Excited State Dipole Moment of PRODAN as Determined from Transient Dielectric Loss Measurements

Fluorescence of Half-Twisted 10-Acyl-1-methyltetrahydrobenzoquinolines

The steric interference of proximal dialkyl amino and acyl groups at the peri (1,8) positions of naphthalene affects the intramolecular charge transfer fluorescence. Previous studies indicate that acyl and freely rotating dimethyl amino groups twist toward coplanarity with the naphthalene ring in the excited state. The present study examines the effect of constraining the amino group in a ring. The photophysical properties of 2,2-dimethyl-1-(1-methyl-1,2,3,4-tetrahydrobenzo[h]quinolin-10-yl)propan-1-one (4), ethyl 1-methyl-1,2,3,4-tetrahydrobenzo[h]quinoline-10-carboxylate (5), and 1-methyl-1,2,3,4-tetrahydrobenzo[h]quinoline-10-carbaldehyde (6) are compared with the dimethyl amino derivatives 2 and 3. Crystal structures of 4-6 show that the amine ring adopts a chair conformation, where the N-methyl group is axial. Computational results suggest that the pyramidal amino group planarizes and twists together with the acyl toward coplanarity in the excited state. The ring structure does not thwart the formation of a planar intramolecular charge transfer (PICT) state.

Novel Fabrication of Basalt Nanosheets with Ultrahigh Aspect Ratios Toward Enhanced Mechanical and Dielectric Properties of Aramid Nanofiber-Based Composite Nanopapers

The rapid development of modern electrical equipment has led to urgent demands for electrical insulating materials with mechanical reliability and excellent dielectric properties. Herein, basalt nanosheets (BSNs) with high aspect ratios (≈780.1) are first exfoliated from basalt scales (BS) through a reliable chemical/mechanical approach. Meanwhile, inspired by the layered architecture of natural nacre, nacre-mimetic composite nanopapers are reported containing a 3D aramid nanofibers (ANF) framework as a matrix and BSNs as ideal building blocks through vacuum-assisted filtration. The as-prepared ANF-BSNs composite nanopapers exhibit considerably enhanced mechanical properties with ultralow BSNs content. These superiorities are wonderfully integrated with exceptional dielectric breakdown strength, prominent volume resistivity, and extremely low dielectric constant and loss, which are far superior to conventional nacre-mimetic composite nanopapers. Notably, the tensile strength and breakdown strength of ANF-BSNs composite nanopapers with a mere 1.0 wt% BSNs reach 269.40 MPa and 77.91 kV mm-1 , respectively, representing an 87% and 133% increase compared to those of the control ANF nanopaper. Their properties are superior to those of previously reported nacre-mimetic composite nanopapers and commercial insulating micropapers, indicating that ANF-BSNs composite nanopapers are a highly promising electrical insulating material for miniaturized high-power electrical equipment.

Photophysical Properties of BADAN Revealed in the Study of GGBP Structural Transitions

The fluorescent dye BADAN (6-bromoacetyl-2-dimetylaminonaphtalene) is widely used in various fields of life sciences, however, the photophysical properties of BADAN are not fully understood. The study of the spectral properties of BADAN attached to a number of mutant forms of GGBP, as well as changes in its spectral characteristics during structural changes in proteins, allowed to shed light on the photophysical properties of BADAN. It was shown that spectral properties of BADAN are determined by at least one non-fluorescent and two fluorescent isomers with overlapping absorbing bands. It was found that BADAN fluorescence is determined by the unsolvated "PICT" (planar intramolecular charge transfer state) and solvated "TICT" (twisted intramolecular charge transfer state) excited states. While "TICT" state can be formed both as a result of the "PICT" state solvation and as a result of light absorption by the solvated ground state of the dye. BADAN fluorescence linked to GGBP/H152C apoform is quenched by Trp 183, but this effect is inhibited by glucose intercalation. New details of the changes in the spectral characteristics of BADAN during the unfolding of the protein apo and holoforms have been obtained.

