Ultrafast Dynamics of the Excited States of Curcumin in Solution

Exploring the World of Curcumin: Photophysics, Photochemistry, and Applications in Nanoscience and Biology

Curcumin is a bright yellow naturally occurring polyphenol which is the principal component of turmeric. It is used as herbal supplement, cosmetics ingredient, and food coloring agent. Over the years, the therapeutic properties of the natural product curcumin have gone unexploited but not unnoticed. Curcumin cannot be employed as a drug due to limitations such as low aqueous solubility and limited bioavailability. Many attempts have been made to overcome these limitations by confining the drug in various confined media to enhance its bioavailability. The biomolecule is emissive and undergoes fundamental excited state processes such as solvation dynamics and excited state intramolecular proton transfer (ESIPT). Curcumin based biomaterials and nanomaterials are also a fast advancing field where curcumin is an intrinsic component necessary for formation of these materials and no longer added as an external free drug. In this review, we will summarize the recent research on the photophysical and photochemical properties of curcumin and its excited state dynamics in various bio-mimicking systems. At the same time we wish to also incorporate the various applications of curcumin, especially in biology. Lastly due to the growing importance of materials science, we will briefly discuss some recent advances on curcumin based biomaterials and nanomaterials. We believe such a compilation of recent research surrounding curcumin will provide an overall understanding of its potentialities in different areas.

Time-resolved keto-enol tautomerization of the medicinal pigment curcumin

Curcumin is a medicinal agent that exhibits anti-cancer and anti-Alzheimer's disease properties. It has a keto-enol moiety that gives rise to many of its chemical properties including metal complexation and acid-base equilibria. A previous study has shown that keto-enol tautomerization at this moiety is implicated in the anti-Alzheimer's disease effect of curcumin, highlighting the importance of this process. In this study, tautomerization of curcumin in methanol, acetone and acetonitrile was investigated using time-resolved 1H nuclear magnetic resonance spectroscopy. Curcumin undergoes hydrogen-deuterium exchange with the solvents and the proton resonance peak corresponding to the hydrogen at the α-carbon position (Cα) decays as a function of time, signifying deuteration at this position. Because tautomerization is the rate limiting step in the deuteration of curcumin at the Cα position, the rate of tautomerization is inferred from the rate of deuteration. The rate constant of tautomerization of curcumin shows a temperature dependence and analysis using the Arrhenius equation revealed activation energies (Ea) of tautomerization of (80.1 ± 5.9), (64.1 ± 1.0) and (68.3 ± 5.5) kJ mol-1 in methanol, D2O/acetone and D2O/acetonitrile, respectively. Insight into the role of water in tautomerization of curcumin was further offered by density functional theory studies. The transition state of tautomerization was optimized in the presence of water molecules. The results show a hydrogen-bonded solvent bridge between the diketo moiety and Cα of curcumin. The Ea of tautomerization of curcumin shows a strong dependence on the number of water molecules in the solvent bridge, indicating the critical role played by the solvent bridge in catalyzing tautomerization of curcumin.

Characteristics of nonconventional hydrogen bonds and stability of dimers of chalcogenoaldehyde derivatives: a noticeable role of oxygen compared to other chalcogens

In this work, twenty-four stable dimers of RCHZ with R = H, F, Cl, Br, CH3 or NH2 and Z = O, S, Se or Te were determined. It was found that the stability of most dimers is primarily contributed by the electrostatic force, except for the dominant role of the induction term in those involving a Te atom, which has been rarely observed. Both electron-donating and -withdrawing groups in substituted formaldehyde cause an increase in the strength of nonconventional Csp2-H⋯Z hydrogen bonds, as well as the dimers, in which the electron donating effect plays a more crucial role. The strength of nonconventional hydrogen bonds decreases in the following order: Csp2-H⋯O ≫ Csp2-H⋯S > Csp2-H⋯Se > Csp2-H⋯Te. Remarkably, a highly significant role of the O atom compared to S, Se and Te in increasing the Csp2-H stretching frequency and strength of the nonconventional hydrogen bonds and dimers is found. A Csp2-H stretching frequency red-shift is observed in Csp2-H⋯S/Se/Te, while a blue-shift is obtained in Csp2-H⋯O. When Z changes from O to S to Se and to Te, the Csp2-H blue-shift tends to decrease and eventually turns to a red-shift, in agreement with the increasing order of the proton affinity at Z in the isolated monomer. The magnitude of the Csp2-H stretching frequency red-shift is larger for Csp2-H⋯Te than Csp2-H⋯S/Se, consistent with the rising trend of proton affinity at the Z site and the polarity of the Csp2-H bond in the substituted chalcogenoaldehydes. The Csp2-H blue-shifting of the Csp2-H⋯O hydrogen bonds is observed in all dimers regardless of the electron effect of the substituents. Following complexation, the electron-donating derivatives exhibit a stronger Csp2-H blue-shift compared to the electron-withdrawing ones. Notably, the stronger Csp2-H blue-shift turns out to involve a less polarized Csp2-H bond and a decrease in the occupation at the σ*(Csp2-H) antibonding orbital in the isolated monomer.

