4) photolyases has still not been fully elucidated, but it is assumed that back rearrangement to the oxetane occurs prior to repair. In this work, a non-steroidal anti-inflammatory drug derivative corresponding to the dechlorinated methyl ester of carprofen (namely methyl 2-(carbazol-2-yl)propanoate, PPMe) has been used to achieve the photosensitized cycloreversion of model oxetanes (formally resulting from photocycloaddition between benzophenone and 1,3-dimethylthymine or 2'-deoxyuridine), by employing fluorescence spectroscopy, laser flash photolysis, HPLC and NMR. Although PPMe is able to photoinduce the cycloreversion of both oxetanes, the fluorescence quenching of PPMe is faster for the 2'-deoxyribose-containing oxetane; this underlines the importance of the structure in such studies. Moreover, PPMe was shown to photoinduce the formation of thymidine cyclobutane dimers through a triplet-triplet energy transfer from a vibrationally excited state, as suggested by the enhanced PPMe triplet quenching by thymidine with increasing temperature. These results reveal a dual role of PPMe in DNA photosensitization, in that it photoinduces either damage or repair.

Electrochemical Approach to the Repair of Oxetanes Mimicking DNA (6−4) Photoproducts

Electrochemical study of oxetanes mimicking DNA (6-4) photoproducts gives new insight into the repair mechanism by (6-4) photolyase. Both electrochemical oxidation and electrochemical reduction at carbon electrodes lead to the cleavage of the oxetanes in a retro-Paterno-Büchi sequence. Within the family of compounds investigated and the range of driving forces offered, transient formation of unstable radical ions is observed, for both oxidative and reductive cleavage. Taking advantage of the electrochemical signature of these mimics, enzymatic assay with Escherichia coli CPD photolyase coupled to electrochemical monitoring of the reaction brings evidence that enzymatic repair of (6-4) DNA photoproducts does involve a catalytic dissociative electron-transfer mechanism at the level of an oxetane intermediate.

Investigation of the Pathways of Excess Electron Transfer in DNA with Flavin-Donor and Oxetane-Acceptor Modified DNA Hairpins

Oxetane is a potential intermediate that is enzymatically formed during the repair of (6-4) DNA lesions by special repair enzymes (6-4 DNA photolyases). These enzymes use a reduced and deprotonated flavin to cleave the oxetane by single electron donation. Herein we report synthesis of DNA hairpin model compounds containing a flavin as the hairpin head and two different oxetanes in the stem structure of the hairpin. The data show that the electron moves through the duplex even over distances of 17 A. Attempts to trap the moving electron with N2O showed no reduction of the cleavage efficiency showing that the electron moves through the duplex and not through solution. The electron transfer is sequence dependent. The efficiency is reduced by a factor of 2 in GC rich DNA hairpins.

Efficient photosensitized splitting of the thymine dimer/oxetane unit on its modifying beta-cyclodextrin by a binding electron donor

Two modified beta-cyclodextrins (beta-CDs) with a thymine dimer and a thymine oxetane adduct respectively, TD-CD and Ox-CD, have been prepared, and utilized to bind an electron-rich chromophore, indole or N,N-dimethylaniline (DMA), to form a supramolecular complex. We have examined the photosensitized splitting of the dimer/oxetane unit in TD-CD/Ox-CD by indole or DMA via an electron-transfer pathway, and observed high splitting efficiencies of the dimer/oxetane unit. On the basis of measurements of fluorescence spectra and splitting quantum yields, it is suggested that the splitting reaction occurs in a supramolecular complex by an inclusion interaction between the modified beta-CDs and DMA or indole. The back electron transfer, which leads low splitting efficiencies for the covalently-linked chromophore-dimer/oxetane compounds, is suppressed in the non-covalently-bound complex, and the mechanism has been discussed.

