New Insights on the Photophysical Behavior of PRODAN in Anionic and Cationic Reverse Micelles: From Which State or States Does It Emit?

PRODAN Photophysics as a Tool to Determine the Bilayer Properties of Different Unilamellar Vesicles Composed of Phospholipids

Vesicles formed by phospholipids are promising candidates for drug delivery. It is known that the lipid composition affects properties such as the rigidity-fluidity of the membrane and that it influences the bilayer permeability, but sometimes sophisticated techniques are selected to monitor them. In this work, we study the bilayer of different unilamellar vesicles composed of different lipids (1,2-dioleoyl-sn-glycero-3-phosphocholine, DOPC, and lecithin) and diverse techniques such as extruder and electrospun templates and using 6-propionyl-2-(N,N-dimethyl) aminonaphthalene (PRODAN) and its photophysics. Moreover, we were able to monitor the influence of cholesterol on the bilayers. We demonstrate that the bilayer properties can be evaluated using the emission feature of the molecular probe PRODAN. This fluorescent probe gives relevant information on the polarity and fluidity of the microenvironment for unilamellar vesicles formed by two different methods. The PRODAN emission at 434 nm suggests that the bilayer properties significantly change if DOPC or lecithin is used in the vesicle preparation especially in their fluidity. Moreover, cholesterol induces alterations in the bilayer's structural and microenvironmental properties to a greater or lesser degree in both vesicles. Thus, we propose an easy and elegant way to evaluate physicochemical properties, which is fundamental for manufacturing vesicles as a drug delivery system, simply by monitoring the molecular probe emission band centered at 434 nm, which corresponds to the PRODAN species deep inside the bilayer.

Multiplicity of solvent environments in lipid bilayer revealed by DAS deconvolution of twin probes: Comparative method of Laurdan and Prodan

Laurdan and Prodan were designed for the evaluation of the surrounding hydration state. When inserted into lipid bilayer systems, both probes are located at different positions and their fluorescence properties are drastically varied, depending on their surrounding environment. In this study, a novel method using the above fluorescence probes was proposed on the basis of fluorescence lifetime (τ) and emission peak (λ), called as τ vs. λ plot, determined by global analysis of their multiple fluorescence decays and deconvolution of these decay-associated spectra. According to the evaluation of τ vs. λ plot, the existence of multiple fluorescence components in the membrane was revealed. In addition, their fluorescence distribution properties, described on τ vs. λ plot, of each probe tended to correspond to the phase state and vertical direction of the lipid membrane. To assess the contribution of environmental effect to each distribution, we defined the region in the τ vs. λ plot, which was modeled from a series of solvent mixtures (hexane, acetone, ethanol and water) to emulate the complex environment in the 1,2-dipalmitoyl-sn-glycero-3-phosphocholine bilayer system. The distributions of fluorescence components of Laurdan and Prodan in lipid membranes were classified into each solvent species, and Prodan partition into bulk water was distinguished. The sensitivity of Prodan to the phase pretransition of the 1,2-dipalmitoyl-sn-glycero-3-phosphocholine bilayer system was also observed in increasing the temperature. Noticeably, most of the fluorescence components was assigned to the solvent model, except for a single component that has longer lifetime and shorter emission wavelength. This component was dominant in solid-ordered phase; hence, it is assumed to be a specific component in lipid membranes that cannot be represented by solvents. Although these are still qualitative analytical methods, the unique approach proposed in this study provides novel insights into the multi-focal property of the membrane.