Theoretical studies on the photo protective mechanism of curcuminoids

In this work, the deactivation pathways of curcuminoids after photoexcitation was studied by employing density functional theory to explore their UVA radiation screening capacity. A comprehensive computational characterization of the excited-state processes of curcumin, demethoxycurcumin, and bis-demethoxycurcumin was done. The molecules exist in diketo and enol forms which are in equilibrium and interconvertible through keto-enol tautomerism. The enolic forms of each of the studied molecules have eight geometric cis-trans isomers as a result of torsion rotation about three different carbon-carbon double bonds across the aliphatic chain. For each geometric isomer, sixteen possible rotamers are found to exist due to rotation about five different carbon-carbon single bond rotations, also across the skeleton of the aliphatic chain. Upon photoexcitation, the studied molecules follow three main pathways of radiationless decay: (a) rotamerism and interconversion between rotamers of comparable energies which are in equilibrium, (b) interconversion between the cis-trans geometrical isomers where an efficient vibrational relaxation path is formed at ∼90° during torsion rotation about carbon-carbon double bond, and (c) excited state intramolecular proton transfer in a single O-H stretching vibration through a cyclic intramolecular hydrogen bonded ring formed at the centre of the molecule giving back the original structure. The absorption and emission spectra of the molecules were also simulated where the theoretically obtained absorption and emission maxima are close to the reported experimental values.

Investigation of ultrafast carrier dynamics in curcumin dye for environment friendly dye-sensitized solar cell

Natural dyes have been widely employed in the fabrication of dye-sensitized solar cells (DSSCs). DSSCs are favored for their cost-effective, and simple fabrication process relies on metal-based and organic dyes. The choice of dyes greatly affects the performance of DSSCs. DSSCs have found a lot of applications in indoor, solar power gadgets with reasonable efficiency up to 13%. Nonetheless, despite advances in DSSC technology, the complex photophysics and excited state dynamics associated with natural dyes employed in DSSCs remain elusive and have not been adequately investigated. This information gap emphasizes the need for more study and analysis into the behavior of these dyes, since understanding their underlying principles might lead to major improvements in DSSC performance and efficiency. In this work, we have investigated the fundamental characteristics and excited-state carrier dynamics of natural dye curcumin using ultrafast transient absorption (TA) spectroscopy technique. The curcumin dye shows delay time-dependent positive and negative signals in the TA spectra, which are related to excited state absorption and stimulated emission. We also found that hydrogen bonding and polarity effect of solvent significantly influence the carrier dynamics of curcumin. Ultrafast lifetime component indicates that hydrogen-bond rearrangements are involved in the kinetics of the relaxation process of the S1 state of curcumin photo-sensitizer.

Excited State Photophysics of Curcumin and its Modulation in Alkaline Non-Aqueous Medium

Curcumin, a well-known medicinal pigment, has seen limited applications in biology despite having great potential as a therapeutic drug. Deprotonation is one of the possible ways to enhance solubility of curcumin in polar solvent. Here, we have explored the effect of deprotonation on the ultrafast dynamics of this biomolecule with the help of the time-resolved fluorescence spectroscopic measurements using the femtosecond fluorescence upconversion technique. The excited state photophysics of fully deprotonated curcumin significantly differs from that of neutral curcumin. We have observed that the completely deprotonated curcumin not only has higher quantum yield, but also higher excited state lifetime and slower solvation dynamics in comparison to neutral curcumin. We propose solvation dynamics and intramolecular charge transfer as the excited state processes associated with the radiative decay of the completely deprotonated molecule, while ruling out the possibility of excited state proton exchange or proton transfer. Our results are well supported by time-dependent density-functional theory calculations. Lastly, we have also demonstrated the possibility of modulating the ultrafast dynamics of fully deprotonated curcumin using non-aqueous alkaline binary solvent mixtures. We believe our results will provide significant physical insight towards unveiling the excited state dynamics of this molecule.

The Role of H-Bonds in the Excited-State Properties of Multichromophoric Systems: Static and Dynamic Aspects

Given their importance, hydrogen bonds (H-bonds) have been the subject of intense investigation since their discovery. Indeed, H-bonds play a fundamental role in determining the structure, the electronic properties, and the dynamics of complex systems, including biologically relevant materials such as DNA and proteins. While H-bonds have been largely investigated for systems in their electronic ground state, fewer studies have focused on how the presence of H-bonds could affect the static and dynamic properties of electronic excited states. This review presents an overview of the more relevant progress in studying the role of H-bond interactions in modulating excited-state features in multichromophoric biomimetic complex systems. The most promising spectroscopic techniques that can be used for investigating the H-bond effects in excited states and for characterizing the ultrafast processes associated with their dynamics are briefly summarized. Then, experimental insights into the modulation of the electronic properties resulting from the presence of H-bond interactions are provided, and the role of the H-bond in tuning the excited-state dynamics and the related photophysical processes is discussed.

How Does Pseudo-Jahn-Teller Effect Induce the Photoprotective Potential of Curcumin?

