Guest/Host Complexes of Octa Acid and Amphiphilic Benzylidene-3-methylimidazolidinones Exchange Hosts within the NMR Time Scale.

Cavitand octa acid (OA) is established to form a stable capsular assembly with one or two hydrophobic guest molecules (1:2 or 2:2 guest/host complex). Examples are known in which the guest molecule tumbles within the capsule without disrupting the structure of the capsuleplex. This process makes the two OA molecules that form the capsule magnetically equivalent.

Conservation of estrogen receptor function in invertebrate reproduction.

Background: Rotifers are microscopic aquatic invertebrates that reproduce both sexually and asexually. Though rotifers are phylogenetically distant from humans, and have specialized reproductive physiology, this work identifies a surprising conservation in the control of reproduction between humans and rotifers through the estrogen receptor. Until recently, steroid signaling has been observed in only a few invertebrate taxa and its role in regulating invertebrate reproduction has not been clearly demonstrated.

A latent reaction in a model GFP chromophore revealed upon confinement: photohydroxylation of ortho-halo benzylidene-3-methylimidazolidiones via an electrocylization process.

Excited state behavior of halogen substituted model GFP chromophores was investigated in an acetonitrile solution and in a confined environment provided by an octa acid capsule in water. Of the ortho, meta, and para halogen substituted GFP chromophores only the ortho compounds gave a new product resulting from an unprecedented photosubstitution of halogens by the hydroxyl group. This unusual reaction highlights the importance of confined spaces in bringing about some unattainable photoreactions.

Chromophore photoreduction in red fluorescent proteins is responsible for bleaching and phototoxicity.

Red fluorescent proteins (RFPs) are indispensable tools for deep-tissue imaging, fluorescence resonance energy transfer applications, and super-resolution microscopy. Using time-resolved optical spectroscopy this study investigated photoinduced dynamics of three RFPs, KillerRed, mRFP, and DsRed. In all three RFPs, a new transient absorption intermediate was observed, which decays on a microsecond-millisecond time scale.

Microcrystals with enhanced emission prepared from hydrophobic analogues of the green fluorescent protein chromophore via reprecipitation.

Certain synthetic analogues of the green fluorescent protein (GFP) chromophore are almost nonfluorescent in dilute solutions but are strongly light-emissive in the solid state, thus exhibiting aggregation-induced emission (AIE) behavior. In the present work, two such hydrophobic derivatives of the GFP chromophore known to be fluorescent in the crystalline state (p-hexyloxy- and p-dodecyloxybenzylideneimidazolinone) were used to prepare aqueous suspensions of particles via a mild solvent-exchange reprecipitation (RP) method. This evolution was monitored at various experimental conditions by UV-vis absorption and fluorescence spectroscopy, fluorescence microscopy, as well as electron transmission and scanning microscopy.

Optically modulatable blue fluorescent proteins.

Blue fluorescent proteins (BFPs) offer visualization of protein location and behavior, but often suffer from high autofluorescent background and poor signal discrimination. Through dual-laser excitation of bright and photoinduced dark states, mutations to the residues surrounding the BFP chromophore enable long-wavelength optical modulation of BFP emission. Such dark state engineering enables violet-excited blue emission to be increased upon lower energy, green coillumination.

Creating graphene p-n junctions using self-assembled monolayers.

3-Aminopropyltriethoxysilane (APTES) and perfluorooctyltriethoxysilane (PFES) were used to modify the interface between transferred CVD graphene films and its supporting dielectric to create n-type and p-type graphene, respectively. A graphene p-n junction was obtained by patterning both modifiers on the same dielectric and verified through the creation of a field effect transistor (FET). Characteristic I-V curves indicate the presence of two separate Dirac points which confirms an energy separation of neutrality points within the complementary regions.

Conformationally locked chromophores as models of excited-state proton transfer in fluorescent proteins.

Members of the green fluorescent protein (GFP) family form chromophores by modifications of three internal amino acid residues. Previously, many key characteristics of chromophores were studied using model compounds. However, no studies of intermolecular excited-state proton transfer (ESPT) with GFP-like synthetic chromophores have been performed because they either are nonfluorescent or lack an ionizable OH group.

