Sequence-specific binding behavior of coralyne toward triplex DNA: An ultrafast time-resolved fluorescence spectroscopy study.

Triplex DNA structure has potential therapeutic application in inhibiting the expression of genes involved in cancer and other diseases. As a DNA-targeting antitumor and antibiotic drug, coralyne shows a remarkable binding propensity to triplex over canonical duplex and thus can modulate the stability of triplex structure, providing a prospective gene targeting strategy. Much less is known, however, about coralyne-binding interactions with triplex. By combining multiple steady-state spectroscopy with ultrafast fluorescence spectroscopy, we have investigated the binding behaviors of coralyne with typical triplexes. Upon binding with a G-containing triplex, the fluorescence of coralyne is markedly quenched owing to the photoinduced electron transfer (PET) of coralyne with the G base. Systematic studies show that the PET rates are sensitive to the binding configuration and local microenvironment, from which the coexisting binding modes of monomeric (full and partial) intercalation and aggregate stacking along the sugar-phosphate backbone are distinguished and their respective contributions are determined. It shows that coralyne has preferences for monomeric intercalation within CGG triplex and pure TAT triplex, whereas CGC+ triplex adopts mainly backbone binding of coralyne aggregates due to charge repulsion, revealing the sequence-specific binding selectivity. The triplex-DNA-induced aggregation of coralyne could be used as a probe for recognizing the water content in local DNA structures. The strong π-π stacking of intercalated coralyne monomer with base-triplets plays an important role in stabilizing the triplex structure. These results provide mechanistic insights for understanding the remarkable propensity of coralyne in selective binding to triplex DNA and shed light on the prospective applications of coralyne-triplex targeted anti-gene therapeutics.

Layered double hydroxides for phosphorus recovery from lipid-rich waste anaerobic fermentation liquor.

This study aimed to extract orthophosphate (ortho-P) from lipid-rich waste AF liquor (AFL) by Mg/Al layered double hydroxides (Mg/Al LDHs) adsorption, evaluate the influence of carbonate and investigate adsorption mechanisms. The carbonate influence experiment using synthetic P-rich wastewater indicated that low carbonate level was favorable for P extraction by LDHs. And then, real AFL rich in volatile fatty acids (VFAs), carbonate and ortho-P was applied as adsorbate to explore the Mg/Al LDHs adsorption performance. Experimental results indicated that 4 g/L Mg/Al LDHs could extract 88.3% of ortho-P from the AFL with low carbonate level (4829.83 mg CaCO3/L), and the adsorption quantity was 62.99 mg P/g LDHs, however, negligible VFAs were extracted. Kinetics and mechanisms analysis indicated that adsorption of P onto Mg/Al LDHs was a rapid physiochemical process, including ion exchange and surface adsorption. Finally, the nutrients release test confirmed the slow-release property of intercalated P.

Sulfur-centered hemi-bond radicals as active intermediates in S-DNA phosphorothioate oxidation.

Phosphorothioate (PS) modifications naturally appear in bacteria and archaea genome and are widely used as antisense strategy in gene therapy. But the chemical effects of PS introduction as a redox active site into DNA (S-DNA) is still poorly understood. Herein, we perform time-resolved spectroscopy to examine the underlying mechanisms and dynamics of the PS oxidation by potent radicals in free model, in dinucleotide, and in S-oligomer. The crucial sulphur-centered hemi-bonded intermediates -P-S∴S-P- were observed and found to play critical roles leading to the stable adducts of -P-S-S-P-, which are backbone DNA lesion products. Moreover, the oxidation of the PS moiety in dinucleotides d[GPSG], d[APSA], d[GPSA], d[APSG] and in S-oligomers was monitored in real-time, showing that PS oxidation can compete with adenine but not with guanine. Significantly, hole transfer process from A+• to PS and concomitant -P-S∴S-P- formation was observed, demonstrating the base-to-backbone hole transfer unique to S-DNA, which is different from the normally adopted backbone-to-base hole transfer in native DNA. These findings reveal the distinct backbone lesion pathway brought by the PS modification and also imply an alternative -P-S∴S-P-/-P-S-S-P- pathway accounting for the interesting protective role of PS as an oxidation sacrifice in bacterial genome.