In this paper, the molecular and electronic structure of curcumin is studied. High-symmetric gas-phase tautomers and their deprotonated forms in various symmetry groups are identified. The stability of lower-symmetry structures was explained by using the Pseudo-Jahn-Teller (PJT) effect. This effect leads to stable structures of different symmetries for the neutral enol and keto forms. The presented analysis demonstrated the potential significance of the PJT effect, which may modulate the setting of electronic and vibrational (vibronic) energy levels upon photodynamic processes. The PJT effect may rationalize the photoprotection action and activity of naturally occurring symmetric dyes.

Exploring Steroidal Surfactants as Potential Drug Carriers for an Anticancer Drug Curcumin: An Insight into the Effect of Surfactants' Structure on the Photophysical Properties, Stability, and Activity of Curcumin

Despite having tremendous medicinal benefits, the practical applications of curcumin are limited, owing to two major challenges: poor aqueous solubility and lack of bioavailability. In this regard, biosurfactant-based micellar systems have surged recently for the development of novel and more effective formulations because of their biological relevance. This study deals with a comprehensive and comparative investigation on the effect of seven structurally different steroidal surfactants on the photophysical properties of curcumin and also evaluates these steroidal surfactants as possible drug delivery media for curcumin. The photophysical properties of curcumin exhibited a strong dependence on the structure of the steroidal surfactant; the extent of excited-state proton transfer between curcumin and the surfactants depends strongly on the type of the side chain in the surfactants, which mostly dictates the photophysics of curcumin in the presence of these structural variants. The solubility of curcumin and its stability at different pHs and temperatures and in the presence of salt are significantly enhanced in the presence of these surfactants. Furthermore, the curcumin-loaded micelles exhibited improved intracellular uptake and cytotoxicity against MCF-7 cancer cells than pristine curcumin. Among these steroidal surfactants, CHAPS, the zwitterionic derivative of cholic acid, was the most efficient one to offer better solubility and stability to curcumin under all conditions, and the death rate of MCF-7 cells by curcumin was found to be the highest in the presence of CHAPS, indicating the enhanced bioavailability of curcumin. Therefore, CHAPS-based colloids are found to be promising candidates as potential drug carriers for curcumin.

Intramolecular Charge Transfer of Curcumin and Solvation Dynamics of DMSO Probed by Time-Resolved Raman Spectroscopy

Intramolecular charge transfer (ICT) of curcumin in dimethyl sulfoxide (DMSO) solution in the excited state was investigated by femtosecond electronic and vibrational spectroscopy. Excited-state Raman spectra of curcumin in the locally-excited and charge-transferred (CT) state of the S₁ excited state were separated due to high temporal (<50 fs) and spectral (<10 cm⁻¹) resolutions of femtosecond stimulated Raman spectroscopy. The ultrafast (0.6–0.8 ps) ICT and subsequent vibrational relaxation (6–9 ps) in the CT state were ubiquitously observed in the ground- and excited-state vibrational modes of the solute curcumin and the ν_CSC and ν_S=O modes of solvent DMSO. The ICT of curcumin in the excited state was preceded by the disruption of the solvation shells, including the breakage of hydrogen bonding between curcumin and DMSO molecules, which occurs at the ultrafast (20–50 fs) time scales.

Don't help them to bury the light. The interplay between intersystem crossing and hydrogen transfer in photoexcited curcumin revealed by surface-hopping dynamics

Curcumin is a natural compound extracted from turmeric (Curcuma longa), which has shown remarkable anti-inflammatory, antibacterial, and possibly anticancer properties. The intense absorption in the visible domain and the possibility of intersystem crossing make curcumin attractive also for photodynamic therapy purposes. In the present contribution, we unravel, thanks to non-adiabatic surface hopping dynamics, the interplay between intersystem crossing and hydrogen transfer in the enol form, i.e. the most stable tautomer of curcumin. Most notably, we show that while hydrogen transfer is ultrafast and happens in the sub-ps regime, intersystem crossing is still present, as shown by the non-negligible population of the triplet state manifold after 2 ps. Hence, while the hydrogen transfer channel can act as a deactivating channel, curcumin, also in the red-shifted absorption enol form, can still be regarded as potentially favorable for photodynamic therapy applications.

An insight into curcumin-based photosensitization as a promising and green food preservation technology

Consumer awareness on the side effects of chemical preservatives has increased the demand for natural preservation technologies. An efficient and sustainable alternative to current conventional preservation techniques should guarantee food safety and retain its quality with minimal side effects. Photosensitization, utilizing light and a natural photosensitizer, has been postulated as a viable and green alternative to the current conventional preservation techniques. The potential of curcumin as a natural photosensitizer is reviewed in this paper as a practical guide to develop a safe and effective decontamination tool for industrial use. The fundamentals of the photosensitization mechanism are discussed, with the main emphasis on the natural photosensitizer, curcumin, and its application to inactivate microorganisms as well as to enhance the shelf life of foods. Photosensitization has shown promising results in inactivating a wide spectrum of microorganisms with no reported microbial resistance due to its particular lethal mode of targeting nucleic acids. Curcumin as a natural photosensitizer has recently been investigated and demonstrated efficacy in decontamination and delaying spoilage. Moreover, studies have shown the beneficial impact of an appropriate encapsulation technique to enhance the cellular uptake of photosensitizers, and therefore, the phototoxicity. Further studies relating to improved delivery of natural photosensitizers with inherent poor solubility should be conducted. Also, detailed studies on various food products are warranted to better understand the impact of encapsulation on curcumin photophysical properties, photo-driven release mechanism, and nutritional and organoleptic properties of treated foods.

