NO formation by neuronal NO-synthase can be controlled by ultrafast electron injection from a nanotrigger

Nitric oxide synthases (NOSs) are unique flavohemoproteins with various roles in mammalian physiology. Constitutive NOS catalysis is initiated by fast hydride transfer from NADPH, followed by slower structural rearrangements. We used a photoactive nanotrigger (NT) to study the initial electron transfer to FAD in native neuronal NOS (nNOS) catalysis. Molecular modeling and fluorescence spectroscopy showed that selective NT binding to NADPH sites close to FAD is able to override Phe1395 regulation. Ultrafast injection of electrons into the protein electron pathway by NT photoactivation through the use of a femtosecond laser pulse is thus possible. We show that calmodulin, required for NO synthesis by constitutive NOS, strongly promotes intramolecular electron flow (6.2-fold stimulation) by a mechanism involving proton transfer to the reduced FAD(-) site. Site-directed mutagenesis using the S1176A and S1176T mutants of nNOS supports this hypothesis. The NT synchronized the initiation of flavoenzyme catalysis, leading to the formation of NO, as detected by EPR. This NT is thus promising for time-resolved X-ray and other cellular applications.

Two photon-induced electron injection from a nanotrigger in native endothelial NO-synthase

We have recently designed a nanotrigger (NT), a photoactive molecule addressing the NADPH sites of proteins. This nanotrigger has a 10(3) times larger two-photon cross-section compared to the ubiquitous NADPH cofactor. In this work, we tested whether two-photon excitation of the bound NT to NADPH sites may be used to initiate enzymatic catalysis by appropriate electron injection. To establish proof of principle, we monitored the ultrafast absorption of NT bound to the fully active endothelial NO-Synthase (eNOS) following excitation by one and two-photons at 405 and 810 nm, respectively. Electron injection from NT* to FAD in eNOS initiated the catalytic cycle in 15+/-3 ps at both exciting wavelengths. The data proved for the first time that electron transfer can be promoted by two-photon excitation. We also show that the nanotrigger decays faster in homogeneous solvents than in the NADPH site of proteins, suggesting that hindered environments modified the natural decay of NT. The nanotrigger provides a convenient way of synchronizing an ensemble of proteins in solution with a femtosecond laser pulse. The ability of NT to initiate NOS catalysis by two-photon excitation may be exploited for controlled and localized release of free NO in cells with enhanced spatial and temporal resolution.

Photoinduced intramolecular charge transfer in push-pull polyenes: effects of solvation, electron-donor group, and polyenic chain length

Subpicosecond absorption spectroscopy is used to characterize the primary photoinduced processes in a class of push-pull polyenes bearing a julolidine end group as the electron donor and a diethylthiobarbituric acid end group as the electron acceptor. The excited-state decay time and relaxation pathway have been studied for four polyenes of increasing chain length (n = 2-5 double bonds) in aprotic solvents of different solvation time, polarity, and viscosity. Intramolecular charge transfer (ICT) leading to a transient state of cyanine-like structure (fully conjugated with no bond length alternation) is observed in all polar solvents at a solvent dependent rate, but the reaction is not observed in cyclohexane, a nonpolar solvent. In polar solvents, the reaction time increases with the average solvation time but remains slightly larger, except in the viscous solvent triacetin. These facts are interpreted as an indication that both solvent reorganization and internal restructuring are involved in the ICT-state formation. The observed photodynamics resemble those we previously found for another class of polyenes bearing a dibutylaniline group as the donor, including a similar charge-transfer rate in spite of the larger electron donor character of the julolidine group. This observation brings further support to the proposal that an intramolecular coordinate is involved in the charge-transfer reaction, possibly a torsional motion of the donor end group. On the other hand, relaxation of the ICT state leads to cis-trans isomerization or crossing to the triplet state, depending on the length of the polyenic chain. In dioxane, tetrahydrofuran, and triacetin, the ICT state of the shorter chains (n = 2, 3) relaxes to the isomer with a viscosity-dependent rate, while that of the longer ones (n = 4, 5) leads to the triplet state with a viscosity-independent rate, as expected. In acetonitrile, the ICT-state lifetime is generally much shorter. A change from photoisomerization to intersystem crossing at n = 4 is also proposed in this solvent, but the formation of a photoproduct at n = 2 is not clear. In cyclohexane, where the ICT state is not formed, the relaxation pathway of the initially excited state is found to lead to an isomer for n = 2. As in polar solvents, a change to intersystem crossing at n = 4 is proposed. The direct relaxation to the ground state found at n = 3 for the series bearing a dibutylaniline group is not observed with the julolidine group. The results clearly illustrate that photoinduced reaction trajectories in push-pull polyenes are controlled by the static and dynamic properties of the solvent, the chemical nature and size of the end groups, and the conjugated-chain length and flexibility.

