A New Cationic Porphyrin Derivative (TMPipEOPP) with Large Side Arm Substituents: A Highly Selective G-Quadruplex Optical Probe

The discovery of uncommon DNA structures and speculation about their potential functions in genes has brought attention to specific DNA structure recognition. G-quadruplexes are four-stranded nucleic acid structures formed by G-rich DNA (or RNA) sequences. G-rich sequences with a high potential to form G-quadruplexes have been found in many important genomic regions. Porphyrin derivatives with cationic side arm substituents are important G-quadruplex-binding ligands. For example, 5,10,15,20-Tetrakis(N-methylpyridinium-4-yl)-21H,23H-porphyrin (TMPyP4), interacts strongly with G-quadruplexes, but has poor selectivity for G-quadruplex versus duplex DNA. To increase the G-quadruplex recognition specificity, a new cationic porphyrin derivative, 5,10,15,20-tetra-{4-[2-(1-methyl-1- piperidinyl)ethoxy]phenyl} porphyrin (TMPipEOPP), with large side arm substituents was synthesized, and the interactions between TMPipEOPP and different DNA structures were compared. The results show that G-quadruplexes cause large changes in the UV-Vis absorption and fluorescence spectra of TMPipEOPP, but duplex and single-stranded DNAs do not, indicating that TMPipEOPP can be developed as a highly specific optical probe for discriminating G-quadruplex from duplex and single-stranded DNA. Visual discrimination is also possible. Job plot and Scatchard analysis suggest that a complicated binding interaction occurs between TMPipEOPP and G-quadruplexes. At a low [G-quadruplex]/[TMPipEOPP] ratio, one G-quadruplex binds two TMPipEOPP molecules by end-stacking and outside binding modes. At a high [G-quadruplex]/[TMPipEOPP] ratio, two G-quadruplexes bind to one TMPipEOPP molecule in a sandwich-like end-stacking mode

Photochemical dihydrogen production using an analogue of the active site of [NiFe] hydrogenase

The photoproduction of dihydrogen (H2) by a low molecular weight analogue of the active site of [NiFe] hydrogenase has been investigated by the reduction of the [NiFe2] cluster, 1, by a photosensitier PS (PS = [ReCl(CO)3(bpy)] or [Ru(bpy)3][PF6]2). Reductive quenching of the 3MLCT excited state of the photosensitiser by NEt3 or N(CH2CH2OH)3 (TEOA) generates PS•−, and subsequent intermolecular electron transfer to 1 produces the reduced anionic form of 1. Time-resolved infrared spectroscopy (TRIR) has been used to probe the intermediates throughout the reduction of 1 and subsequent photocatalytic H2 production from [HTEOA][BF4], which was monitored by gas chromatography. Two structural isomers of the reduced form of 1 (1a•− and 1b•−) were detected by Fourier transform infrared spectroscopy (FTIR) in both CH3CN and DMF (dimethylformamide), while only 1a•− was detected in CH2Cl2. Structures for these intermediates are proposed from the results of density functional theory calculations and FTIR spectroscopy. 1a•− is assigned to a similar structure to 1 with six terminal carbonyl ligands, while calculations suggest that in 1b•− two of the carbonyl groups bridge the Fe centres, consistent with the peak observed at 1714 cm−1 in the FTIR spectrum for 1b•− in CH3CN, assigned to a ν(CO) stretching vibration. The formation of 1a•− and 1b•− and the production of H2 was studied in CH3CN, DMF and CH2Cl2. Although the more catalytically active species (1a•− or 1b•−) could not be determined, photocatalysis was observed only in CH3CN and DMF

Photophysical characteristics of diphenyl carbazide-laser flash photolysis and fluorescence studies

The photophysical characteristics of diphenyl carbazide (DPC) have been investigated in various protic and aprotic polar solvents. Shifts in fluorescence spectra showed good linear fits with the ET(30) polarity scale and the solvent polarity function Δf. The emitter state is identified as having charge transfer character and is estimated to have a dipole moment of ~8.6 D. Excited state lifetimes determined from transient absorption and fluorescence lifetime studies match well. Triplet-triplet absorption and phosphorescence emission (at 77 K) have been characterized and the triplet state energy is estimated to be ~80 kcal/mol. Our experimental results show that DPC behaves more like a keto compound than a hydrazide derivative

On the exciplex formation in the energy transfer from aromatic hydrocarbons to crystal violet

The interaction of a triphenyl methane (TPM) dye: crystal violet (CV) with the triplet state (3S∗ of the aromatic molecules biphenyl, p-terphenyl or anthracene was studied by pulse radiolytic kinetic spectrophotometry. In each case, studies of time-resolved spectra and kinetics showed that an exciplex between sensitiser triplet and ground state of CV is formed as an intermediate before the former dissociates to give triplet state of CV, [3S*+CV kf⇋kb3(S..CV)*→k3S+3CV*]. A detailed kinetic analysis has been carried out to evaluate various kinetic parameters. The equilibrium constant, K (kf/kb), was found to be 1.8 × 104 dm3 mol-1, 1.53 × 104 dm3 mol-1 and 1.32 × 104 dm3 mol-1 in the case of biphenyl, p-terphenyl and anthracene sensitiser triplet states, respectively