Self-assembled hydrophobic Ala-Aib peptide encapsulating curcumin: a convenient system for water insoluble drugs

The exploitation of self-assembled systems to improve the solubility of drugs is getting more and more attention. Among the different types of self-assembled biomaterials, peptides and in particular peptides containing non-coded amino acids (NCAPs) are promising because their use opens the door to more stable materials inducing increased stability to proteolysis. New classes of NCAP, Ac-Ala-X-Ala-Aib-AlaCONH2 (X = alpha-aminoisobutyric acid (Aib) or X = cyclopentane amino acid (Ac5c)) have been prepared and the correlation between the different secondary peptide structure and solvent (i.e. CD3CN, CD3OH, H2O/D2O) verified by NMR. Furthermore, the formation of a nanocolloidal system in water was deeply studied by DLS and the morphology of the obtained spherical aggregates with nanometric dimensions was assessed by TEM. Aib containing pentapeptide was selected for greater ease of synthesis. Its ability to encapsulate curcumin, as a model insoluble drug molecule, was investigated using fluorescence emission and confocal microscopy analyses. Two different approaches were used to study the interaction between curcumin and peptide aggregates. In the first approach peptide aggregates were formed in the presence of curcumin, while in the second approach curcumin was added to the already formed peptide aggregates. We succeeded in our challenge by using the second approach and 53.8% of added curcumin had been encapsulated.

Introducing Tween-curcumin niosomes: preparation, characterization and microenvironment study

In this work, we report unusual niosomes (non-ionic surfactant based vesicles), prepared using non-ionic surfactant Tween 80 (T80) as well as Tween 20 (T20) and curcumin. Conventional niosomes consist of non-ionic surfactant and cholesterol. We found that, despite being a probiotic, curcumin plays a similar role to cholesterol in the formation and stabilization of niosomes. The prepared Tween-curcumin niosomes were characterised using Dynamic Light Scattering (DLS), zeta potential, Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), and Transmission Electron Microscopy (TEM) techniques. The curcumin-induced micelle to vesicle transition in the Tween surfactants was investigated by DLS, zeta potential, fluorescence anisotropy, and fluorescence lifetime studies. At room temperature (298 K), the prepared niosomes were found to be stable; however, at a higher temperature (333 K), the niosomes degrade gradually and irreversibly to form micelles. The temperature-dependent vesicle to micelle degradation was monitored using fluorescence anisotropy, absorption, DLS and Differential Scanning Calorimetry (DSC) measurements. Further, the Tween-curcumin niosomes show a controlled release of curcumin, which could open up the possibility of multidrug therapy.

Interaction of carbon nanotubes with curcumin: Effect of temperature and pH on simultaneous static and dynamic fluorescence quenching of curcumin using carbon nanotubes

Curcumin (Cur) has medicinal properties, undergoes hydrolysis, and has low water solubility that limits its bioavailability and industrial usage. Different host molecules such as carbon nanotubes (CNT) can be useful in improving solubility and stabilizing Cur, therefore understanding the interaction of Cur with host molecules such as CNT is crucial. In this study, UV-visible light absorption and fluorescence spectroscopic techniques have been applied to reveal the interaction of Cur with CNT. Visible light absorption of Cur increases with CNT concentration, whereas fluorescence intensity of Cur is quenched in the presence of CNT. The obtained results confirm that fluorescence reduction is due to both static and dynamic quenching and is a result of the ground state and excited-state complex formation. The pH environment influences the quenching rate due to deprotonation of Cur at higher pH; excess OH- ion concentration in the solution further discourages electrostatic interaction between the deprotonated form of Cur with CNT. It is found that at lower temperatures (<35°C) dynamic quenching is much more dominant and at higher temperatures (45°C) static quenching is more dominant. The interaction is further supported using X-ray diffraction patterns and Fourier transform infrared spectra in the solid state, and suggests encapsulation of curcumin within the CNT. It is further evident that fluorescence quenching of Cur using CNT is further enhanced in the presence of several salts, as increase in ionic strength of the solution pushes the hydrophobic Cur molecule towards the CNT core by increasing the proximity between them.

A short review on chemical properties, stability and nano-technological advances for curcumin delivery

Introduction: Curcumin is a polyphenol found in turmeric that is derived from the rhizomes of Curcuma longa. Curcumin has received a worldwide attention due to being a major constituent of the traditional Chinese and Indian holistic systems, and due to its well-documented pharmacological effects against various diseases.Areas covered: In order to provide a better understanding of curcumin's biological activities, its chemical, structural, spectral and photophysical properties should be studied. Also, it is crucial to study the aqueous, spectral, photophysical, photochemical, and thermal stability. Such studies indicated that curcumin suffers from bioavailability problems such as low serum levels, limited tissue distribution, and excessive metabolism which all limit its therapeutic efficacy. This review summarizes different properties of curcumin, its stability, bioavailability problems, and recent nanotechnological approaches with special highlight on nanocapsules for curcumin delivery.Expert opinion: Poor bioavailability of curcumin could be overcome through recently emerging and promising nanotechnological approaches.