Two-photon transitions in quadrupolar and branched chromophores: experiment and theory

A combined experimental and theoretical study is conducted on a series of model compounds in order to assess the combined role of branching and charge symmetry on absorption, photoluminescence, and two-photon absorption (TPA) properties. The main issue of this study is to examine how branching of quadrupolar chomophores can lead to different consequences as compared to branching of dipolar chromophores. Hence, three structurally related pi-conjugated quadrupolar chromophores symmetrically substituted with donor end groups and one branched structure built from the assembly of three quadrupolar branches via a common donor moiety are used as model compounds. Their photophysical properties are studied using UV-vis spectroscopy, and the TPA spectra are determined through two-photon excited fluorescence experiments using femtosecond pulses in the 500-1000 nm range. Experimental studies are complemented by theoretical calculations. The applied theoretical methodology is based on time-dependent density functional theory, the Frenkel exciton model, and analysis in terms of the natural transition orbitals of relevant electronic states. Theory reveals that a symmetrical intramolecular charge transfer from the terminal donating groups to the middle of the molecule takes place in all quadrupolar chromophores upon photoexcitation. In contrast, branching via a central electron-donating triphenylamine moiety breaks the quadrupolar symmetry of the branches. Consequently, all Frank-Condon excited states have significant asymmetric multidimensional charge-transfer character upon excitation. Subsequent vibrational relaxation of the branched chromophore in the excited state leads to a localization of the excitation and fluorescence stemming from a single branch. As opposed to what was earlier observed when dipolar chromophores are branched via the same common electron-donating moiety, we find only a slight enhancement of the maximum TPA response of the branched compound with respect to an additive contribution of its quadrupolar branches. In contrast, substantial modifications of the spectral shape are observed. This is attributed to the subtle interplay of interbranch electronic coupling and asymmetry caused by branching.

Quenching of molecular fluorescence on the surface of monolayer-protected gold nanoparticles investigated using place exchange equilibria

The insertion of fluorescently labeled thiols into the protecting self-assembled monolayer on the surface of gold nanoparticles through place exchange reactions and the effects of this insertion on the photophysical properties of the fluorophores are investigated. Analysis of solution-phase fluorescence data using a dynamic equilibrium model yields the equilibrium constant for the place exchange equilibrium, as well as the relative fluorescence brightness of the fluorophores on the particle surface. In all cases we find a significant quenching of the fluorescence, and potential reasons for this quenching are discussed. In the case of these relatively small particles (4.5 nm diameter), the quenching appears to be mainly related to enhanced nonradiative deactivation pathways. The place exchange equilibrium constant reveals a reduced affinity of the fluorescently labeled thiols for insertion into the nonfluorescent alkylthiol monolayer (K(eq) approximately 0.2) compared to unlabeled alkylthiols.

Effect of Branching on Two-Photon Absorption in Triphenylbenzene Derivatives

The photophysical and linear and nonlinear spectral properties of octupolar compounds with a triphenylbenzene core are investigated and compared with properties of corresponding dipolar branches. A correlation is found between the solvatochromic behavior and the two-photon absorption cross section. Moreover, the nature of the core is found to be responsible for the nature of the coupling between branches; in the studied case only (weak) electrostatic interactions are effective, while other cores, like the triphenylamine moiety, are able to promote coherent coupling between the branches, leading to strongly nonadditive properties.

