Femtosecond Dynamics of Piroxicam Structures in Solutions

Ultrafast dynamics of the antibiotic Rifampicin in solution

Ultrafast time-resolved studies demonstrate that intra- and intermolecular H-bonds with water molecules act synergistically to stabilize the active zwitterionic form of Rifampicin, an effective antibiotic against mycobacterial infections.

Structural and photodynamic properties of the anti-cancer drug irinotecan in aqueous solutions of different pHs

This work reports on photophysical studies of the irinotecan (IRT) anti-cancer drug in water solutions of different acidities (pH = 1.11-9.46). We found that IRT co-exists as mono-cationic (C1), di-cationic (C2), or neutral (N) forms. The population of each prototropic species depends on the pH of the solution. At pH = 1.11-3.01, the C1 and C2 structures are stabilized. At pH = 7.00, the most populated species is C1, while at pH values larger than 9.46 the N form is the most stable species. In the 1.11-2.61 pH range, the C1* emission is efficiently quenched by protons to give rise to the emission from C2*. The dynamic quenching constant, KD, is ∼32 M-1. While the diffusion governs the rate of excited-state proton-transfer (ESPT) under these conditions, the reaction rate increases with the proton concentration. A two-step diffusive Debye-Smoluchowski model was applied at pH = 1.11-2.61 to describe the protonation of C1*. The ESPT time constants derived for C1* are 382 and 1720 ps at pH = 1.11 and 1.95, respectively. We found that one proton species is involved in the protonation of C1* to give C2*, in the analyzed acidic pH range. Under alkaline conditions (pH = 9.46), the N form is the most stable structure of IRT. These results indicate the influence of the pH of the medium on the structural and dynamical properties of IRT in water solution. They may help to provide a better understanding on the relationship between the structure and biological activity of IRT.

Solubility and Preferential Solvation of Piroxicam in Neat Solvents and Binary Systems

The solubilization and solvatochromic behavior of piroxicam (PRX) were analyzed using UV-vis spectroscopy in neat (protic and aprotic) and binary solvent mixtures. The effects of solvent dipolarity/polarizability and solvent–solute hydrogen bonding interactions on the absorption maxima were evaluated by means of the linear solvation energy relationship concept of Kamlet and Taft. This analysis indicated that both interactions play an important role in the position of the absorption maxima in neat solvents. While, the PRX solubility depends on the solute–solvent specific interactions, polarizability and the cohesive forces of the solvent, manifested mainly by means of the Hildebrand’s solubility parameter. Preferential solvation (PS) was studied in 10 binary mixtures. A non-ideal behavior of the wavenumber curve as the function of analytical mole fraction of co-solvent was detected. Index of preferential solvation, as well as the influence of solvent parameters were calculated. The process of dissolution was analyzed in aqueous binary mixtures of ethanol, ethylene glycol and propylene glycol. They were not spontaneous in all proportions, but when water concentration decreases in the mixtures, the process becomes more spontaneous.

Photodynamics of a Proton-Transfer Dye in Solutions and Confined Within NaX and NaY Zeolites

We report on steady-state, picosecond and femtosecond time-resolved emission studies of 2-(2'-hydroxyphenyl)benzoxazole (HBO) in solutions and interacting with NaX and NaY zeolites. In solutions, an ultrafast (less than 150 fs) excited-state intramolecular proton-transfer (ESIPT) reaction takes place in syn-enol form, and leads to keto-type tautomer. We also observed a torsional motion in the keto form (~20 ps in dichloromethane, DCM). For NaX and NaY DCM suspensions, anionic forms interacting with the zeolites at S0 and S1 states are generated. They show two fluorescence lifetimes in both zeolites (720 ps and 2.4 ns for NaY and 960 ps and 2.7 ns for NaX), while those of the enol bonded to the zeolite framework and of the free keto forms are ~100 and 250 ps, respectively. The ultrafast dynamics of the anion in alkaline solutions reveals two deactivation pathways: an intramolecular charge transfer (ICT, 1.2 ps) and a twisting motion, affected by the viscosity of the solvent (12 and 20 ps for MeOH and ethylene glycol). When HBO is interacting with NaX and NaY the twisting motion is cancelled, while the ICT becomes slower as a result of a combination of several environment effects. HBO anions within the faujasite framework show also a ~ 30 ps decay associated to a non-fluorescent (n, π*) state. Our results demonstrate how intermolecular H-bonds, the confinement and the electrostatic interactions of HBO with the used materials, affect its ground as well as its excited state properties. Our findings add new knowledge on the interactions of silica-based nanomaterials containing the H-bonding guests.