Intramolecular Interactions in the Triplet Excited States of Benzophenone–Thymine Dyads

Time-resolved and product studies on the synthesized dyads 1 and 2 have provided evidence that the benzophenone-to-thymine orientation strongly influences intramolecular photophysical and photochemical processes. The prevailing reaction mechanism has been established as a Paterno-Büchi cycloaddition to give oxetanes 3-6; however, the ability of benzophenone to achieve a formal hydrogen abstraction from the methyl group of thymidine has also been evidenced by the formation of photoproducts 7 and 8. These processes have been observed only in the case of the cisoid dyad 1. Adiabatic photochemical cycloreversion of the oxetane ring is achieved upon direct photolysis to give the starting dyad 1 in its excited triplet state. The photobiological implications of the above results are discussed with respect to benzophenone-photosensitized damage of thymidine.

An efficient carbonyl-alkene metathesis of bicyclic oxetanes: photoinduced electron transfer reduction of the Paternò–Büchi adducts from 2,3-dihydrofuran and aromatic aldehydes

The bicyclic oxetanes and resulting from photocycloaddition of aromatic aldehydes to 2,3-dihydrofuran, were efficiently cleaved by means of electron-transfer reduction, photoinduced by the electronically excited reductants 1-methoxynaphthalene (MN) and 2,7-dimethoxynaphthalene (DMN) in acetonitrile. The fluorescence quenching rates of DMN/MN by and were determined by static methods, the triplet quenching rates were determined by means of laser flash photolysis (LFP). The product analysis established a "photo-photo metathesis" where both cycloaddition and cycloreversion processes are induced by photochemical processes.

Model studies of the (6-4) photoproduct photoreactivation: efficient photosensitized splitting of thymine oxetane units by covalently linked tryptophan in high polarity solvents

Three covalently linked tryptophan-thymine oxetane compounds used as a model of the (6-4) photolyase-substrate complex have been prepared. Under 290 nm light, efficient splitting of the thymine oxetane with aromatic carbonyl compounds gives the thymine monomer and the corresponding carbonyl compounds by the covalently linked tryptophan via an intramolecular electron transfer, and exhibits a strong solvent dependence: the quantum yield (Phi) is ca. 0.1 in dioxane, and near 0.3 in water. Electron transfer from the excited tryptophan residue to the oxetane unit is the origin of fluorescence quenching of the tryptophan residue, and is more efficient in strong polar solvents. The splitting efficiency of the oxetane radical anion within the tryptophan.+-oxetane.- species is also solvent-dependent, ranging from ca. 0.2 in dioxane to near 0.35 in water. Thus, the back electron transfer reaction in the charge-separated species would be suppressed in water, but is still a main factor causing low splitting efficiencies in the tryptophan-oxetane systems. In contrast to the tryptophan-oxetane system, fast nonradiation processes are the main causes of low efficiency in the flavin-oxetane system. Hence, nonradiative processes of the excited FADH-, rather than electron transfer to oxetane, may be an important factor for the low repair efficiency of (6-4) photolyase.

Excited State Enantiodifferentiating Interactions between a Chiral Benzophenone Derivative and Nucleosides

Time-resolved measurements using nanosecond laser flash photolysis have revealed significant enantiodifferentiation in the interaction between ketoprofen (a chiral benzophenone derivative) and two relevant nucleosides, namely, thymidine and 2'-deoxyguanosine. In both cases, the highest quenching rate constants have been observed for (R)-ketoprofen, the enantiomer with lower pharmacological activity. Photoproduct studies performed in the case of thymidine suggest that the enantiodifferentiating process corresponds to a Paterno-Büchi reaction, leading to the formation of oxetanes. With 2'-deoxyguanosine, the quenching is associated with an electron-transfer process monitored through the generation of a ketyl radical.