Charge instability in quadrupolar chromophores: symmetry breaking and solvatochromism

We present a joint theoretical and experimental work aimed to understand the spectroscopic behavior of multipolar dyes of interest for nonlinear optics (NLO) applications. In particular, we focus on the occurrence of broken-symmetry states in quadrupolar organic dyes and their spectroscopic consequences. To gain a unified description, we have developed a model based on a few-state description of the charge-transfer processes characterizing the low-energy physics of these systems. The model takes into account the coupling between electrons and slow degrees of freedom, namely, molecular vibrations and polar solvation coordinates. We predict the occurrence of symmetry breaking in either the ground or first excited state. In this respect, quadrupolar chromophores are classified in three different classes, with distinctively different spectroscopic behavior. Cases of true and false symmetry breaking are discriminated and discussed by making resort to nonadiabatic calculations. The theoretical model is applied to three representative quadrupolar chromophores: their qualitatively different solvatochromic properties are connected to the presence or absence of broken-symmetry states and related to two-photon absorption (TPA) cross-sections. The proposed approach provides useful guidelines for the synthesis of dyes for TPA application and represents a general and unifying reference frame to understand energy-transfer processes in multipolar molecular systems, offering important clues to understand basic properties of materials of interest for NLO and energy-harvesting applications.

Synthesis, Fluorescence, and Two-Photon Absorption of a Series of Elongated Rodlike and Banana-Shaped Quadrupolar Fluorophores: A Comprehensive Study of Structure–Property Relationships

An extensive series of push-push and pull-pull derivatives was prepared from the symmetrical functionalization of an ambivalent core with conjugated rods made from arylene-vinylene or arylene-ethynylene building blocks, bearing different acceptor or donor end-groups. Their absorption and photoluminescence, as well as their two-photon-absorption (TPA) properties in the near infrared (NIR) region, were systematically investigated to derive structure-property relationships and to lay the guidelines for both spectral tuning and amplification of molecular TPA in the target region. Whatever the nature of the core or of the connectors, push-push systems were found to be more efficient than pull-pull systems, and planarization of the core (fluorene versus biphenyl) always leads to an increase in the TPA cross sections. In contrast, increasing the conjugation length as well as replacement of a phenylene moiety by a thienylene moiety in the conjugated rods did not necessarily lead to increased TPA responses. The present study also demonstrated that the topology of the conjugated rods can dramatically influence the TPA properties. This is of particular interest in terms of molecular engineering for specific applications, as both TPA properties and photoluminescence characteristics can be considerably affected. Thus, it becomes possible to optimize the transparency/TPA and fluorescence/TPA efficiency trade-offs for optical limiting in the red-NIR region (700-900 nm) and for two-photon-excited fluorescence (TPEF) microscopy applications, respectively.

Synchronous photoinitiation of endothelial NO synthase activity by a nanotrigger targeted at its NADPH site

We designed a new nanotrigger to synchronize and monitor an enzymatic activity interacting specifically with the conserved NADPH binding site. The nanotrigger (NT) combines a docking moiety targeting the NADPH site and a chromophore moiety responsive to light excitation for efficient electron transfer to the protein. Specific binding of the nanotrigger to the reductase domain of the endothelial nitric oxide synthase (eNOSred) was demonstrated by competition between NADPH and the nanotrigger on the reduction of eNOSred flavin. A micromolar Ki was estimated. We had monitored initiation of eNOSred activity by ultrafast transient spectroscopy. The transient absorption spectrum recorded at 250 ps fits the expected sum of the reduced and oxidized species, independently obtained by other chemical methods, in agreement with a photoinduced electron transfer from the excited nanotrigger to the flavin moiety of eNOSred. The rate of electron transfer from the excited state of the nanotrigger (NT*) to the protein is estimated to be k(ET) = (7 +/- 2) x 10(9) s(-1) using the decay of oxidized eNOSred-bound nanotrigger compared against prereduced eNOSred or glucose 6-P dehydrogenase as controls. This fast electron transfer bypasses the slow hydride transfer to initiate NOS catalysis as shown by ultrafast kinetics using the eNOSred mutated in the regulatory F1160 residue. The selective targeting of the nanotrigger to NADPH sites should allow controlled initiation of the enzymatic activity of numerous proteins containing an NADPH site.