Local Electric Fields in Aqueous Electrolytes

Vibrational Stark shifts were explored in aqueous solutions of organic molecules with carbonyl- and nitrile-containing constituents. In many cases, the vibrational resonances from these moieties shifted toward lower frequency as salt was introduced into solution. This is in contrast to the blue-shift that would be expected based upon Onsager's reaction field theory. Salts containing well-hydrated cations like Mg²⁺ or Li⁺ led to the most pronounced Stark shift for the carbonyl group, while poorly hydrated cations like Cs⁺ had the greatest impact on nitriles. Moreover, salts containing I^- gave rise to larger Stark shifts than those containing Cl^-. Molecular dynamics simulations indicated that cations and anions both accumulate around the probe in an ion- and probe-dependent manner. An electric field was generated by the ion pair, which pointed from the cation to the anion through the vibrational chromophore. This resulted from solvent-shared binding of the ions to the probes, consistent with their positions in the Hofmeister series. The "anti-Onsager" Stark shifts occur in both vibrational spectroscopy and fluorescence measurements.

Modulation of the Belousov-Zhabotinsky Reaction with Lipid Bilayers: Effects of Lipid Head Groups and Membrane Properties

The Belousov-Zhabotinsky (BZ) reaction is an oscillating reaction due to periodic oscillations that happen in the concentration of some intermediates. Such systems can be applied together with hydrophobic membranes to create an autonomous behavior in artificial systems. However, because of a complex set of reactions happening in such systems, the interferences caused by hydrophobic membranes are not easily understood. In this study, we tested lipid membranes composed of trimethylammonium-propane (TAP) and phosphate (PA) lipids in an attempt to break down how the polar region of phosphatidylcholine (PC) lipid membranes affect the BZ reaction. According to our findings, the trimethylammonium group and membrane fluidity are crucial to change the frequency of oscillations in the reaction. In addition, the results also indicate a possible complexation of cerium ions with membranes with a phosphate head group.

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.

Ultrafast Formation of the Charge Transfer State of Prodan Reveals Unique Aspects of the Chromophore Environment

Lipophilic dyes such as laurdan and prodan are widely used in membrane biology due to a strong bathochromic shift in emission that reports the structural parameters of the membrane such as area per molecule. Disentangling of the factors which control the spectral shift is complicated by the stabilization of a charge-transfer-like excitation of the dye in polar environments. Predicting the emission therefore requires modeling both the relaxation of the environment and the corresponding evolution of the excited state. Here, an approach is presented in which (i) the local environment is sampled by a classical molecular dynamics (MD) simulation of the dye and solvent, (ii) the electronically excited state of prodan upon light absorption is predicted by numerical quantum mechanics (QM), (iii) the iterative relaxation of the environment around the excited dye by MD coupled with the evolution of the excited state is performed, and (iv) the emission properties are predicted by QM. The QM steps are computed using the many-body Green's function in the GW approximation and the Bethe-Salpeter equation with the environment modeled as fixed point charges, sampled in the MD simulation steps. The comparison to ultrafast time-resolved transient absorption measurements demonstrates that the iterative molecular mechanics (MM)/QM approach agrees quantitatively with both the polarity-dependent shift in emission and the time scale over which the charge transfer state is stabilized. Together the simulations and experimental measurements suggest that the evolution into the charge transfer state is slower in amphiphilic solvents.

Fluorescence Quenching of Carbonyl-Twisted 5Acyl-1-dimethylaminonaphthalenes by Alcohols

Derivatives of 1-dimethylamino-5-propionylnaphthalene that constrain the carbonyl group into a five-, six-, and seven-membered ring were prepared, and their fluorescence quenching in protic solvents was studied. Evidence for enhanced quenching due to carbonyl twisting out of the molecular plane is presented, but this effect is heavily masked by the strong quenching by all of the derivatives and by the ring size-dependent deactivation seen in polar, aprotic solvents. Calculations show strong, ring size-dependent vibrational coupling between the carbonyl group and the naphthalene ring in the first excited state.