Solvent-Dependent Excited-State Evolution of Prodan Dyes

Excited-state character and dynamics of two 6-(dimethylamino)-2-acylnaphthalene dyes (Prodan and Badan-SCH₂CH₂OH) were studied by picosecond time-resolved IR spectroscopy (TRIR) in solvents of different polarity and relaxation times: hexane, CD₃OD, and glycerol-d₈. In all these solvents, near-UV excitation initially produced the same S₁(ππ*) excited state characterized by a broad TRIR signal. A very fast decay (3, ∼100 ps) followed in hexane, whereas conversion to a distinct IR spectrum with a ν(C═O) band downshifted by 76 cm^-1 occurred in polar/H-bonding solvents, slowing down on going from CD₃OD (1, 23 ps) to glycerol-d₈ (5.5, 51, 330 ps). The final relaxed excited state was assigned as planar Me₂N → C═O intramolecular charge transfer S₁(ICT) by comparing experimental and TDDFT-calculated spectra. TRIR conversion kinetics are comparable to those of early stages of multiexponential fluorescence decay and dynamic fluorescence red-shift. This work presents a strong evidence that Prodan-type dyes undergo solvation-driven charge separation in their S₁ state, which is responsible for the dynamic fluorescence Stokes shift observed in polar/H-bonding solvents. The time evolution of the optically prepared S₁(ππ*) state to the S₁(ICT) final state reflects environment relaxation and solvation dynamics. This finding rationalizes the widespread use of Prodan-type dyes as probes of environment dynamics and polarity.

A new interpretation of the absorption and the dual fluorescence of Prodan in solution

Remarkable interest is associated with the interpretation of the Prodan fluorescent spectrum. A sequential hybrid Quantum Mechanics/Molecular Mechanics method was used to establish that the fluorescent emission occurs from two different excited states, resulting in a broad asymmetric emission spectrum. The absorption spectra in several solvents were measured and calculated using different theoretical models presenting excellent agreement. All theoretical models [semiempirical, time dependent density functional theory and and second-order multiconfigurational perturbation theory] agree that the first observed band at the absorption spectrum in solution is composed of three electronic excitations very close in energy. Then, the electronic excitation around 340 nm-360 nm may populate the first three excited states (π-π^*L_b, n-π^*, and π-π^*L_a). The ground state S₀ and the first three excited states were analyzed using multi-configurational calculations. The corresponding equilibrium geometries are all planar in vacuum. Considering the solvent effects in the electronic structure of the solute and in the solvent relaxation around the solute, it was identified that these three excited states can change the relative order depending on the solvent polarity, and following the minimum path energy, internal conversions may occur. A consistent explanation of the experimental data is obtained with the conclusive interpretation that the two bands observed in the fluorescent spectrum of Prodan, in several solvents, are due to the emission from two independent states. Our results indicate that these are the n-π^* S₂ state with a small dipole moment at a lower emission energy and the π-π^*L_b S₁ state with large dipole moment at a higher emission energy.

Discriminatory Interaction Behavior of Lipid Vesicles toward Diversely Emissive Carbon Dots Synthesized from Ortho, Meta, and Para Isomeric Carbon Precursors

Photoluminescent carbon dots (C-dots) are widely used for bioimaging techniques to study different cellular processes. However, biocompatibility of C-dots is crucial because the wrong selection of C-dots may lead to an adverse effect on a particular cellular process. Herein, we investigate the interaction of zwitterionic lipid vesicles with photoluminescent C-dots derived from different isomeric (ortho, meta, and para) precursors of phenylenediamine (PDA) by spectroscopic and microscopic imaging techniques as well as dynamic light scattering methods. The study reveals that interaction of lipid vesicles with C-dots is highly dependent on the properties of the isomeric precursors. We find that vesicles retain their morphology upon interaction with ortho C-dots (oCD). The microscopic images reveal that oCD are selectively embedded in the lipid vesicles and can effectively be used for imaging purpose. On the contrary, meta and para C-dots (mCD and pCD) being located on the interfacial region induce aggregation in the vesicles. We explain the observation in terms of the location of the C-dots on the lipid vesicles, their electrostatic attraction at the vesicle interface, possible cross-linking with other vesicles and different hydration features of the isomeric precursors of the C-dots. The study may be helpful in understanding the interactions and attachment processes of C-dots at the interface of biological membranes.