Relaxation dynamics in the excited states of LDS-821 in solution

Relaxation dynamics in the excited electronic state (S1) of LDS-821 have been studied in aprotic and alcoholic solvents with pico- and subpicosecond time resolution using pump-supercontinuum probe transient absorption technique. Steady-state absorption and fluorescence as well as the time-resolved transient absorption/stimulated emission spectra and the temporal dynamics monitored at different wavelengths mainly reveal the features of a two-mode kinetic process in the S1 stateconversion of the locally excited (LE) state to the twisted intramolecular charge-transfer (TICT) state. A short (<1 ps) rise time of stimulated emission monitored at the shorter wavelength band, is the signature of the barrierless skeletal stretching motion along the "valley-like" region of the potential energy surface (PES) of the S1 state in the Franck-Condon (FC) region to attain a metastable untwisted configuration, called the LE state. The decay time of the LE state is equal to the rise time measured at the longer wavelength band, assigned to the TICT state. The linear dependence of the rate of the LE → TICT conversion process on the inverse of viscosity of the solvent indicates that this low-energy barrier crossing process is accompanied by a torsional motion about the free double bond of the molecule. The solvation time of the TICT state in different solvents has been seen to be nearly equal to the growth lifetime of the TICT state but much shorter than the longitudinal relaxation time of the solvent. These facts indicate that the rate of the LE → TICT process is mainly controlled by the contribution from the intramolecular modes rather than the solvation

Study of spectral characteristics, kinetics, and equilibria of radicals derived from hydroxy benzophenones

Studies on radical equilibria and the spectral evaluation have been carried out for three monohydroxy-substituted benzophenones (HOBP's) in aqueous solution. The transient ketyl or anion radicals were generated via hydrated electron (eaq-) reaction or dimethyl ketyl radical reaction in a pulse radiolysis experiment. The reactivity of both the undissociated (HOBP) as well as dissociated (-OBP) forms toward these reducing agents were studied. eaq- was found to react with the HOBP's and -OBP's with diffusion-controlled rates which are of the order 3 × 1010 dm3 mol-1 s-1. In the case of the para derivative, two acid-base equilibria were identified (pKa 9 and 11) in the pH range 6-13, one corresponding to the protonation-deprotonation at the OH site of the ketyl radical and the other for the protonation-deprotonation at the carbonyl site. For the ortho derivative, the intramolecularly hydrogen bonded structure of the radical anion has been found to be stable even in strongly alkaline solution (pH 13). Three different radical forms for the meta derivative have been identified at different pH conditions. The spectral evaluation of these radical species at different pH conditions was carried out. Reactions of dimethyl ketyl radicals with HOBP's and -OBP's were found to be different at different pH conditions. At acidic pH, it forms an adduct with the HOBP's, whereas at alkaline pH, the reaction with the ortho and meta derivatives occurs by H atom transfer. However, no reaction is seen with the para derivative in alkaline solution. Detailed spectral and kinetic data on the formation and decay of the various transient intermediates have been obtained

Photoionization of crystal violet in aqueous solution

Laser flash photolysis studies were carried out on a triphenylmethane dye, crystal violet (CV+), at 248 nm in aqueous solutions. The results show that CV+ undergoes photoionization and the resulting transients CV-2+, hydrated electrons (e-aq) and CV+ radical formed by the reaction of e-aq with CV+ have been characterized. Studies using suitable scavengers were done to support the characterization of the transient species. Laser intensity effects show that the ionization is biphotonic. Two mechanisms are proposed to explain the observed photoionization involving higher excited singlet state and/ or another long-lived excited state of the dye

Laser flash photolysis studies on the monohydroxy derivatives of benzophenone

Time-resolved studies in the pico- and nanosecond time domain have been performed to characterize the triplet states of monohydroxy-substituted benzophenones, namely, para- (p-), meta- (m-), and ortho- (o-) hydroxybenzophenones (HOBP). Due to a very fast intersystem crossing (ISC) process, only the triplet states have been detected in the subnanosecond time domain. Spectral characteristics and lifetimes of the triplet states of HOBP have been seen to be extremely sensitive to the position of the OH group in the phenyl ring as well as the solvent characteristics. In case of m-HOBP and p-HOBP, the excited triplet state in non-hydrogen-bond-forming solvents has an nπ∗ configuration and is capable of abstracting a hydrogen atom from another unexcited molecule to form ketyl and phenoxy type radicals. But in hydrogen-bond-forming solvents, the triplet state, which is strongly associated with the solvent molecules as a hydrogen-bonded complex, is very short-lived due to fast nonradiative relaxation via hydrogen-stretching vibrations in intermolecular hydrogen bonds with the solvents and is capable of abstracting a hydrogen atom neither from the solvent molecule nor from another unexcited HOBP molecule. In the case of o-HOBP, due to strong intramolecular hydrogen bonding, the internal conversion in the excited singlet state is very efficient and hence the yield of the triplet state is low (<15%) and also the triplet state is very short-lived. However, in methanol and DMSO, due to disruption of the intramolecular hydrogen bond, the triplet yield is higher and also the generation of the phenolate ion via excited-state proton transfer is a significant process. Deprotonation reactions probably taking place from both the excited singlet state as well as the triplet state of the other two derivatives also have been found to be significant in polar solvents