1,5-Prodan Emits from a Planar Intramolecular Charge-Transfer Excited State

1-Propionyl-5-dimethylaminonaphthalene (8, 1,5-Prodan) and two derivatives where the amino group is constrained in a seven-membered (9) and five-membered (10) ring are prepared. All three exhibit strong fluorescence and similar degrees of solvatochromism. Their fluorescence is strongly quenched in alcohol solvents. Because the amino group in 9 and especially 10 is forced to be coplanar with the naphthalene ring, the similar photophysical behavior of all three suggests that emission arises from a planar excited state (planar intramolecular charge transfer).

PRODAN differentially influences its local environment

PRODAN influences its local environment at the nanoscale differently between ordered and disordered phases as shown by MD simulations.

Spectroscopic evidence for hydration and dehydration of lipid bilayers upon interaction with metal ions: a new physical insight

In this manuscript, we investigate the interactions of different metal ions with zwitterionic phospholipid bilayers of different chain lengths using the well-known membrane probe PRODAN and steady state and time resolved fluorescence spectroscopy.

Hydrogen bond strengthening induces fluorescence quenching of PRODAN derivative by turning on twisted intramolecular charge transfer

Researchers have proposed different effective mechanisms of hydrogen bonding (HB) on the fluorescence of 6-propionyl-2-dimethylaminonaphthalene (PRODAN) and its derivatives. Herein, excited state transition and dynamics analysis confirm that the fluorescence of PD (a derivative of PRODAN with ethyl replaced by 3-hydroxy-2,2-dimethylpropan) emits from the planar intramolecular charge transfer (PICT) state rather than twist ICT (TICT) state, because the fluorescence emission and surface hopping from the TICT state to the twist ground (T-S₀) state is energy forbidden. Nevertheless, the strengthening of intramolecular-HB (intra-HB) and intermolecular-HB (inter-HB) of PD-(methanol)₂ smooth the pathway of surface hopping from TICT to T-S₀ state and the external conversion going to planar ground state by decreasing the energy difference of the two states. This smoothing changes the fluorescence state of PD-(methanol)₂ to the TICT state in which fluorescence emission does not occur but surface hopping, leading to the partial fluorescence quenching of PD in methanol solvent. This conclusion is different from previous related reports. Moreover, the inter-HB strengthening of PD-methanol in PICT state induces the cleavage of intra-HB and a fluorescence red-shift of 54nm compared to PD. This red-shift increases to 66nm for PD-(methanol)₂ for the strengthening of the one intra-HB and two inter-HBs. The dipole moments of PD-methanol and PD-(methanol)₂ respectively increase about 10.3D and 8.1D in PICT state compared to PD. The synergistic effect of intra-HB and inter-HB induces partial quenching of PD in methanol solvent by turning on the TICT state and fluorescence red-shift. This work gives a reasonable description on the fluorescence red-shift and partial quenching of PD in methanol solvent, which will bring insight into the study of spectroscopic properties of molecules owning better spectral characteristics.

Supramolecular guest relay using host-protein nanocavities: an application of host-induced guest protonation

Small drug molecules and other important metabolites are delivered via a suitable carrier protein-mediated transport through a specific receptor.

Two-photon voltmeter for measuring a molecular electric field

We present a new approach for determining the strength of the dipolar solute-induced reaction field, along with the ground- and excited-state electrostatic dipole moments and polarizability of a solvated chromophore, using exclusively one-photon and two-photon absorption measurements. We verify the approach on two benchmark chromophores N,N-dimethyl-6-propionyl-2-naphthylamine (prodan) and coumarin 153 (C153) in a series of toluene/dimethyl sulfoxide (DMSO) mixtures and find that the experimental values show good quantitative agreement with literature and our quantum-chemical calculations. Our results indicate that the reaction field varies in a surprisingly broad range, 0-10(7)  V cm(-1) , and that at close proximity, on the order of the chromophore radius, the effective dielectric constant of the solute-solvent system displays a unique functional dependence on the bulk dielectric constant, offering new insight into the close-range molecular interaction.

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.