One-electron oxidation of TAT-motif triplex DNA and the ensuing Hoogsteen hydrogen-bonding dissociation.

One-electron oxidation of adenine (A) leads initially to the formation of adenine radical cation (A•+). Subsequent deprotonation of A•+ can provoke deoxyribonucleic acid (DNA) damage, which further causes senescence, cancer formation, and even cell death. However, compared with considerable reports on A•+ reactions in free deoxyadenosine (dA) and duplex DNA, studies in non-B-form DNA that play critical biological roles are rare at present. It is thus of vital importance to explore non-B-form DNA, among which the triplex is an emerging topic. Herein, we investigate the deprotonation behavior of A•+ in the TAT triplex with continuous A bases by time-resolved laser flash photolysis. The rate constants for the one-oxidation of triplex 8.4 × 108 M-1 s-1 and A•+ deprotonation 1.3 × 107 s-1 are obtained. The kinetic isotope effect of A•+ deprotonation in the TAT triplex is 1.8, which is characteristic of a direct release of the proton into the solvent similar to free base dA. It is thus elucidated that the A•+ proton bound with the third strand is most likely to be released into the solvent because of the weaker Hoogsteen H-bonding interaction and the presence of the highly mobile hydration waters within the third strand. Additionally, it is confirmed through Fourier transform infrared spectroscopy that the deprotonation of A•+ results in the dissociation of the third strand and disruption of the secondary structure of the triplex. These results provide valuable kinetic data and in-depth mechanistic insights for understanding the adenine oxidative DNA damage in the triplex.

Degradation of Cytosine Radical Cations in 2'-Deoxycytidine and in i-Motif DNA: Hydrogen-Bonding Guided Pathways.

Radical cations of nucleobases are key intermediates causing genome mutation, among which cytosine C•+ is of growing importance because the ensuing cytosine oxidation causes GC → AT transversions in DNA replication. Although the chemistry and biology of steady-state C oxidation products have been characterized, time-resolved study of initial degradation pathways of C•+ is still at the preliminary stage. Herein, we choose i-motif, a unique C-quadruplex structure composed of hemiprotonated base pairs C(H)+:C, to examine C•+ degradation in a DNA surrounding without interference of G bases. Comprehensive time-resolved spectroscopy were performed to track C•+ dynamics in i-motif and in free base dC. The competing pathways of deprotonation (1.4 × 107 s-1), tautomerization (8.8 × 104 s-1), and hydration (5.3 × 103 s-1) are differentiated, and their rate constants are determined for the first time, underlining the strong reactivity of C•+. Distinct pathway is observed in i-motif compared with dC, showing the prominent features of C•+ hydration forming C(5OH)• and C(6OH)•. By further experiments of pH-dependence, comparison with single strand, and with Ag+ mediated i-motif, the mechanisms of C•+ degradation in i-motif are disclosed. The hydrogen-bonding within C(H)+:C plays a significant role in guiding the reaction flux, by blocking the tautomerization of C(-H)• and reversing the equilibrium from C(-H)• to C•+. The C radicals in i-motif thus retain more cation character, and are mainly subject to hydration leading to lesion products that can induce disruption of i-motif structure and affect its critical roles in gene-regulation.

Binding Interactions of Zinc Cationic Porphyrin with Duplex DNA: From B-DNA to Z-DNA.