Excited-state proton transfer in N-methyl-6-hydroxyquinolinium salts: solvent and temperature effects.

The excited-state proton transfer (ESPT) reaction of the "super"photoacid N-methyl-6-hydroxyquinolinium (MHQ) was studied using both fluorescence upconversion and time-correlated single photon counting (TCSPC) techniques. The ultrafast ESPT kinetics were investigated in various alcohols and water and determined to be solvent-controlled. The ESPT temperature dependence of MHQ was also studied in various alcohols and compared to that observed for another "super"photoacid, 5,8-dicyano-2-naphthol (DCN2).

Water-soluble distyrylbenzenes: one core with two sensory responses--turn-on and ratiometric.

The synthesis of four water-soluble distyrylbenzenes (compounds 1-4) is reported. Their acidochromicity in aqueous media was investigated. Blue shifts and increases in the quantum yields were observed as a general response.

Collapse and recovery of green fluorescent protein chromophore emission through topological effects.

Housed within the 11-stranded β-barrel of the green fluorescent protein (GFP) is the arylideneimidazolidinone (AMI) chromophore, the component responsible for fluorescence. This class of small-molecule chromophore has drawn significant attention for its remarkable photophysical and photochemical properties, both within the intact protein and after its denaturation. All of the proteins so far isolated that have visible light fluorescence have been found to contain an AMI chromophore.

Recapture of GFP chromophore fluorescence in a protein host.

When encapsulated by human serum albumin (HSA), certain derivatives of the green fluorescent protein (GFP) chromophore recover their fluorescence due to inhibition of torsional motion. These derivatives show remarkable sensitivity and selectivity as well as favorable spectroscopic properties toward HSA, thus providing selective probes for this and similar proteins and demonstrating the use of GFP chromophores as topological fluorophores.

Fluorescence turn on by cholate aggregates.

Bile salts, including sodium cholate (NaCh), are amphiphilic molecules with a concave hydrophilic side and a convex hydrophobic side. By forming aggregates in aqueous solution, these natural surfactants fulfill vital biological roles in the solubilization of cholesterol, lipids, and fat-soluble vitamins and thus are involved in the transport and absorption of important biological molecules. Following our success with the encapsulation of fluorescent protein chromophore (FP) analogs by synthetic hydrophobic and hydrophilic hosts, based upon substitution patterns, we now report the binding and turn on of other analogs by bile salt aggregates, observations which may lead to new tools for studying trafficking in these important systems.

Reactive oxygen species in photochemistry of the red fluorescent protein "Killer Red".

The fluorescent protein aptly named "Killer Red" (KRed) is capable of killing transfected cells and inactivating fused proteins upon exposure to visible light in the presence of oxygen. We have investigated the source of the bioactive species through a variety of photophysical and photochemical techniques. Our results indicate a Type I (electron transfer mediated) photosensitizing mechanism.

Hydroxydialkylamino cruciforms: amphoteric materials with unique photophysical properties.

Two amphoteric cruciforms 6 and 7 (XF; 4,4'-[(1E,1'E)-(2,5-bis{[4-(dibutylamino)phenyl]ethynyl}-1,4-phenylene)bis(ethene-2,1-diyl)]diphenol, 4,4'-[{2,5-bis[(E)-4-(dibutylamino)styryl]-1,4-phenylene}bis(ethyne-2,1-diyl)]diphenol) were prepared by a Horner reaction followed by a Sonogashira coupling and subsequent deprotection. The XFs display significant changes in absorption and emission when exposed to trifluoroacetic acid, tetrabutylammonium hydroxide, and metal triflates. The substitution pattern of 6 and 7 leads to spatial separation of the frontier molecular orbitals, which allows the HOMO or LUMO of the XF to be addressed independently by acidic or basic agents.

Chemically modulating the photophysics of the GFP chromophore.