Quantitative Design of Bright Fluorophores and AIEgens by the Accurate Prediction of Twisted Intramolecular Charge Transfer (TICT)

Inhibition of TICT can significantly increase the brightness of fluorescent materials. Accurate prediction of TICT is thus critical for the quantitative design of high-performance fluorophores and AIEgens. TICT of 14 types of popular organic fluorophores were modeled with time-dependent density functional theory (TD-DFT). A reliable and generalizable computational approach for modeling TICT formations was established. To demonstrate the prediction power of our approach, we quantitatively designed a boron dipyrromethene (BODIPY)-based AIEgen which exhibits (almost) barrierless TICT rotations in monomers. Subsequent experiments validated our molecular design and showed that the aggregation of this compound turns on bright emissions with ca. 27-fold fluorescence enhancement, as TICT formation is inhibited in molecular aggregates.

Interaction of aliphatic amino acids with zwitterionic and charged lipid membranes: hydration and dehydration phenomena

Aliphatic amino acids interact differently in order to induce gelation or fluidization in zwitterionic and charged lipid membranes as a result of hydration or dehydration of the membrane surface.

Effect of Surface Ligand and Temperature on Lipid Vesicle-Gold Nanoparticle Interaction: A Spectroscopic Investigation

We herein investigate the interactions of differently functionalized anionic and cationic gold nanoparticles (AuNPs) with zwitterionic phosphocholine (PC) as well as inverse phosphocholine (iPC) lipid bilayers via spectroscopic measures. In this study, we used PC lipids with varying phase-transition temperatures, i.e., DMPC ( T_m = 24 °C), DOPC ( T_m = -20 °C), and iPC lipid DOCP ( T_m = -20 °C) to study their interactions with AuNPs functionalized with anionic ligands citrate, 3-mercaptopropionic acid, glutathione, and cationic ligand cysteamine. We studied the interactions by steady-state and time-resolved spectroscopic studies using membrane-sensitive probes 6-propionyl-2-dimethylaminonaphthalene (PRODAN) and 8-anilino-1 naphthalenesulfonate (ANS), as well as by confocal laser scanning microscopy (CLSM) imaging and dynamic light scattering (DLS) measurements. We observe that AuNPs bring in stability to the lipid vesicle, and the extent of interaction differs with the different surface ligands on the AuNPs. We observe that AuNPs functionalized with citrate effectively increase the phase-transition temperature of the vesicles by interacting with them. Our study reveals that the extent of interaction depends on the bulkiness of the ligands attached to the AuNPs. The bulkier ligands exert less van der Waals force, resulting in a weaker interaction. Moreover, we find that the interactions are more strongly pronounced when the vesicles are near the phase-transition temperature of the lipid.  The CLSM imaging and DLS measurements demonstrate the surface modifications in the vesicles as a result of these interactions.

Excited-state prototropism of 7-hydroxy-4-methylcoumarin in [Cnmim][BF4] series of ionic liquid–water mixtures: insights on reverse micelle-like water nanocluster formation

This study explores the excited-state prototropic behavior of 7-hydroxy-4-methylcoumarin dye in ionic liquid–water media, to reveal the intriguing reverse micelle formation in these solvent systems.

Determination of Benzyl-hexadecyldimethylammonium 1,4-Bis(2-ethylhexyl)sulfosuccinate Vesicle Permeability by Using Square Wave Voltammetry and an Enzymatic Reaction

This report describes the studies performed to determine the permeability coefficient value (P) of 1-naphthyl phosphate (1-NP) through the benzyl-hexadecyldimethylammonium 1,4-bis(2-ethylhexyl)sulfosuccinate (AOT-BHD) vesicle bilayer. 1-NP was added in the external phase and must cross the bilayer of the vesicle to react with the encapsulated enzyme (alkaline phosphatase) to yield 1-naphtholate (NPh^-), the product of the enzymatic hydrolysis. This product is electrochemically detected, at basic pH value, by a square wave voltammetry technique, which can be a good alternative over the spectroscopic one, to measure the vesicle solutions because scattering (due to its turbidity) does not make any influence in the electrochemical signal. The experimental data allow us to propose a mathematical model, and a value of P = (1.00 ± 0.15) × 10^-9 cm s^-1 was obtained. Also, a value of P = (2.0 ± 0.5) × 10^-9 cm s^-1 was found by using an independent technique, ultraviolet-visible spectroscopy, for comparison. It is evident that the P values obtained from both the techniques are comparable (within the experimental error of both techniques) under the same experimental conditions. This study constitutes the first report of the 1-NP permeability determination in this new vesicle. We want to highlight the importance of the introduction of a new method and the electrochemical response of the product generated through an enzymatic reaction that occurs in the inner aqueous phase of the vesicle, where the enzyme is placed.