Recognition of unusual left-handed Z-DNA by specific binding of small molecules is crucial for understanding biological functions in which this particular structure participates. Recent investigations indicate that zinc cationic porphyrin (ZnTMPyP4) is promising as a probe for recognizing Z-DNA due to its characteristic chiroptical properties upon binding with Z-DNA. However, binding mechanisms of the ZnTMPyP4/Z-DNA complex remain unclear. By employing time-resolved UV-visible absorption spectroscopy in conjunction with induced circular dichroism (ICD), UV-vis, and fluorescence measurements, we examined the binding interactions of ZnTMPyP4 towards B-DNA and Z-DNA. For the ZnTMPyP4/Z-DNA complex, two coexisting binding modes were identified as the electrostatic interaction between pyridyl groups and phosphate backbones, and the major groove binding by zinc(II) coordinating with the exposed guanine N7. The respective contribution of each mode is assessed, allowing a complete scenario of binding modes revealed for the ZnTMPyP4/Z-DNA. These interaction modes are quite different from those (intercalation and partial intercalation modes) for the ZnTMPyP4/B-DNA complex, thereby resulting in explicit differentiation between B-DNA and Z-DNA. Additionally, the binding interactions of planar TMPyP4 to DNA were also investigated as a comparison. It is shown that without available virtual orbitals to coordinate, TMPyP4 binds with Z-DNA solely in the intercalation mode, as with B-DNA, and the intercalation results in a structural transition from Z-DNA to B-ZNA. These results provide mechanistic insights for understanding ZnTMPyP4 as a probe of recognizing Z-DNA and afford a possible strategy for designing new porphyrin derivatives with available virtual orbitals for the discrimination of B-DNA and Z-DNA.

Impurity Sub-Band in Heavily Cu-Doped InAs Nanocrystal Quantum Dots Detected by Ultrafast Transient Absorption.

The effect of Cu impurities on the absorption cross section, the rate of hot exction thermalization, and on exciton recombination processes in InAs quantum dots was studied by femtosecond transient absorption. Our findings reveal dynamic spectral effects of an emergent impurity sub-band near the bottom of the conduction band. Previously hypothesized to explain static photophysical properties of this system, its presence is shown to shorten hot carrier relaxation. Partial redistribution of interband oscillator strength to sub-band levels reduces the band edge bleach per exciton progressively with the degree of doping, even though the total linear absorption cross section at the band edge remains unchanged. In contrast, no doping effects were detected on absorption cross sections high in the conduction band, as expected due to the relatively high density of sates of the undoped QDs.

Long-Term Outcomes of Simultaneous Skin and Bowel Flaps for Esophageal Reconstruction.

Esophageal reconstruction can be performed with skin or bowel flaps. The choice of flap remains controversial, as the long-term outcomes of skin flaps cannot always be assessed in patients with limited life expectancies due to advanced malignancy, unlike the pediatric and benign cases which have had esophageal reconstruction using bowel flaps. We report the long-term clinical and histopathological outcomes in a series of 45 cases repaired with combined skin and bowel flaps.Four patients developed symptomatic strictures after corrosive esophageal injuries were repaired with a combination of a tubed free radial forearm fasciocutaneous flap and a pedicled bowel flap. On average, 24 years had passed since uneventful initial esophageal reconstructions. Barium esophagograms were obtained in all cases and pathological examination was performed upon all surgical specimens.The cutaneous portions of the reconstructed esophagus exhibited a variety of findings on barium examination. Each of the 4 cases developed an esophagocutaneous fistula after revision; an average of 4 surgeries was required to close these fistulae. The inner surfaces of the portion of esophagus repaired with skin flaps showed extensive ulceration, polypoid lesions, and fibrosis. Pathology specimens from skin flaps showed extensive acute and chronic inflammation, microabscesses, fibrosis, and acanthosis, with depletion and degeneration of the pilosebaceous units. By contrast, adjacent parts of the esophagus repaired with bowel were widely patent with normal appearing mucosa.Our findings indicate that a bowel flap is durable with good tolerance to gastrointestinal content over long periods, whereas skin flaps often developed morphological changes and could not maintain long-term esophageal function without eventual stricture and dysphagia. We therefore recommend use of bowel flaps for esophageal reconstruction in patients with long life expectancy.

Dual Functionality of 6-Methylthioguanine: Synergistic Effects Enhancing the Photolability of DNA Nucleobases.