3-(7-Dimethylamino)coumarin N-phenylsemicarbazones in solution and polymer matrices: Tuning their fluorescence via para-phenyl substitution

The photo-physical properties of five new para-phenyl substituted derivatives of 3-(7-dimethylamino)coumarin N-phenylsemicarbazone with various electron-withdrawing substituents R (RF, Br, CF3, CN or NO2) in the para-position on the phenyl ring were investigated in solvents and in polymer matrices. Tuning their fluorescent properties via para-substitution is discussed in terms of Twisted Intra-molecular Charge-Transfer (TICT) state formation, specific solute-solvent interactions (hydrogen bonding), fluorescent H-aggregates formation, and the solvent polarity and polymer matrix effects.

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.

7-(Dimethylamino)coumarin-3-carbaldehyde and its phenylsemicarbazone: TICT excited state modulation, fluorescent H-aggregates, and preferential solvation

The photophysical properties of 7-(dimethylamino)coumarin-3-carbaldehyde 3 and its phenylsemicarbazone 4 were investigated in solvents of various polarity and in differing solvent mixtures. The different fluorescent quantum yield (ΦF) behavior of 3 and 4 in highly polar solvents is discussed in terms of Twisted Intramolecular Charge-Tranfer (TICT) state formation and the specific solute-solvent interactions. Because of the weak intermolecular hydrogen bonding ability of both the radiative ICT and nonradiative TICT excited state of 3 and the linear steep decrease in ΦF from a medium to high polarity region, coumarin 3 could be a useful polarity probe for microenvironments containing hydrogen bonding groups. Compared to 3, coumarin 4 exhibits the highest ΦF values in highly polar solvents with strong hydrogen bond acceptor ability. The high quantum yield of fluorescence in DMSO, DMF, and alcohols qualifies coumarin 4 as a laser dye in the given medium, with kF higher than k(nr). Contrary to previous reports that many H-aggregates are nonfluorescent in nature, coumarin 3 forms highly fluorescent H-aggregates in MeOH and EtOH. On the basis of the restrictions of the Kasha-exciton theory model, we assume that the formation of fluorescent H-dimer aggregates of 3 is driven by π(+)-π(-) interactions. To the best of our knowledge, this is the first report on aggregation of coumarin dye in alcoholic solutions. In addition, restrictions in the fitting procedure relating to determination of the solvation number, n, using the Covington-Newman model of preferential solvation and also the solvent nonideality parameter, h', are discussed in this article.

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.

Highly Solvatochromic 7-Aryl-3-hydroxychromones

Introduction of the dialkylaminophenyl group in position 7 of 3-hydroxychromone changes the orientation of the excited-state dipole moment and leads to superior solvatochromic properties (>170 nm emission shift in aprotic media). The excited-state intramolecular proton-transfer (ESIPT) reaction of 7-aryl-3-hydroxychromones is almost completely inhibited in most solvents. Methylation of the 3-OH abolishes ESIPT completely and also leads to improved photostability. The probes exhibit a ∼100-fold increase in fluorescence intensity and large Stokes shifts upon binding to membranes, reflecting differences in membrane phase and charge by a >40 nm spread in the emission band position.

Polarity-sensitive fluorescent probes in lipid bilayers: bridging spectroscopic behavior and microenvironment properties

We have studied the emission features of the fluorescent polarity-sensitive probes known as Prodan and Laurdan in a liquid-crystalline DPPC bilayer. To this purpose, we have combined high-level quantum mechanical electronic structure calculations with a molecular field theory for the positional-orientational-conformational distribution of the probes, in their ground and excited states, inside of the lipid bilayer, taking into account at both levels the nonuniformity and anisotropy of the environment. Thus, we can interpret the features of the fluorescence spectra of Prodan and Laurdan in relation to the position and orientation of their chromophore in the bilayer. We have found that the environment polarity is not sufficient to explain the large red shifts experimentally observed and that specific effects due to hydrogen bonding must be considered. We show that the orientation of the probe is important in determining the accessibility to water of the H-bond-acceptor group; in the case of Laurdan interesting conformational effects are highlighted.

What is solvatochromism?

