Enhanced photocatalytic activity of gold nanoparticles driven by supramolecular host-guest chemistry.

Functionalization of gold nanoparticles with supramolecular hosts allows their plasmon-based photocatalytic activity to be enhanced. This is mainly ascribed to the formation of labile host-guest complexes with the reagent molecules on the metal surface, thus promoting nanoparticle-substrate approximation without interfering with the light-induced catalytic process.

Combining nanosecond and millisecond time scale techniques: determination of thermodynamic and kinetic data of primary alkyl amine cation radicals.

Primary alkyl amines are one of the most commonly used and effective reagents in CO2 capture. Most of the amines used for CO2 capture are recycled, but a minor portion of the amines are degraded after one electron oxidation process, leading to highly toxic substances. The combination of the complementary information obtained from photoinduced electron transfer (flash photolysis) and heterogeneous electron transfer (electrochemistry) appears to be very attractive to fully characterize the electron transfer reaction mechanism of reactive species in general, as well as for determining important thermodynamic properties, such as standard potentials (E°) or pKa values. It is particularly difficult to determine these crucial data accurately in the cases of alkyl primary amines. Hence, in this manuscript we focus on the establishment of the several alkyl primary amines oxidation mechanism in organic aprotic solvents. In order to achieve this, this work combines information provided by flash photolysis (nanosecond), cyclic voltammetry (millisecond), and digital simulation (nanomile-second). Moreover, the accuracy of the E° values calculated using the nanosecond equilibrium method allows not only revising them, but also estimating new important thermodynamic data concerning the bond dissociation energies (BDEs) of ammonium cations (N(+)-H) and of the amine cation radicals (α-C-H), as well as their corresponding pKa values.

Encapsulation and release mechanisms in coordination polymer nanoparticles.

The interplay of guest encapsulation and release mechanisms in nanoscale metal-organic vehicles and its effect on the drug-delivery kinetics of these materials were investigated through a new multidisciplinary approach. Two rationally-designed molecular guests were synthesized, which consist of a red-fluorescent benzophenoxazine dye covalently tethered to a coordinating catechol group and a protected, non-coordinating catechol moiety. This allowed loading of the guests into compositionally and structurally equivalent coordination polymer particles through distinct encapsulation mechanisms: coordination and mechanical entrapment. The two types of particles delivered their fluorescent cargo with remarkably different kinetic profiles, which could be satisfactorily modeled considering degradation- and diffusion-controlled release processes. This demonstrates that careful selection of the method of guest incorporation into coordination polymer nanoparticles allows selective tuning of the rate of drug delivery from these materials and, therefore, of the time window of action of the encapsulated therapeutic agents.

Color stability of white mineral trioxide aggregate.

OBJECTIVE: This study aims to evaluate the color stability of white mineral trioxide aggregate (WMTA) after irradiation with three different curing lights and with a fluorescent lamp in an oxygen-free environment. MATERIAL AND METHODS: Thirty samples of WMTA were divided into four experimental groups (three curing light and one fluorescent lamp) and one negative control group. The samples in the curing light groups were immersed in glycerine and were irradiated for 20, 60, and 120 s with a curing light. The samples in the fluorescent lamp group were immersed in glycerine and left on a laboratory shelf below a fluorescent lamp, whereas the negative control group was irradiated with a curing light without immersion in glycerine. A spectrophotometer was used to determine the color of each specimen before and after each light exposure and after 5 days. Data were analyzed using analysis of variance and Fisher's least significant difference test. RESULTS: All the groups showed discoloration except for the negative control group. At 20, 60, and 120 s, there were no significant differences between the Optilux and Bluephase groups (which were the darkest). The Demi group was the curing light experimental group that showed the lowest degree of discoloration (P = 0.0001). No differences were observed between the fluorescent lamp and the negative control groups. After 5 days, the fluorescent lamp group also showed darkening of the sample surface and there were no significant differences between this group and the other three experimental groups (P > 0.05). CONCLUSIONS: WMTA showed dark discoloration after irradiation with a curing light or fluorescent lamp in an oxygen-free environment. CLINICAL RELEVANCE: WMTA may cause tooth discoloration when it is used in a coronal position.