A NADPH substitute for selective photo-initiation of reductive bioprocesses via two-photon induced electron transfer

A NADPH substitute where the nicotinamide moiety is replaced by a chromophoric unit having much larger two-photon absorption cross-section and able to transfer electrons to flavins only upon excitation is described as an effective two-photon nanotrigger for selective photo-activation of electron transfer in bioreductive processes.

Effects of Dipolar Interactions on Linear and Nonlinear Optical Properties of Multichromophore Assemblies: A Case Study

Interchromophore interactions in flexible multidipolar structures for nonlinear optics were addressed by a combined experimental and theoretical study on two series of one-, two-, and three-chromophore systems in which identical push-pull chromophores are assembled through covalent and flexible linkers in close proximity. The photophysical and nonlinear optical properties (quadratic hyperpolarizability) of the multichromophore systems were investigated and compared to those of the monomeric chromophores. Multimers have larger dipole moments than their monomeric analogues, that is, the dipolar subchromophores self-orientate within the multimeric structures. This effect was found to depend on the intersubchromophore distance in a nontrivial manner, which confirms that molecular engineering of such flexible systems is more complex than in completely geometrically controlled systems. Electric-field-induced second-harmonic generation (EFISHG) measurements in solution revealed increased figures of merit as compared to the monomeric analogue. This effect increases with increasing number and polarity of the individual subchromophores in the nanoassembly and increasing spacing between dipolar subchromophores. Experimental results are interpreted by a theoretical model for interacting polar and polarizable chromophores. The properties of multidipolar assemblies are shown to be related to the relative orientation of chromophores, which is imposed by interchromophore interactions. The supramolecular structure is thus a result of self-organization. The proposed theoretical model was also used to predict the properties of multichromophore structures made up of more polar and polarizable push-pull chromophores, and showed that stronger interchromophore interactions can heavily affect the individual optical responses. This suggests new routes for engineering highly NLO responsive multichromophore systems.

Linear and Two-Photon Absorption Properties of Interacting Polar Chromophores: Standard and Unconventional Effects

Investigations into the effects of electrostatic interactions on linear and two-photon absorption (TPA) properties are probed using a reference polar chromophore and different related compounds. Compounds in which a passive charge is present near the active chromophore, as well as dimers of the reference molecule, were studied. The combined experimental and theoretical investigation demonstrates that strong modulation of the properties can be attained. In particular, the TPA response of dimers is heavily affected by unconventional excitonic effects, that is, beyond the standard Heitler-London approximation. These effects, which stem from purely electrostatic interactions, lower the nonlinear optical response in the investigated case by an additional amount (up to 20%), and are expected to enhance the TPA cross section in other supramolecular alignments. Electrostatic interactions cannot be overlooked when modelling or investigating highly concentrated and/or confined samples, as are usually needed in many applications. The correct knowledge of their effects can be exploited to guide engineering at the molecular and supramolecular level.

Distance-dependent fluorescence quenching on gold nanoparticles ensheathed with layer-by-layer assembled polyelectrolytes

We report on the preparation, characterization, and photophysical study of new fluorescent core/shell nanoparticles fabricated by electrostatic layer-by-layer assembly. On the basis of gold cores with a diameter of 13 nm, these nanocolloids possess different fluorescently labeled polymer corona layers at various distances from the surface of the core metal using nonfluorescent polyelectrolytes as spacer layers. UV-visible spectroscopy and transmission electron microscopy confirm that the particle suspensions of fluorescently labeled core/shell nanoparticles are stable at all stages of their construction. Photophysical investigations reveal strongly distance-dependent fluorescence quenching in these particle systems. The contribution of the metal core to this quenching can be assesed precisely after the gentle dissolution of the gold cores by potassium cyanide. The photophysical measurements reveal clearly that the gold nanoparticles decrease the transition probability for radiative transitions.