There is growing interest in engineering the properties of fluorescent proteins through modifications to the chromophore structure utilizing mutagenesis with either natural or unnatural amino acids. This entails an understanding of the photophysical and photochemical properties of the modified chromophore. In this work, a range of GFP chromophores with different alkyl substituents are synthesized and their electronic spectra, pH dependence, and ultrafast fluorescence decay kinetics are investigated.

Fluorescence response profiling for small molecule sensors utilizing the green fluorescent protein chromophore and its derivatives.

Using a fluorescence response profile, a systematic examination was performed for synthetic chromophores of the green fluorescent protein (GFP) to discover new small molecule sensors. A group of 41 benzylideneimidazolinone compounds (BDI) was prepared and screened toward 94 biologically relevant analytes to generate fluorescence response profiles. From the response pattern, compounds containing aminobenzyl and heteroaromatic cyclic substructures revealed a pH dependent emission decrease effect, and unlike other fluorescence scaffolds, most BDIs showed fluorescence quenching when mixed with proteins.

Steric and electronic effects in capsule-confined green fluorescent protein chromophores.

The turn-on of emission in fluorescent protein chromophores sequestered in an "octaacid" capsule is controlled by stereoelectronic effects described by a linear free energy relationship. The stereochemical effects are governed by both the positions and volumes of the aryl substituents, while the electronic effects, including ortho effects, can be treated with Hammett σ parameters. The use of substituent volumes rather than A values reflects packing of the molecule within the confines of the capsule.

Diferrocenyl molecular wires. The role of heteroatom linkers.

Diferrocenyl molecular "wires", in which two ferrocenes are linked by a conjugated chain, allowing the communication of redox information between the ferrocenes, are a versatile platform on which to investigate notions of molecular conductivity. In this paper, we examine the role of heteroatoms--including O, P, S, and Se, as well as C atoms in various oxidation states--separated from the ferrocenes by intervening double bonds which minimize any steric effects. Surprisingly, oxygen is a better electronic mediator than sulfur, a phenomenon we attribute to superior molecular orbital overlap.

Inhibition of twisting of a green fluorescent protein-like chromophore by metal complexation.

Substitution of a pyridyl for the hydroxyphenyl moiety in the Green Fluorescent Protein analog p-hydroxybenzylidene-dimethylimidiazolinone produces a chromophore which "turns on" fluorescence in the presence of Zn(2+) or Cd(2+) ions. Such a phenomenon provides "proof of principle" for using GFP chromophores in a variety of sensing applications.

Topochemistry and photomechanical effects in crystals of green fluorescent protein-like chromophores: effects of hydrogen bonding and crystal packing.

To obtain insight into the effects of the environment on the photophysics and photochemistry of the green fluorescence protein (GFP), eight crystal structures of six synthetic aryl-substituted analogues (2-fluoro, 2-methyl, 3-hydroxy, 3-methoxy, 2,4-dimethyl and 2,5-dimethyl) of the GFP chromophore (4-hydroxy-benzylidenedimethylimidazolinone) were determined and correlated with their two-dimensional steady-state and time-resolved solid-state excitation-emission spectra. The stacking between the molecules greatly affected the emission energy and the lifetime of the emission of the chromophore, implying that pi-pi interactions could be critical for the photophysics of GFP. The reaction pathways were dependent on the excitation energy, resulting either in [2 + 2] photodimerization at the bridging double bond (UV excitation) or flipping of the imidazolone ring (visible excitation).

Activation of fluorescent protein chromophores by encapsulation.

Chromophores related to fluorescent proteins, when sequestered into the "octaacid" capsule, recover their fluorescence. The fluorescence recovery is related to the inhibition of torsional motions within the cavity, implicating the single-bond torsion as an important contributor to internal conversion within this important class of chromophores.

Excited-State Proton Transfer in Chiral Environments: Photoracemization of BINOLs.

We have studied excited-state proton transfer (ESPT) from chiral proton donors to chiral and achiral acceptors. The key role of the exergonicity of the reaction and the transition-state position along the reaction coordinate for the existence of an enantiomeric effect was established. This effect was observed for "super" photoacids (ΔG ≪ 0) and vanished for endergonic reactions (ΔG > 0) where a "late" transition state similar to planar achiral binaphtholate anion occurs.