Ultrafast intramolecular proton transfer reactions and solvation dynamics of DMSO

Ultrafast intramolecular proton transfers of 1,2-dihydroxyanthraquinone (alizarin-h2) and its deuterated product (alizarin-d2) in dimethyl sulfoxide (DMSO) have been investigated by femtosecond stimulated Raman spectroscopy. The population dynamics in the solute vibrational mode of νC=O and the coherent oscillations observed in all of the skeletal vibrational modes νC=O and νC=C clearly showed the ultrafast excited-state intramolecular proton transfer dynamics of 110 and 170 fs for alizarin-h2 and alizarin-d2, respectively. Interestingly, we have observed that the solvent vibrational modes νS=O and νCSC may also represent ultrafast structural dynamics at the frequencies for its "free" or "aggregated" species. From the kinetic analysis of the νS=O and νCSC modes of DMSO, the ultrafast changes in the solvation or intermolecular interactions between DMSO molecules initiated by the structural changes of solute molecules have been thoroughly investigated. We propose that the solvent vibrational modes νS=O and νCSC of DMSO can be used as a "sensor" for ultrafast chemical reactions accompanying the structural changes and subsequent solute-solvent interactions.

Ultra-High to Ultra-Low Drug-Loaded Micelles: Probing Host-Guest Interactions by Fluorescence Spectroscopy

Polymer micelles are an attractive means to solubilize water insoluble compounds such as drugs. Drug loading, formulations stability and control over drug release are crucial factors for drug-loaded polymer micelles. The interactions between the polymeric host and the guest molecules are considered critical to control these factors but typically barely understood. Here, we compare two isomeric polymer micelles, one of which enables ultra-high curcumin loading exceeding 50 wt.%, while the other allows a drug loading of only 25 wt.%. In the low capacity micelles, steady-state fluorescence revealed a very unusual feature of curcumin fluorescence, a high energy emission at 510 nm. Time-resolved fluorescence upconversion showed that the fluorescence life time of the corresponding species is too short in the high-capacity micelles, preventing an observable emission in steady-state. Therefore, contrary to common perception, stronger interactions between host and guest can be detrimental to the drug loading in polymer micelles.

DNA-binding and in vitro cytotoxic activity of platinum(II) complexes of curcumin and caffeine

Three Pt(II) complexes containing the natural ligands curcumin and caffeine, namely [Pt(curc)(PPh₃)₂]Cl (1), [PtCl(curc)(DMSO)] (2) (curc = deprotonated curcumin) and trans-[Pt(caffeine)Cl₂(DMSO)] (3), were synthesized and fully characterized. The data obtained suggest that, for both 1 and 2, the anion of curcumin is coordinated to the platinum ion via the oxygen atoms of the β-diketonate moiety. Spectroscopic features reveal that in 2 and 3, a DMSO molecule is S-bonded to the metal centre. For 3, all data indicate a square-planar geometry formed by a 9-N bonded caffeine, two trans chloride anions and a DMSO. The three complexes undergo changes in solution upon incubation for 24 h; 1 and 2 release curcumin while 3 isomerizes from trans to cis configuration. The DNA-binding and cytotoxic properties of 1-3 were evaluated in vitro. Despite their structural similarity, curcuminate-containing 1 and 2 exhibit distinct DNA interactions. While 1 appears to intercalate between nucleobase pairs, inducing the oxidative degradation of the biomolecule, 2 behaves as a groove binder, by means of electrostatic forces. Caffeine-containing 3 exhibits a behaviour that is comparable to that of 2. Complexes 1 and 2 showed moderate to high cytotoxicity and selectivity against several cancer cell lines, while 3 is inactive. Compounds 1 and 2 can be further activated by visible-light irradiation.

Quantitative micro-Raman analysis of micro-particles in drug delivery

Polymeric micro and nanoconstructs are emerging as promising delivery systems for therapeutics and contrast agents in microcirculation. Excellent assets associated with polymeric particulates of tunable shape, size, mechanical and chemical properties may improve the efficiency of delivery and represent the basis of personalized medicine and treatment. Nevertheless, lack of effective techniques of analysis may limit their use in biomedicine and bioengineering. In this paper, we demonstrated Raman Spectroscopy for quantitative characterization of poly lactic-co-glycolic acid (PLGA) micro-plate drug delivery systems. To do so, we (i) acquired bi-dimensional Raman maps of PLGA micro-plates loaded with curcumin at various times of release over multiple particles. We (ii) realized an exploratory analysis of data using the principal component analysis (PCA) technique to find hidden patterns in the data and reduce the dimensionality of the system. Then, we (iii) used an innovative univariate method of analysis of the reduced system to derive quantitative drug release profiles. High performance liquid chromatography (HPLC), the consolidated method of analysis of macro-sized systems, was used for comparison. We found that our system is as efficient as HPLC but, differently from HPLC, it enables quantitative analysis of systems at the single particle level.