A small chemical modification of the nucleobase structure can significantly enhance the photoactivity of DNA, which may incur DNA damage, thus holding promising applications in photochemotherapy treatment of cancers or pathogens. However, single substitution confers only limited phototoxicity to DNA. Herein, we combine femtosecond and nanosecond time-resolved spectroscopy with high-level ab initio calculations to disentangle the excited-state dynamics of 6-methylthioguanine (me6-TG) under variable wavelength UVA excitation (310-330 nm). We find that double substitution of nucleobases (thionation and methylation) boosts the photoactivity by introducing more reactive channels. Intriguingly, 1nNπ*, rather than 1nSπ*, acts as the doorway state engendering the formation of the long-lived reactive triplet state in me6-TG. The 1nNπ* induces a low spin-orbit coupling of 8.3 cm-1, which increases the intersystem crossing (ISC) time (2.91 ± 0.14 ns). Despite the slowed ISC, the triplet quantum yield (ΦT) still accounts for a large fraction (0.6 ± 0.1), consistent with the potential energy surface that favors excited-state bifurcation to 1nNπ*min (3.36 ± 0.15 ps) rather than 1ππ*min (5.05 ± 0.26 ps), such that the subsequent ISC to triplet via 1nNπ*min constitutes the main relaxation pathway in me6-TG. Although this ΦT is inferior to its single-substituted predecessor 6-thioguanine (6-TG, 0.8 ± 0.2), the effect of thionation in synergy with methylation opens a unique C-S bond cleavage pathway through crossing to a repulsive 1πσ* state, generating thiyl radicals as highly reactive intermediates that may invoke biological damage. This photodissociation channel is extremely difficult for conventional nucleobases. These findings demonstrate the synergistic effects of double functionality substitution in modulating excited-state dynamics and enhancing the photolabile character of DNA nucleobases, providing inspirations for the rational design of advanced photodynamic and photochemotherapy approaches.

Fluorescence Quenching Dynamics of 2-Amino-7-methyl-1,8-naphthyridine in Abasic-Site-Containing DNA Duplexes for Nucleobase Recognition.

Dramatic fluorescence quenching of small heterocyclic ligands trapped in the abasic site (AP) of DNA has been implemented as an unprecedented strategy recognizing single-base mutations in sequence analysis of cancer genes. However, the key mechanisms governing selective nucleobase recognition remain to be disentangled. Herein, we perform fluorescence quenching dynamics studies for 2-amino-7-methyl-1,8-naphthyridine (AMND) in well-designed AP-containing DNA single/double strands. The primary mechanism is discovered, showing that AMND only targets cytosine to form a pseudo-base pair, and therefore, fluorescence quenching of AMND arises through the DNA-mediated electron transfer (ET) between excited state AMND* and flanking nucleobases, most favorably with flanking guanines. Subtle dynamic conformational variations induced by different flanking nucleobases are revealed and found to modulate efficiencies of electron transfer and fluorescence quenching. These findings provide critical mechanistic insights for guiding the design of photoinduced electron transfer (PET)-based fluorescent ligands as sensitive single-base recognition reporters.

Primary pulmonary epithelioid angiosarcoma presenting as a solitary pulmonary nodule on image.

Primary angiosarcoma of lung is a rare condition. Only about 20 cases have appeared in English published reports so far. Its rarity and consequent low index of suspicion makes clinical diagnosis difficult. Pathological diagnosis of the epithelioid variant of pulmonary angiosarcoma is particularly challenging. We report a case of primary pulmonary epithelioid angiosarcoma as a solitary pulmonary nodule in image study in a 41-year-old man with a brief review, to contribute it to the sparse literature on this disease.

Ultrafast formation of the benzoic acid triplet upon ultraviolet photolysis and its sequential photodissociation in solution.