Light-driven enzymatic catalysis of DNA repair: a review of recent biophysical studies on photolyase

More than 50 years ago, initial experiments on enzymatic photorepair of ultraviolet (UV)-damaged DNA were reported [Proc. Natl. Acad. Sci. U. S. A. 35 (1949) 73]. Soon after this discovery, it was recognized that one enzyme, photolyase, is able to repair UV-induced DNA lesions by effectively reversing their formation using blue light. The enzymatic process named DNA photoreactivation depends on a non-covalently bound cofactor, flavin adenine dinucleotide (FAD). Flavins are ubiquitous redox-active catalysts in one- and two-electron transfer reactions of numerous biological processes. However, in the case of photolyase, not only the ground-state redox properties of the FAD cofactor are exploited but also, and perhaps more importantly, its excited-state properties. In the catalytically active, fully reduced redox form, the FAD absorbs in the blue and near-UV ranges of visible light. Although there is no direct experimental evidence, it appears generally accepted that starting from the excited singlet state, the chromophore initiates a reductive cleavage of the two major DNA photodamages, cyclobutane pyrimidine dimers and (6-4) photoproducts, by short-distance electron transfer to the DNA lesion. Back electron transfer from the repaired DNA segment is believed to eventually restore the initial redox states of the cofactor and the DNA nucleobases, resulting in an overall reaction with net-zero exchanged electrons. Thus, the entire process represents a true catalytic cycle. Many biochemical and biophysical studies have been carried out to unravel the fundamentals of this unique mode of action. The work has culminated in the elucidation of the three-dimensional structure of the enzyme in 1995 that revealed remarkable details, such as the FAD-cofactor arrangement in an unusual U-shaped configuration. With the crystal structure of the enzyme at hand, research on photolyases did not come to an end but, for good reason, intensified: the geometrical structure of the enzyme alone is not sufficient to fully understand the enzyme's action on UV-damaged DNA. Much effort has therefore been invested to learn more about, for example, the geometry of the enzyme-substrate complex, and the mechanism and pathways of intra-enzyme and enzyme <-->DNA electron transfer. Many of the key results from biochemical and molecular biology characterizations of the enzyme or the enzyme-substrate complex have been summarized in a number of reviews. Complementary to these articles, this review focuses on recent biophysical studies of photoreactivation comprising work performed from the early 1990s until the present.

Origin of a Large Temperature Dependence of Regioselectivity Observed for [2 + 2] Photocycloaddition (Paternò-Büchi Reaction) of 1,3-Dimethylthymine with Benzophenone and Its Derivatives: Conformational Property of the Intermediary Triplet 1,4-Diradicals

[reaction: see text] The [2 + 2] photochemical additions of 1,3-dimethylthymine (DMT) with benzophenone and its 4,4'-substituted derivatives (BPs), difluoro, di-tert-butyl, and dimethoxy benzophenones, have been investigated at a temperature range from -40 to 70 degrees C. The photochemical reactions, which are cycloaddition of the 5-6 double bond of DMT with the carbonyl group of BPs, the so-called the Paternò-Büchi (PB) reaction, reveal largely temperature-dependent regioselectivity. The chemical yields of one series of regioisomers, 2, decrease with the increase of the reaction temperature, but those of another regioisomer series, 3, increase, and thus the ratio of 2/3 is strongly dependent on the temperature (2/3 = ca. 70:30 to 30:70). The temperature dependence of the regioselectivity yields two linear functions in the corresponding Eyring diagrams. The Eyring plot with changed slopes is clearly indicative of the change for the selectivity-determining step in the PB reaction, in which the triplet 1,4-diradicals play a crucial role. Computational studies reveal the conformational equilibrium structures of the triplet 1,4-diradicals, energy barriers between the conformers, and the conjectural equilibrium constants from relative potential energies of the stable conformers. A proposed mechanism can reasonably explain the temperature-dependent regioselectivity and chemical yields of two regioisomers varying with the reaction temperature.