Interplay between conformational and solvent effects in UV-visible absorption spectra: curcumin tautomers as a case study

We present a combined theoretical and experimental study on the UV-vis spectra of enol-keto (EK) and keto-keto (KK) tautomeric forms of curcumin dissolved in aqueous solution. Solvent effects have been investigated by resorting to the implicit polarizable continuum model (QM/PCM) and non-polarizable and fully polarizable QM/MM approaches, the latter based on the fluctuating charges (FQ) force-field. In particular, all methods are challenged to rationalize the contribution of conformational, electrostatic and polarization effects in the calculation of the vertical excitation spectra of curcumin tautomers. The obtained results highlight that for both tautomers specific solute-solvent hydrogen-bond interactions play a minor role with respect to conformational and electrostatic effects.

Unravelling the Excellent Chemical Stability and Bioavailability of Solvent Responsive Curcumin-Loaded 2-Ethyl-2-oxazoline-grad-2-(4-dodecyloxyphenyl)-2-oxazoline Copolymer Nanoparticles for Drug Delivery

A new gradient copolymer has been synthesized by the living cationic ring-opening polymerization of hydrophilic 2-ethyl-2-oxazoline with lipophilic 2-(4-dodecyloxyphenyl)-2-oxazoline (EtOx-grad-DPOx). The prepared copolymer is capable of assembling in water to yield polymeric nanoparticles that are successfully loaded with an anticancer agent, curcumin. Self-assembly of the copolymer was found to be tuned by the polarity as well as the hydrogen bonding ability of solvents. Solvent took distinctive role in the preparation of unloaded and curcumin-loaded nanoparticles. The stability of the nanoparticles was increased by curcumin loading promoted by curcumin-polymer interactions. Further, the chemical stability of curcumin in water is largely enhanced inside the polymeric nanoparticles. Curcumin-loaded (EtOx-grad-DPOx) copolymer nanoparticles showed excellent stability in the biological medium, low cytotoxicity, and concentration dependent uptake by U87 MG and HeLa cells, which indicate the possibility of their efficient application in drug delivery.

Excited-state dynamics of the medicinal pigment curcumin in a hydrogel

Curcumin is a yellow polyphenol with multiple medicinal effects. These effects, however, are limited due to its poor aqueous stability and solubility. A hydrogel of 3% octadecyl randomly substituted polyacrylate (PAAC18) has been shown to provide high aqueous stability for curcumin under physiological conditions, offering a route for photodynamic therapy. In this study, the excited-state photophysics of curcumin in the PAAC18 hydrogel is investigated using a combination of femtosecond transient absorption and fluorescence upconversion spectroscopy. The transient absorption results reveal a multiexponential decay in the excited-state kinetics with fast (1 ps & 15 ps) and slow (110 ps & ≈5 ns) components. The fast decay component exhibits a deuterium isotope effect with D₂O in the hydrogel, indicating that the 15 ps decay component is attributable to excited-state intramolecular hydrogen atom transfer of curcumin in the PAAC18 hydrogel. In addition, solvent reorganisation of excited-state curcumin is investigated using multiwavelength femtosecond fluorescence upconversion spectroscopy. The results show that the dominant solvation response (τ = 0.08 ps) is a fast inertial motion owing to the presence of bulk-like water in the vicinity of the hydrophobic octadecyl substituents of the PAAC18 hydrogel. The results also show an additional response with longer time constants of 1 and 6 ps, which is attributable to translational diffusion of confined water molecules in the three-dimensional, cross-linking network of the octadecyl substituents of PAAC18. Overall, we show that excited-state intramolecular hydrogen atom transfer and solvent reorganisation are major photophysical events for curcumin in the PAAC18 hydrogel.

Anomalous Dynamics in tert-Butyl Alcohol-Water and Trimethylamine N-Oxide-Water Binary Mixtures: A Femtosecond Transient Absorption Study

In this article, we have investigated the unusual dynamics of tert-butyl alcohol (TBA)-water and trimethylamine N-oxide (TMAO)-water binary mixtures using solvation dynamics as a tool. For this purpose, femtosecond transient absorption spectroscopy has been employed. Although these two molecules are isosteres to each other, a significant difference in water dynamics has been observed. The solvation times in TBA-water binary mixtures are found to be between 1.5 and 15.5 ps. On the contrary, we have observed very fast dynamics in TMAO-water binary mixtures (between 210 and 600 fs). Interestingly, unusual retardation in dynamics is observed at 0.10 mole fraction of TBA and TMAO in both the binary mixtures.