Time-resolved infrared (TR-IR) absorption spectroscopy in both the femtosecond and nanosecond time domain has been applied to examine the photolysis of benzoic acid in acetonitrile solution following either 267 nm or 193 nm excitation. By combining the ultrafast and nanosecond TR-IR measurements, both the excited states and the photofragments have been detected and key mechanistic insights were obtained. We show that the solvent interaction modifies the excited state relaxation pathways and thus the population dynamics, leading to different photolysis behavior in solution from that observed in the gas phase. Vibrational energy transfer to solvents dissipates excitation energy efficiently, suppressing the photodissociation and depopulating the excited S(2) or S(3) state molecules to the lowest T(1) state with a rate of ∼2.5 ps after a delayed onset of ∼3.7 ps. Photolysis of benzoic acid using 267 nm excitation is dominated by the formation of the T(1) excited state and no photofragments could be detected. The results from TR-IR experiments using higher energy of 193 nm indicate that photodissociation proceeds more rapidly than the vibrational energy transfer to solvents and C-C bond fission becomes the dominant relaxation pathway in these experiments as featured by the prominent observation of the COOH photofragments and negligible yield of the T(1) excited state. The measured ultrafast formation of T(1) excited state supports the existence of the surface intersections of S(2)/S(1), S(2)/T(2), and S(1)/T(1)/T(2), and the large T(1) quantum yield of ∼0.65 indicates the importance of the excited state depopulation to triplet manifold as the key factor affecting the photophysical and photochemical behavior of the monomeric benzoic acid.

Free-Radical-Mediated Photoinduced Electron Transfer between 6-Thioguanine and Tryptophan Leading to DNA-Protein-Like Cross-Link.

The nucleobase analog 6-thioguanine (6-TG) has emerged as important immunosuppressant, anti-inflammatory, and anticancer drug in the past few decades, but its unique photosensitivity of absorbing strongly ultraviolet UVA light elicits photochemical hazards in many ways. The particularly intriguing yet unresolved question is whether the direct photoreaction of 6-TG can promote DNA-protein cross-links (DPCs) formation, which are large DNA adducts blocking DNA replication and physically impede DNA-related processes. Herein, by real-time observation of radical intermediates using time-resolved UV-vis absorption spectroscopy in conjunction with product analysis by HPLC-MS, we discover that UVA excitation of 6-TG triggers direct covalent cross-linking with tryptophan (TrpH) via an exquisite radical mechanism of electron transfer. The photoexcitation prepares the redox-active triplet 36-TG*, which initiates electron transfer with TrpH, creating TrpH•+ and 6-TG•- in the first step. The deprotonated Trp• undergoes radical-recombination with its geminate partner 6-TG•- and eliminates a H2S, leading to the cross-linking product 6-TG-Trp. The photoadduct structures (two chiral isomers and one constitutional isomer) are identified unambiguously, validating further the mechanism. These findings pinpoint the exact amino acid that is vulnerable to photo-cross-linking with 6-TG and establish a mechanistic framework for understanding mutagenic DPCs formation and developing photoprobes based on this new type of photo-cross-linking.

Competitive reaction pathways of C2Cl3 + NO via four-membered ring and bicyclic ring intermediates.

The products and mechanisms of the atmospherically and environmentally important reaction, C(2)Cl(3) + NO, are investigated comprehensively by step-scan time-resolved Fourier transform infrared emission spectroscopy and the CCSD(T)/6-311+G(d)//B3LYP/6-311G(d) level of electronic structure calculations. Vibrationally excited products of Cl(2)CO, ClNCO, CCl(3)NCO and NCO have been observed in the IR emission spectra. Cyclic intermediates are found to play important roles leading to the rich variety of the chemical transformations of the reaction. Mainly two competitive reaction pathways are revealed: the four-membered ring intermediate pathway leading to the products Cl(2)CO + ClCN which is essentially barrierless and the bicyclic ring intermediate pathway leading to the product channels of ClNCO + CCl(2,) CCl(3)NCO and CCl(3) + NCO which is rate-limited by a barrier of 42.9 kJ mol(-1) higher than the reactants. By photolyzing the precursor at 248 and 193 nm, respectively, C(2)Cl(3) radicals with different internal energy are produced to observe the product branching ratios as a function of reactant energy. The Cl(2)CO channel via the four-membered ring intermediate pathway is shown to be overwhelmingly dominant at low energy (temperature) but become less important at high energy while the ClNCO and CCl(3)NCO channels via the bicyclic ring intermediate pathway are greatly enhanced and compete effectively. The experimental observation of the products and their branching ratios varying with reactant energy is well consistent with the calculated potential energy profiles.

[2+2] Photocycloaddition reaction dynamics of triplet pyrimidines.