Dynamics of different steps of the photopyrolytic cycle of an eminent anticancer drug topotecan inside biocompatible lyotropic liquid crystalline systems

Dynamics of different steps of photopyrolytic processes of an eminent anticancer drug topotecan have been investigated inside different lyotropic liquid crystalline systems.

F. Gold nanoparticles optical properties induced by water and an ionic liquid (bmimBF4) inside cationic reverse micelles

Polar solvent interactions at the interface are a key factor affecting the final properties of nanoparticles, including size, shape and crystallinity.

Electrochemical and photophysical behavior of 1-naphthol in benzyl-n-hexadecyldimethylammonium 1,4-bis(2-ethylhexyl)sulfosuccinate large unilamellar vesicles

The behavior of 1-naphthol and its partition process in LUVs formed from a new catanionic surfactant were studied by electrochemical and spectroscopic techniques.

New insights on the fluorescent emission spectra of Prodan and Laurdan

Prodan and Laurdan are fluorescent probes largely used in biological systems. They were synthetized to be sensitive to the environment polarity, and their fluorescent emission spectrum shifts around 120 nm, from cyclohexane to water. Although accepted that their emission spectrum is composed by two emission bands, the origin of these two bands is still a matter of discussion. Here we analyze the fluorescent spectra of Prodan and Laurdan in solvents of different polarities, both by decomposing the spectrum into two Gaussian bands and by computing the Decay Associated Spectra (DAS), the latter with time resolved fluorescence. Our data show that the intensity of the lower energy emission band of Prodan and Laurdan (attributed, in the literature, to the decay of a solvent relaxed state) is higher in cyclohexane than in water, showing a decrease as the polarity of the medium increases. Moreover, in all solvents studied here, the balance between the two emission bands is not dependent on the temperature, strongly suggesting two independent excited states. Both bands were found to display a red shift as the medium polarity increases. We propose here a new interpretation for the two emission bands of Prodan and Laurdan in homogeneous solvents: they would be related to the emission of two independent states, and not to a pair of non-relaxed and solvent relaxed states.

Singularities in the physicochemical properties of spontaneous AOT-BHD unilamellar vesicles in comparison with DOPC vesicles

AOT-BHD vesicles present a bilayer completely different to the traditional DOPC vesicles, with low polarity, high viscosity and more electron donor capacity.

Fluorescence quenching of (dimethylamino)naphthalene dyes Badan and Prodan by tryptophan in cytochromes P450 and micelles

Fluorescence of 2-(N,N-dimethylamino)-6-propionylnaphthalene dyes Badan and Prodan is quenched by tryptophan in Brij 58 micelles as well as in two cytochrome P450 proteins (CYP102, CYP119) with Badan covalently attached to a cysteine residue. Formation of nonemissive complexes between a dye molecule and tryptophan accounts for about 76% of the fluorescence intensity quenching in micelles, the rest is due to diffusive encounters. In the absence of tryptophan, fluorescence of Badan-labeled cytochromes decays with triexponential kinetics characterized by lifetimes of about 100 ps, 700–800 ps, and 3 ns. Site mutation of a histidine residue in the vicinity of the Badan label by tryptophan results in shortening of all three decay lifetimes. The relative amplitude of the fastest component increases at the expense of the two slower ones. The average quenching rate constants are 4.5 × 10⁸ s^–1 (CYP102) and 3.7 × 10⁸ s^–1 (CYP119), at 288 K. Cyclic voltammetry of Prodan in MeCN shows a reversible reduction peak at −1.85 V vs NHE that becomes chemically irreversible and shifts positively upon addition of water. A quasireversible reduction at −0.88 V was observed in an aqueous buffer (pH 7.3). The excited-state reduction potential of Prodan (and Badan) is estimated to vary from about +0.6 V (vs NHE) in polar aprotic media (MeCN) to approximately +1.6 V in water. Tryptophan quenching of Badan/Prodan fluorescence in CYPs and Brij 58 micelles is exergonic by ≤0.5 V and involves tryptophan oxidation by excited Badan/Prodan, coupled with a fast reaction between the reduced dye and water. Photoreduction is a new quenching mechanism for 2-(N,N-dimethylamino)-6-propionylnaphthalene dyes that are often used as solvatochromic polarity probes, FRET donors and acceptors, as well as reporters of solvation dynamics.