Unveiling Structural Motions of a Highly Fluorescent Superphotoacid by Locking and Fluorinating the GFP Chromophore in Solution

Superphotoacidity involves ultrafast proton motions implicated in numerous chemical and biological processes. We used conformational locking and strategic addition of electron-withdrawing substituents to synthesize a new GFP chromophore analogue: p-HO-3,5-diF-BDI:BF₂ (diF). It is highly fluorescent and exhibits excited-state proton transfer (ESPT) in various solvents, placing it among the strongest photoacids. Tunable femtosecond stimulated Raman spectroscopy with unique resonance conditions and transient absorption are complementarily employed to elucidate the structural basis for superphotoacidity. We reveal a multistep ESPT reaction from diF to methanol with an initial proton dissociation on the ∼600 fs time scale that forms a charge-separated state, stabilized by solvation, and followed by a diffusion-controlled proton transfer on the ∼350 ps time scale. A ∼1580 cm^-1 phenolic ring motion is uncovered to accompany ESPT before 1 ps. This study provides a vivid movie of the photoinduced proton dissociation of a superphotoacid with bright fluorescence, effectively bridging fundamental mechanistic insights to precise control of macroscopic functions.

Optical Spectroscopic and Morphological Characterizations of Curcuminized Silk Biomaterials: A Perspective from Drug Stabilization

Silk protein fibroins have gained remarkable attention in recent years as a potential drug carrier in the developing medicinal field of research. In this work, the stability of anticancer agent curcumin in the presence of two different silk protein fibroins from nonmulberry Antheraea mylitta (Am) and mulberry Bombyx mori (Bm) has been examined, and the possible mechanism of stabilization in a physiologically relevant medium has also been explored. In solution phase, upon treatment with curcumin, the predominated β-sheet structure of Am is marginally altered, whereas in the case of Bm, a substantial structural changeover has been observed (from coil to β-sheet) to accommodate the hydrophobic drug. Also, the morphological assessments suggest that curcumin is nicely housed in the nanoscaffold of silk fibroin (SF). Consequently, the extent of degradation of curcumin is remarkably suppressed upon encapsulation with the SF. The dissimilarity in the binding patterns of curcumin with these silk proteins could be responsible for the observed difference in the stability orders. Curcumin binds the surface of Bm, whereas in Am, the drug is incorporated in the hydrophobic cavity, and as a consequence, the drug is effectively sequestered out of the aqueous medium. The increase in the fluorescence quantum yield upon interaction with the protein greatly modulates the excited-state intermolecular hydrogen atom transfer (ESIPT) process, which is in tune with a substantial increase in the lifetime of the excited-state of curcumin. The ESIPT is known to play a crucial role in the degradation of curcumin under physiological pH conditions, which perhaps implies its potential therapeutic activity in the presence of silk. The in-depth spectroscopic analyses of curcumin-SF complexes in aqueous medium can provide useful insights for further applicative developments in bioengineering.

J-Type Heteroexciton Coupling Effect on an Asymmetric Donor-Acceptor-Donor-Type Fluorophore

The novel donor-acceptor-donor (D-A-D)-type fluorophore with an asymmetric structure is reported. The twisted-induced charge transfer (TICT) luminescence was observed. The degree of charge transfer and radiative rate constant in the luminescence increased simultaneously with increase in orientational polarizability of solvents. In contrast to the numerous CT fluorophore researches, this behavior has never been previously observed. This characteristic behavior reveals the existence of an effective exciton coupling between the CT states in the donor-acceptor-donor-type fluorophore for the first time.

Localization and dynamics of the anticarcinogenic curcumin with GM1 and other miceller assemblies

Structural transitions involving shape changes play an important role in cellular physiology and enhance the bioavailability of the natural food like curcumin in surfactant aggregates. In this work, we have studied the localization, dynamics and stability of curcumin in various miceller assemblies using a combination of absorbance and fluorescence spectroscopic approaches. The measurements of absorption and fluorescence spectra of curcumin revealed that the nature of interactions of ionic and nonionic surfactants and the glycosphingolipid, GM₁ with curcumin is significantly different with surfactant concentrations. At low concentrations of SDS and the GM₁ the head group of SDS and GM₁ binds to the central β-diketone group of curcumin to form SDS-curcumin or GM₁-curcumin complexes. At high concentrations, both formed micelles with curcumin completely solubilized inside. Cucurmin is solubilized in the stern layer of SDS micelles. Compared to spherical micelles, rod shaped micelles allow more curcumin to bind through hydrophobic interactions indicated by higher absorption and fluorescence, enhanced partition coefficient and stability. Whereas curcumin associates with GM₁ micelles with lower partition coefficient, solubility and remain closer to aqueous phase decreasing its bioavailability and stability. While cucurmin is solubilized in the palisade layer of deoxycholate and octyl glucopyranoside micelles through the alkyl chains providing more hydrophobic microenvironment to curcumin with enhanced stability and bioavailability. Graphical abstract Schematic diagram of the two different types of detergent micelles and larger GM1 micelles.