Taking the 266 nm excited pyrimidine (uracil or thymine) with cyclopentene as model reaction systems, we have examined the photoproduct formation dynamics from the [2 + 2] photocycloaddition reactions of triplet pyrimidines in solution and provided mechanistic insights into this important DNA photodamage reaction. By combining two compliment methods of nanosecond time-resolved transient IR and UV-vis laser flash-photolysis spectroscopy, the photoproduct formation dynamics as well as the triplet quenching kinetics are measured. Characteristic IR absorption bands due to photoproduct formation have been observed and product quantum yields are determined to be ∼0.91% for uracil and ∼0.41% for thymine. Compared to the measured large quenching rate constants of triplet uracil (1.5 × 10(9) M(-1)s(-1)) or thymine (0.6 × 10(9) M(-1)s(-1)) by cyclopentene, the inefficiency in formation of photoproducts indicates competitive physical quenching processes may exist on the route leading to photoproducts, resulting in very small product yields eventually. Such an energy wasting process is found to be resulted from T(1)/S(0) surface crossings by the hybrid density functional calculations, which compliments the experiments and reveals the reaction mechanism.

Ultrafast excited state dynamics and light-switching of [Ru(phen)2(dppz)]2+ in G-quadruplex DNA.

The triplet metal to ligand charge transfer (3MLCT) luminescence of ruthenium (II) polypyridyl complexes offers attractive imaging properties, specifically towards the development of sensitive and structure-specific DNA probes. However, rapidly-deactivating dark state formation may compete with 3MLCT luminescence depending on different DNA structures. In this work, by combining femtosecond and nanosecond pump-probe spectroscopy, the 3MLCT relaxation dynamics of [Ru(phen)2(dppz)]2+ (phen = 1,10-phenanthroline, dppz = dipyridophenazine) in two iconic G-quadruplexes has been scrutinized. The binding modes of stacking of dppz ligand on the terminal G-quartet fully and partially are clearly identified based on the biexponential decay dynamics of the 3MLCT luminescence at 620 nm. Interestingly, the inhibited dark state channel in ds-DNA is open in G-quadruplex, featuring an ultrafast picosecond depopulation process from 3MLCT to a dark state. The dark state formation rates are found to be sensitive to the content of water molecules in local G-quadruplex structures, indicating different patterns of bound water. The unique excited state dynamics of [Ru(phen)2(dppz)]2+ in G-quadruplex is deciphered, providing mechanistic basis for the rational design of photoactive ruthenium metal complexes in biological applications.

Photodissociation and photoisomerization dynamics of CH2=CHCHO in solution.

By means of time-resolved Fourier transform infrared absorption spectroscopy, we have investigated the 193 nm photodissociation and photoisomerization dynamics of the prototype molecule of alpha,beta-enones, acrolein (CH(2)=CHCHO) in CH(3)CN solution. The primary photolysis channels and absolute branching ratios are determined. The most probable reaction mechanisms are clarified by control experiments monitoring the product yields varied with the triplet quencher addition. The predominant channel is the 1,3-H migration yielding the rearrangement product CH(3)CH=C=O with a branching ratio of 0.78 and the less important channel is the alpha cleavage of C-H bond yielding radical fragments CH(2)=CHCO+H with a branching ratio of only 0.12. The 1,3-H migration is strongly suggested to correlate with the triplet (3)(pipi*) state rather than the ground S(0) state and the alpha cleavage of C-H bond is more likely to proceed in the singlet S(1) (1)(npi*) state. From the solution experiments we have not only acquired clues clarifying the previous controversial mechanisms, but also explored different photochemistry in solution. Compared to the gas phase photolysis which is dominated by photodissociation channels, the most important channel in solution is the photoisomerization of 1,3-H migration. The reason leading to the different photochemistry in solution is further ascribed to the solvent cage effect.

Adiabatic and nonadiabatic reaction pathways of the O(3P) with propyne.