Solvatochromism is commonly used in many fields of chemical and biological research to study bulk and local polarity in macrosystems (membranes, etc.), or even the conformation and binding of proteins. Despite its wide use, solvatochromism still remains a largely unknown phenomenon due to the extremely complex coupling of many different interactions and dynamical processes which characterize it. In this study we analyze the influence of different solvents on the photophysical properties of selected charge-transfer probes (4-AP, PRODAN, and FR0). The purpose is to achieve a microscopic understanding of the intermolecular effects which govern the absorption and fluorescence properties of solvated molecular probes, such as solvent-induced structural modifications, polarization effects, solubility, solute-solvent hydrogen-bonding interactions, and solute aggregation. To this aim we have exploited a time dependent density functional theory (TDDFT) approach coupled to complementary solvation approaches (continuum, discrete and mixed discrete and continuum). Such an integration has allowed us to clearly disentangle the complex interplay between specific and nonspecific interactions of the solvent with the probes and show that strong H-bonding effects not only can lead to large solvatochromic shifts but also can affect the nature of the emitting species with resulting reduction of the quantum yield.

Numerical studies of the membrane fluorescent dyes dynamics in ground and excited states

Fluorescence methods are widely used in studies of biological and model membranes. The dynamics of membrane fluorescent markers in their ground and excited electronic states and correlations with their molecular surrounding within the fully hydrated phospholipid bilayer are still not well understood. In the present work, Quantum Mechanical (QM) calculations and Molecular Dynamics (MD) simulations are used to characterize location and interactions of two membrane polarity probes (Prodan; 6-propionyl-2-dimethylaminonaphthalene and its derivative Laurdan; 2-dimethylamino-6-lauroylnaphthalene) with the dioleoylphosphatidylcholine (DOPC) lipid bilayer model. MD simulations with fluorophores in ground and excited states are found to be a useful tool to analyze the fluorescent dye dynamics and their immediate vicinity. The results of QM calculations and MD simulations are in excellent agreement with available experimental data. The calculation shows that the two amphiphilic dyes initially placed in bulk water diffuse within 10 ns towards their final location in the lipid bilayer. Analysis of solvent relaxation process in the aqueous phase occurs on the picoseconds timescale whereas it takes nanoseconds at the lipid/water interface. Four different relaxation time constants, corresponding to different relaxation processes, where observed when the dyes were embedded into the membrane.

Profiling of dynamics in protein-lipid-water systems: a time-resolved fluorescence study of a model membrane protein with the label BADAN at specific membrane depths

Profiles of lipid-water bilayer dynamics were determined from picosecond time-resolved fluorescence spectra of membrane-embedded BADAN-labeled M13 coat protein. For this purpose, the protein was labeled at seven key positions. This places the label at well-defined locations from the water phase to the center of the hydrophobic acyl chain region of a phospholipid model membrane, providing us with a nanoscale ruler to map membranes. Analysis of the time-resolved fluorescence spectroscopic data provides the characteristic time constant for the twisting motion of the BADAN label, which is sensitive to the local flexibility of the protein–lipid environment. In addition, we obtain information about the mobility of water molecules at the membrane–water interface. The results provide an unprecedented nanoscale profiling of the dynamics and distribution of water in membrane systems. This information gives clear evidence that the actual barrier of membranes for ions and aqueous solvents is located at the region of carbonyl groups of the acyl chains.

Spectral heterogeneity of PRODAN fluorescence in isotropic solvents revealed by multivariate photokinetic analysis

This paper describes a multivariate analysis of the fluorescence emission of 6-propionyl-2-dimethylaminonaphthalene (PRODAN) in a series of isotropic solvents of differing polarity and hydrogen-bonding ability. Multivariate methods distill the essential features from spectral data matrices so that the structural details that are embedded within the data are revealed to the analyst. In the aprotic solvents investigated, the analysis reveals a pair of emission components that have emission maxima that scale with the orientational polarizability. In the alcohols, short-lived, polarity-independent blue bands tentatively attributed to neutral hydrogen-bonded solute-solvent complexes form and relax prior to emission from paired bands that have Stokes shifts that scale with the solvent hydrogen-bonding ability rather than the polarity. In water, the short-lived blue bands were not observed, but the shift in the paired bands did scale with the solvent hydrogen-bonding ability.