Electric dipole moments of the fluorescent probes Prodan and Laurdan: experimental and theoretical evaluations

Several experimental and theoretical approaches can be used for a comprehensive understanding of solvent effects on the electronic structure of solutes. In this review, we revisit the influence of solvents on the electronic structure of the fluorescent probes Prodan and Laurdan, focusing on their electric dipole moments. These biologically used probes were synthesized to be sensitive to the environment polarity. However, their solvent-dependent electronic structures are still a matter of discussion in the literature. The absorption and emission spectra of Prodan and Laurdan in different solvents indicate that the two probes have very similar electronic structures in both the ground and excited states. Theoretical calculations confirm that their electronic ground states are very much alike. In this review, we discuss the electric dipole moments of the ground and excited states calculated using the widely applied Lippert-Mataga equation, using both spherical and spheroid prolate cavities for the solute. The dimensions of the cavity were found to be crucial for the calculated dipole moments. These values are compared to those obtained by quantum mechanics calculations, considering Prodan in vacuum, in a polarizable continuum solvent, and using a hybrid quantum mechanics-molecular mechanics methodology. Based on the theoretical approaches it is evident that the Prodan dipole moment can change even in the absence of solute-solvent-specific interactions, which is not taken into consideration with the experimental Lippert-Mataga method. Moreover, in water, for electric dipole moment calculations, it is fundamental to consider hydrogen-bonded molecules.

Interaction of human serum albumin with liposomes of saturated and unsaturated lipids with different phase transition temperatures: a spectroscopic investigation by membrane probe PRODAN

The interaction of human serum albumin (HSA) with liposomes made of saturated and unsaturated phosphocholines has been studied using circular dichroism (CD), steady state and time resolved fluorescence spectroscopic techniques.

PRODAN dual emission feature to monitor BHDC interfacial properties changes with the external organic solvent composition

We have investigated the water/benzyl-n-hexadecyldimethylammonium chloride (BHDC)/n-heptane:benzene reverse micelles (RMs) interfaces properties using 6-propionyl-2-(N,N-dimethyl)aminonaphthalene, PRODAN, as molecular probe. We have used absorption and emission (steady-state and time-resolved) spectroscopy of PRODAN to monitor the changes in the RMs interface functionalities upon changing the external organic solvent blend. We demonstrate that PRODAN is a useful probe to investigate how the external solvent composition affects the micelle interface properties. Our results show that changes in the organic solvent composition in water/BHDC/n-heptane:benzene RMs have a dramatic effect on the photophysics of PRODAN. Thus, increasing the aliphatic solvent content over the aromatic one produces PRODAN partition and PRODAN intramolecular electron transfer (ICT) processes. Additionally, the water presence in these RMs makes the PRODAN ICT process favored with the consequent decreases in the LE emission intensity and a better definition of the charge transfer (CT) band. All this evidence suggests that the benzene molecules are expelled out of the interface, and the water-BHDC interactions are stronger with more presence of water molecules in the polar part of the interface. Thus, we demonstrate that a simple change in the composition of the external phase promotes remarkable changes in the RMs interface. Finally, the results obtained with PRODAN together with those reported in a previous work in our lab reveal that the external phase is important when trying to control the properties of RMs interface. It should be noted that the external phase itself, besides the surfactant and the polar solvent sequestrated, is a very important control variable that can play a key role if we consider smart application of these RMs systems.