Effect of Bilayer Partitioning of Curcumin on the Adsorption and Transport of a Cationic Dye Across POPG Liposomes Probed by Second-Harmonic Spectroscopy

The effect of Curcumin partitioning into the bilayer during the adsorption and transport of a cationic dye, LDS, across a negatively charged POPG bilayer was investigated by the interface-selective second-harmonic (SH) spectroscopic technique. The intensity of SH electric field (E_2ω) arising due to LDS adsorbed on the outer bilayer of the POPG liposome was observed to increase instantaneously (<1 s) following the addition of Curcumin. The fractional increase in the SH electric field (E^f_2ω) and the bilayer transport rates (k_T) of LDS were studied with respect to the pH of the solution and also with the Curcumin content in the lipid bilayer. Results obtained indicate that compared with the anionic form of the drug, its neutral form is more conducive of increasing the E^f_2ω of LDS. With increasing Curcumin content in the lipid bilayer, two distinct regimes could be observed in terms of E^f_2ω and k_T values of LDS. For Curcumin:Lipid (C/L) ratio ≤0.02, the E^f_2ω of LDS increases rapidly, while k_T remains unchanged; and for C/L ratio ≥0.02, the E^f_2ω values remains more or less constant, while there is a significant (∼40 times) increase followed by a modest increase in the k_T values of LDS. The observed results support an earlier two-state binding model of Curcumin with the POPG bilayer. In addition, it is further proposed that at low C/L ratio Curcumin binds to the surface of the bilayer replacing the counterions (Na⁺) bound to the lipid head groups, which changes the bilayer surface charge density, thereby causing more LDS cations to adsorb on the bilayer surface. At high C/L ratio, Curcumin intercalates within the hydrophobic domain of the bilayer, altering its hydrophobicity and inducing enhanced transport of the LDS cation. Results presented in this work provide further insights into how Curcumin alters bilayer properties when it partitions from the aqueous to the bilayer phase.

Nanoprecipitation and Spectroscopic Characterization of Curcumin-Encapsulated Polyester Nanoparticles

Curcumin-encapsulated polyester nanoparticles (Cur-polyester NPs) of approximately 100 nm diameter with a negatively charged surface were prepared using a one-step nanoprecipitation method. The Cur-polyester NPs were prepared using polylactic acid, poly(D,L-lactic-co-glycolic acid) and poly(ϵ-caprolactone) without any emulsifier or surfactant. The encapsulation of curcumin in these polyester NPs greatly suppresses curcumin degradation in the aqueous environment due to its segregation from water. In addition, the fluorescence of curcumin in polyester NPs has a quantum yield of 4 to 5%, which is higher than that of curcumin in micellar systems and comparable to those in organic solvents, further supporting the idea that the polyester NPs are capable of excluding water from curcumin. Furthermore, the results from femtosecond fluorescence upconversion spectroscopy reveal that there is a decrease in the signal amplitude corresponding to solvent reorganization of excited state curcumin in the polyester NPs compared with curcumin in micellar systems. The Cur-polyester NPs also show a lack of deuterium isotope effect in the fluorescence lifetime. These results indicate that the interaction between curcumin and water in the polyester NPs is significantly weaker than that in micelles. Therefore, the aqueous stability of curcumin is greatly improved due to highly effective segregation from water. The overall outcome suggests that the polyester NPs prepared using the method reported herein are an attractive system for encapsulating and stabilizing curcumin in the aqueous environment.

Use of Time-Resolved Fluorescence to Monitor Bioactive Compounds in Plant Based Foodstuffs

The study of compounds that exhibit antioxidant activity has recently received much interest in the food industry because of their potential health benefits. Most of these compounds are plant based, such as polyphenolics and carotenoids, and there is a need to monitor them from the field through processing and into the body. Ideally, a monitoring technique should be non-invasive with the potential for remote capabilities. The application of the phenomenon of fluorescence has proved to be well suited, as many plant associated compounds exhibit fluorescence. The photophysical behaviour of fluorescent molecules is also highly dependent on their microenvironment, making them suitable probes to monitor changes in pH, viscosity and polarity, for example. Time-resolved fluorescence techniques have recently come to the fore, as they offer the ability to obtain more information, coupled with the fact that the fluorescence lifetime is an absolute measure, while steady state just provides relative and average information. In this work, we will present illustrative time-resolved measurements, rather than a comprehensive review, to show the potential of time-resolved fluorescence applied to the study of bioactive substances. The aim is to help assess if any changes occur in their form, going from extraction via storage and cooking to the interaction with serum albumin, a principal blood transport protein.

Effect of encapsulation of curcumin in polymeric nanoparticles: how efficient to control ESIPT process?

This paper demonstrates the photophysics of curcumin inside polymeric nanoparticles (NPs), which are being recently used as targeted drug delivery vehicles. For this purpose, we have prepared three polymeric NPs by ultrasonication method from three well-defined water-insoluble random copolymers. These copolymers having various degrees of hydrophobicity were synthesized via reversible addition-fragmentation transfer (RAFT) method using styrene and three different functional monomers, namely, 2-hydroxyethyl acrylate, 4-formylphenyl acrylate, and 4-vinylbenzyl chloride. The photophysics of the curcumin molecules inside the polymeric NPs have been monitored by applying tools like steady state and time-resolved fluorescence spectroscopy. An increase in fluorescence intensity along with an increase in the lifetime values indicated a perturbation of the excited state intramolecular proton transfer (ESIPT) process of curcumin inside the polymeric NPs.