Stereodifferentiation in the Photochemical Cycloreversion of Diastereomeric Methoxynaphthalene−Oxetane Dyads

[structure: see text] Intramolecular PET cycloreversion of oxetanes 1 and 2 has been achieved in acetonitrile and chloroform as solvents. Interestingly, a higher photoreactivity has been found in acetonitrile, while a significant stereodifferentiation has been found in chloroform. This stereodifferentiation can be attributed to the folded conformation which predominates in 2, with the naphthalene ring directed toward the oxetane region, allowing for the intramolecular electron transfer. Accordingly, intramolecular fluorescence quenching is also more efficient in acetonitrile, whereas stereodifferentiation is markedly higher in chloroform. Thus, a good correlation can be established between the results from steady-state irradiations and fluorescence measurements.

(6-4)-Photolyase activity requires a charge shift reaction

A model compound containing a thymine oxetane moiety linked to a flavin chromophore was investigated regarding (6-4)-photolyase activity. The need for a charge shift reaction was demonstrated by a detailed pH-dependent kinetic analysis.

Model compounds for (6–4) photolyases: a comparative flavin induced cleavage study of oxetanes and thietanes

Thietanes were used in the past as mimics for an unstable oxetane intermediate formed during the repair of mutagenic (6-4) lesions. The thietane derivatives were found to be not repaired, raising the question of how well thietanes are cleaved by single electron donation compared to oxetanes. We have prepared two flavin-containing oxetane and thietane model compounds for the (6-4) photolyase catalyzed repair process and we show that both are efficiently cleaved by a reduced and deprotonated flavin. Thietanes are therefore excellent models. The lack of their repair can be attributed to lack of binding.

Dissociative electron transfer to and from pyrimidine cyclobutane dimers: an electrochemical study

Cyclic voltammetry was used to study the reduction and oxidation behaviour of several pyrimidine cyclobutane dimers mimicking UV induced lesion in DNA strands in polar solvents (N,N-dimethylformamide and acetonitrile). Both electron injection and removal to and from the dimers, respectively, lead to their cleavage and reformation of the monomeric base. The influence of stereochemistry and substitution pattern at the cyclobutane motif on the reactivity has been studied. It appears that the repair process always proceeds in a sequential fashion with initial formation of a dimer ion radical intermediate, which then undergoes ring opening by homolytic cleavage of the two C-C bonds. Standard redox potentials for the formation of both radical anion and radical cation state of the dimers were determined. Quantum calculations on simplified model compounds reveal the reason for the finding that the exergonic homolytic cleavages of the carbon-carbon bonds are endowed with sizeable activation barriers. The consequences of these mechanistic studies on the natural enzymatic repair by photolyase enzyme are discussed.

Reductive PET Cycloreversion of Oxetanes: Singlet Multiplicity, Regioselectivity, and Detection of Olefin Radical Anion

Cycloreversion of 2-(p-cyanophenyl)-4-methyl-3-phenyloxetane (1) is achieved using 1-methoxynaphthalene (2) as electron-transfer photosensitizer. The experimental results are consistent with the reaction taking place from the singlet excited state of the sensitizer. Ring splitting of the radical anion 1*- occurs with cleavage of O-C2 and C3-C4 bonds, leading to products (acetaldehyde and p-cyanostilbene) different from the reagents used in the Paterno-Büchi synthesis of 1. The olefin radical anion involved in the electron-transfer process has been detected by means of laser flash photolysis.

Model studies of the (6-4) photoproduct photoreactivation: synthesis and photosensitized splitting of uracil oxetane adducts

Uracil oxetane adducts, which are model compounds for the oxetane intermediates generated during the formation of (6-4) photoproducts or in their photoenzymatic repair, have been synthesized using 1,3-dimethyluracil with carbonyl compounds. On the basis of fluorescence measurements and photolysis experiments, it is demonstrated that the oxetane adducts can be split into the nucleotide base and carbonyl compounds via an electron transfer reaction from photosensitizer. The reaction is more efficient for a stronger electron donor.