Förster resonance energy transfer among a structural isomer of adenine and various Coumarins inside a nanosized reverse micelle

In this article we have studied Förster Resonance Energy Transfer (FRET) using 2-aminopurine (2-AP), a structural isomer of adenine as donor and various Coumarins as acceptors inside AROSOL-OT (AOT)-water reverse micelles (RM) using steady-state and time-resolved fluorescence spectroscopies. We have used three sets of FRET and all the pairs except 2-AP-Coumarin-480 exhibited quite efficient FRET. For the efficient pairs, overlap integral J(λ) and Förster distance (R0) are of high values but the rate constant of energy transfer (kET) are quite low. The rate is gradually amplified with increase in water content for the 2-AP-Coumarin-440 pair while the reverse is observed for 2-AP-Coumarin-460. In future our FRET pair can be used in more modified and sophisticated confined media such as biomembranes of varying size, physical properties and chemical compositions etc.

Influence of heterogeneity of confined water on photophysical behavior of acridine with amines: a time-resolved fluorescence and laser flash photolysis study

The photophysical behavior of acridine (Acr) shows facilitated water-assisted protonation equilibrium between its deprotonted (Acr* ∼ 10 ns) and protonated forms (AcrH(+*) ∼ 28 ns) within confined region of ordered water molecules inside AOT/H(2)O/n-heptane reverse micelles (RMs). The time-resolved-area-normalized-emission spectra confirm both Acr* and AcrH(+*), while time-resolved-emission spectra depict time evolution between them. Quenching of AcrH(+*) with N,N-dimethylaniline (DMA) is a purely diffusion-controlled bimolecular quenching with linear Stern-Volmer (S-V) plot, while nonlinearity arises with triethylamine (TEA) that forms ground state complex with AcrH(+) (AcrH(+)··H(2)O··TEA) indicating both static and dynamic quenching. Transient intermediates, DMA(•+) and AcrH(•) infer photoinduced electron transfer from DMA to Acr, while those from AcrH(+)··H(2)O··TEA complex suggest water mediated excited-state proton transfer (ESPT) between AcrH(+) and TEA. The ESPT becomes faster in larger RMs due to enhanced mobility of hydronium ions in AcrH(+)··H(2)O··TEA, which reduces in smaller RMs as water becomes much more constrained owing to stronger complexation by excess confinement.

Microviscosity inside a nanocavity: a femtosecond fluorescence up-conversion study of malachite green

Femtosecond fluorescence up-conversion measurements of malachite green (MG) have been carried out to confirm the relaxation pathway and subsequently to probe the microviscosity of water trapped in a nanoconfined environment using an AOT (sodium dioctylsulfosuccinate, aerosol-OT) reverse micelle as a model system. The experimental results reveal a strong dependence of S(1) state relaxation dynamics of MG on solvent viscosity while a very weak dependence has been observed for the S(2) state relaxation. The time-dependent density functional theory (TD-DFT) calculations have been used to construct potential energy surfaces of MG by pursuing an intramolecular rotation along the torsional coordinate of the phenyl rings. On synchronization with the experimental observations, the computational results comprehend the existence of a conical intersection along the S(1) and S(0) potential energy surfaces, which leads to mixed vibrational levels of S(1) and S(0) characteristics. The results suggest that the conical intersection is along the torsional coordinate of N,N-dimethyl substituted phenyl ring. Correlating the observed dynamics of MG in a confined system with the relaxation time of MG in different glycerol-water mixtures, we assert the determination of the microviscosity of water inside the AOT reverse micelle. The data confer that the microviscosity of water in an AOT water pool of w(0) = 2 (9 cP) is almost 9 times higher than the bulk water. As we increase the w(0) from 2 to 40, the microviscosity decreases monotonically to 5.68 cP, and the decrease is observed to be exponential in nature.