Solvent induced shifts of electronic spectra IV. Computational study on PRODAN fluorescence and implications to the excited state structure

Vertical S(1)-S(0) electronic transitions of the highly solvent-sensitive fluorescence label 2-propionyl-6-dimethylamino naphthalene (PRODAN) are modeled by semiempirical CISD AM1 and TD DFT calculations in a large number of solvents of different polarity and hydrogen donating ability. Calculations correctly reproduce the observed solvent induced shifts of the emission maxima. The fluorescence Frank-Condon transition energies in solvent can be predicted quantitatively at the AM1 SM5.42 OPEN(2,2) C.I.=5 CISD level. For the planar PRODAN emitting state at the latter level we obtain a regression with practically unit slope and zero intercept for aprotic solvents. The respective relationship for the O-twisted S(1) state has a slope of 0.59 and intercept of 9100 cm(-1). These results support the concept that no geometry twist in the S(1) state of PRODAN is necessary to explain the observed solvent effects on fluorescence.

Site-directed fluorescence labeling of a membrane protein with BADAN: probing protein topology and local environment

The work presented here describes a new and simple method based on site-directed fluorescence labeling using the BADAN label that permits the examination of protein-lipid interactions in great detail. We applied this technique to a membrane-embedded, mainly alpha-helical reference protein, the M13 major coat protein. Using a high-throughput approach, 40 site-specific cysteine mutants were prepared of the 50-residues long protein. The steady-state fluorescence spectra were analyzed using a three-component spectral model that enabled the separation of Stokes shift contributions from water and internal label dynamics, and protein topology. We found that most of the fluorescence originated from BADAN labels that were hydrogen-bonded to water molecules even within the hydrophobic core of the membrane. Our spectral decomposition method revealed the embedment and topology of the labeled protein in the membrane bilayer under various conditions of headgroup charge and lipid chain length, as well as key characteristics of the membrane such as hydration level and local polarity, provided by the local dielectric constant.

Photokinetic analysis of PRODAN and LAURDAN in large unilamellar vesicles from multivariate frequency-domain fluorescence

This paper describes a multivariate photokinetic analysis of the membrane phase dependence of PRODAN and LAURDAN photokinetics in DMPC vesicles. Decay data, arranged in the form of Fourier transformed emission-decay matrices (FT-EDMs), were collected as a function of temperature around the gel phase transition temperature. Each matrix was partitioned into the emission spectra and decay profiles of the underlying emission components using methods based on principal components analysis. The analysis revealed that both probes typically emit at least three spectral components, which vary in intensity as the membrane undergoes gel to liquid-crystalline phase transitions: a locally excited species (lambda max approximately 415 nm), a charge-transfer species (lambda max approximately 435 nm), and a solvent relaxed species (lambda max approximately 490 nm). In contrast to previous reports, the most red-shifted species is not photoexcited, but evolves from the locally excited species and does not exhibit the dynamic Stokes' shifts associated with conventional solvent relaxation. The primary difference in the emission of the two probes is the prominence of the charge-transfer species in the LAURDAN emission.