Ultrafast photodynamics of drugs in nanocavities: cyclodextrins and human serum albumin protein

In this feature article, we discuss recent advances in studying ultrafast dynamic and structural aspects of host-guest interactions. Steady-state and time-resolved techniques exploring events from the femto- to nanosecond regime were used to examine the ultrafast photodynamics and subsequent events in selected nanostructures of the formed complexes. These consist of aromatic systems, biologically relevant molecules, and drugs trapped within cyclodextrins (CD) and human serum albumin (HSA) protein pockets. We examine the effects exerted by these chemical and biological cavitands on internal twisting motions, proton transfer and charge transfer, and cis-trans isomerization reactions that may occur in the confined molecular systems. In addition, the influence of a restricting environment on the interaction of guest molecules with biological water is considered. The dynamic details of the complexes (diffusion, early interactions, formation, stability, internal guest diffusion, and conformational changes) and the excited-state relaxation pathways, rate constants of the involved processes, and changes in the electronic distribution within encapsulated guests gave clues to elucidate their photobehavior and are relevant to the photostability and delivery of drugs when using nanocarriers.

Existence of a new emitting singlet state of proflavine: femtosecond dynamics of the excited state processes and quantum chemical studies in different solvents

Proflavine (3,6-diaminoacridine) shows fluorescence emission with lifetime, 4.6 ± 0.2 ns, in all the solvents irrespective of the solvent polarity. To understand this unusual photophysical property, investigations were carried out using steady state and time-resolved fluorescence spectroscopy in the pico- and femtosecond time domain. Molecular geometries in the ground and low-lying excited states of proflavine were examined by complete structural optimization using ab initio quantum chemical computations at HF/6-311++G** and CIS/6-311++G** levels. Time dependent density functional theory (TDDFT) calculations were performed to study the excitation energies in the low-lying excited states. The steady state absorption and emission spectral details of proflavine are found to be influenced by solvents. The femtosecond fluorescence decay of the proflavine in all the solvents follows triexponential function with two ultrafast decay components (τ(1) and τ(2)) in addition to the nanosecond component. The ultrafast decay component, τ(1), is attributed to the solvation dynamics of the particular solvent used. The second ultrafast decay component, τ(2), is found to vary from 50 to 215 ps depending upon the solvent. The amplitudes of the ultrafast decay components vary with the wavelength and show time dependent spectral shift in the emission maximum. The observation is interpreted that the time dependent spectral shift is not only due to solvation dynamics but also due to the existence of more than one emitting state of proflavine in the solvent used. Time resolved area normalized emission spectral (TRANES) analysis shows an isoemissive point, indicating the presence of two emitting states in homogeneous solution. Detailed femtosecond fluorescence decay analysis allows us to isolate the two independent emitting components of the close lying singlet states. The CIS and TDDFT calculations also support the existence of the close lying emitting states. The near constant lifetime observed for proflavine in different solvents is suggested to be due to the similar dipole moments of the ground and the evolved emitting singlet state of the dye from the Franck-Condon excited state.

The role of triplet state keto-enol tautomerism in the photodeamination of metamitron

Substituted 4-amino-1,2,4-triazin-5-ones undergo photodeamination through cleavage of the N-NH(2) bond in the presence of oxygen and water. To elucidate the mechanism of this reaction, we investigated the photolysis of metamitron (4-amino-6-phenyl-3-methyl-1,2,4-triazin-5-one) by nanosecond laser flash photolysis, steady-state irradiation, and ab initio calculations. Upon pulsed laser excitation of deoxygenated aqueous metamitron, two transient species are clearly detected. The predictions of ab initio results are consistent with experimental results: (i) it is proposed here that the transient species are, respectively, the keto and diradical forms of the metamitron keto-enol tautomerism in the triplet state, and (ii) in water, the activation free energy barrier of enolization is drastically decreased. Thus, the formation of the diradical triplet is enabled in aqueous solvent. A detailed analysis of the intermediate structures that lead to the final products (HNO(2) and deaminometamitron) is provided.