For the reaction of O((3)P) with propyne, the product channels and mechanisms are investigated both theoretically and experimentally. Theoretically, the CCSD(T)//B3LYP/6-311G(d,p) level of calculations are performed for both the triplet and singlet potential energy surfaces and the minimum energy crossing point between the two surfaces are located with the Newton-Lagrange method. The theoretical calculations show that the reaction occurs dominantly via the O-addition rather than the H-abstraction mechanism. The reaction starts with the O-addition to either of the triple bond carbon atoms forming triplet ketocarbene (3)CH(3)CCHO or (3)CH(3)COCH which can undergo decomposition, H-atom migration or intersystem crossing from which a variety of channels are open, including the adiabatic channels of CH(3)CCO + H (CH(2)CCHO + H), CH(3) + HCCO, CH(2)CH + HCO, CH(2)CO + CH(2), CH(3)CH + CO, and the nonadiabatic channels of C(2)H(4) + CO, C(2)H(2) + H(2) + CO, H(2) + H(2)CCCO. Experimentally, the CO channel is investigated with TR-FTIR emission spectroscopy. A complete detection of the CO product at each vibrationally excited level up to v = 5 is fulfilled, from which the vibrational energy disposal of CO is determined and found to consist with the statistical partition of the singlet C(2)H(4) + CO channel, but not with the triplet CH(3)CH + CO channel. In combination with the present calculation results, it is concluded that CO arises mainly from the singlet methylketene ((1)CH(3)CHCO) dissociation following the intersystem crossing of the triplet ketocarbene adduct ((3)CH(3)CCHO). Fast intersystem crossing via the minimum energy crossing point of the triplet and singlet surfaces is shown to play significant roles resulting into nonadiabatic pathways for this reaction. Moreover, other interesting questions are explored as to the site selectivity of O((3)P) atom being added to which carbon atom of the triple bond and different types of internal H-atom migrations including 1,2-H shift, 3,2-H shift, and 3,1-H shift involved in the reaction.

Reaction mechanisms of a photo-induced [1,3] sigmatropic rearrangement via a nonadiabatic pathway.

Time-resolved Fourier transform infrared absorption spectroscopy measurements and B3LYP/cc-pVDZ calculations have been conducted to characterize the reaction dynamics of a remarkable photoinduced 1,3-Cl sigmatropic rearrangement reaction upon 193 or 266 nm excitation of the model systems acryloyl chloride (CH(2)CHCOCl) and crotonyl chloride (CH(3)CHCHCOCl) in solution. The reaction is elucidated to follow nonadiabatic pathways via two rapid ISC processes, S(1) --> T(1) and T(1) --> S(0), and the S(1)/T(1) and T(1)/S(0) surface intersections are found to play significant roles leading to the nonadiabatic pathways. The S(1) --> T(1) --> S(0) reaction pathway involving the key participation of the T(1) state is the most favorable, corresponding to the lowest energy path. It is also suggested that the photoinduced 1,3-Cl migration reaction of RCHCHCOCl (R = H, CH(3)) proceeds through a stepwise mechanism involving radical dissociation-recombination, which is quite different from the generally assumed one-step concerted process for pericyclic reactions.

Preferential Binding of π-Ligand Porphyrin Targeting 5'-5' Stacking Interface of Human Telomeric RNA G-Quadruplex Dimer.

Human telomeric RNA (TERRA) containing thousands of G-rich repeats has the propensity to form parallel-stranded G-quadruplexes. The emerging crucial roles of TERRA G-quadruplexes in RNA biology fuel increasing attention for studying anticancer ligand binding with such structures, which, however, remains scarce. Here we utilized multiple steady-state and time-resolved spectroscopy analyses in conjunction with NMR methods and investigated thoroughly the binding behavior of TMPyP4 to a TERRA G-quadruplex dimer formed by the 10-nucleotide sequence r(GGGUUAGGGU). It is clearly identified that TMPyP4 intercalates into the 5'-5' stacking interface of two G-quadruplex blocks with a binding stoichiometry of 1:1 and binding constant of 1.92 × 106 M-1. This is consistent with the unique TERRA structural features of the enlarged π-π stacking plane of the A·(G·G·G·G)·A hexad at 5'-ends of each G-quadruplex block. The preferential binding of π-ligand porphyrin to the 5'-5' stacking interface of the native TERRA G-quadruplex dimer is first ascertained by the combination of dynamics and structural characterization.