Measurement of solvation responses at multiple sites in a globular protein

Proteins respond to electrostatic perturbations through complex reorganizations of their charged and polar groups, as well as those of the surrounding media. These solvation responses occur both in the protein interior and on its surface, though the exact mechanisms of solvation are not well understood, in part because of limited data on the solvation responses for any given protein. Here, we characterize the solvation kinetics at sites throughout the sequence of a small globular protein, the B1 domain of streptococcal protein G (GB1), using the synthetic fluorescent amino acid Aladan. Aladan was incorporated into seven different GB1 sites, and the time-dependent Stokes shift was measured over the femtosecond to nanosecond time scales by fluorescence upconversion and time-correlated single photon counting. The seven sites range from buried within the protein core to fully solvent-exposed on the protein surface, and are located on different protein secondary structures including beta-sheets, helices, and loops. The dynamics in the protein sites were compared against the free fluorophore in buffer. All protein sites exhibited an initial, ultrafast Stokes shift on the subpicosecond time scale similar to that observed for the free fluorophore, but smaller in magnitude. As the probe is moved from the surface to more buried sites, the dynamics of the solvation response become slower, while no clear correlation between dynamics and secondary structure is observed. We suggest that restricted movements of the surrounding protein residues give rise to the observed long time dynamics and that such movements comprise a large portion of the protein's solvation response. The proper treatment of dynamic Stokes shift data when the time scale for solvation is comparable to the fluorescence lifetime is discussed.

Spectrin organization and dynamics: new insights

Spectrin is the major constituent protein of the erythrocyte cytoskeleton which forms a filamentous network on the cytoplasmic face of the membrane by providing a scaffold for a variety of proteins. In this review, several aspects of spectrin organization are highlighted, particularly with respect to its ability to bind hydrophobic ligands and its interaction with membrane surfaces. The characteristic binding of the fluorescent hydrophobic probes Prodan and pyrene to spectrin, which allows an estimation of the polarity of the hydrophobic probe binding site, is illustrated. In addition, the contribution of uniquely localized and conserved tryptophan residues in the 'spectrin repeats' in these processes is discussed. A functional implication of the presence of hydrophobic binding sites in spectrin is its recently discovered chaperone-like activity. Interestingly, spectrin exhibits residual structural integrity even after denaturation which could be considered as a hallmark of cytoskeletal proteins. Future research could provide useful information about the possible role played by spectrin in cellular physiology in healthy and diseased states.

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

6-propionyl-2-(N,N-dimethyl)aminonaphtahalene, PRODAN, is widely used as a fluorescent molecular probe because of its significant Stokes shift in polar solvents. It is an aromatic compound with intramolecular charge-transfer states (ICT) that can be particularly useful as a sensor. The nature of the emissive states has not yet been established despite the detailed experimental and theoretical investigations done on this fluorophore. In this work, we performed absorption, steady-state, time-resolved fluorescence (TRES) and time-resolved area normalized emission (TRANES) spectroscopies on the molecular probe PRODAN in the anionic water/sodium 1,4-bis-2-ethylhexylsulfosuccinate (AOT)/n-heptane and the cationic water/benzyl-n-hexadecyl dimethylammonium chloride (BHDC)/benzene reverse micelles (RMs). The experiments were done by varying the surfactant concentrations at a fixed molar ratio (W = [H2O]/[Surfactant]) and changing the water content at a constant surfactant concentration. The results obtained varying the surfactant concentration at W = 0 show a bathochromic shift and an increase in the intensity of the PRODAN emission band due to the PRODAN partition process between the external solvent and the RMs interface. The partition constants, Kp, are quantified from the changes in the PRODAN emission spectra and the steady-state anisotropy (<r>) with the surfactant concentration in both RMs. The Kp value is larger in the BHDC than the AOT RMs, probably due to the interaction between the cationic polar head of the surfactant and the aromatic ring of PRODAN. The partition process is confirmed with the TRES experiments, where the data fit to a continuous model, and with the time-resolved area normalized emission spectroscopy (TRANES) spectra, where only one isoemissive point is detected. On the other hand, the emission spectra at W = 10 and 20 show a dual fluorescence with a new band that emerges in the low-energy region of the spectra, a band that was previously assigned to the PRODAN emission from the water pool of RMs. Our studies demonstrate that this band is due to the emission from an ICT state of the molecular probe PRODAN located at the interface of the RMs. These results are also confirmed by the lifetime measurements, the TRES experiments where the results fit to a two-state model, and the time-resolved area normalized emission spectroscopy (TRANES) spectra where three or two isoemissive points are detected in the AOT and BHDC RMs, respectively. In the AOT RMs, Kp values obtained at W = 10 and 20 are almost independent of the water content; the values are higher for the BHDC RMs due to the higher micropolarity of this interface.