Probing a polar cluster in the retinal binding pocket of bacteriorhodopsin by a chemical design approach.

Bacteriorhodopsin has a polar cluster of amino acids surrounding the retinal molecule, which is responsible for light harvesting to fuel proton pumping. From our previous studies, we have shown that threonine 90 is the pivotal amino acid in this polar cluster, both functionally and structurally. In an attempt to perform a phenotype rescue, we have chemically designed a retinal analogue molecule to compensate the drastic effects of the T90A mutation in bacteriorhodopsin. This analogue substitutes the methyl group at position C(13) of the retinal hydrocarbon chain by and ethyl group (20-methyl retinal). We have analyzed the effect of reconstituting the wild-type and the T90A mutant apoproteins with all-trans-retinal and its 20-methyl derivative (hereafter, 13-ethyl retinal). Biophysical characterization indicates that recovering the steric interaction between the residue 90 and retinal, eases the accommodation of the chromophore, however it is not enough for a complete phenotype rescue. The characterization of these chemically engineered chromoproteins provides further insight into the role of the hydrogen bond network and the steric interactions involving the retinal binding pocket in bacteriorhodopsin and other microbial sensory rhodopsins.

Surface-structured molecular sensor for the optical detection of acidity.

In this letter, we report on the development of a surface molecular sensor for the detection of acidity. Lithographically controlled wetting deposition has been applied to form the nanostructure of a new fluorescent compound with three protonation states featuring different optical properties on a glass substrate. Atomic force microscopy demonstrates the functionalization of the surface with ordered arrays of the sensor molecules. The fluorescence properties of the resulting nanopattern at different pH values have been investigated by confocal fluorescene microsopy, thus revealing the fast, sensitive, reversible response of the prepared nanosensor to gas flows of varying acidity.

Investigation of an acid-base and redox molecular switch: from bulk to the single-molecule level.

In this work we investigate a new fluorescent molecular switch based on the interconversion between the fluorescent zwitterionic form (ZW1) and the non-fluorescent anionic state (MC2) of a spirocyclic Meisenheimer complex of 1,3,5-trinitrobenzene. Density functional theory molecular orbital calculations reveal that photo-induced electron transfer from a guanidine group to the trinitrocyclohexadiene fluorophore of the complex quenches the emission from MC2. Protonation, as well as coordination of other Lewis acids to the guanidine group, suppress the quenching mechanism and allow the complex to fluoresce. In agreement with the calculations, reversible on-off fluorescence switching of the ZW1-MC2 bulk system occurs by protonation-deprotonation of the guanidine moiety upon acid-base addition. Interestingly, spectroelectrochemical ensemble measurements show that switching of the ZW1-MC2 pair can also be attained electrochemically, thus unraveling the versatile functioning of this system. The ultimate limit of monitoring the reversible on-off operation of individual switch molecules is reached by means of single-molecule fluorescence spectroscopy, which demonstrates the potential of the ZW1-MC2 system to be used as a true single-molecule switch on the nanometer scale.

Inter-helical hydrogen bonds are essential elements for intra-protein signal transduction: the role of Asp115 in bacteriorhodopsin transport function.

The behavior of the D115A mutant was analyzed by time-resolved UV-Vis and Fourier transformed infrared (FTIR) spectroscopies, aiming to clarify the role of Asp115 in the intra-protein signal transductions occurring during the bacteriorhodopsin photocycle. UV-Vis data on the D115A mutant show severely desynchronized photocycle kinetics. FTIR data show a poor transmission of the retinal isomerization to the chromoprotein, evidenced by strongly attenuated helical changes (amide I), the remarkable absence of environment alterations and protonation/deprotonation events related to Asp96 and direct Schiff base (SB) protonation form the bulk. This argues for the interactions of Asp115 with Leu87 (via water molecule) and Thr90 as key elements for the effective and vectorial proton path between Asp96 and the SB, in the cytoplasmic half of bacteriorhodopsin. The results strongly suggest the presence of a regulation motif enclosed in helices C and D (Thr90-Pro91/Asp115) which drives properly the dynamics of helix C through a set of interactions. It also supports the idea that intra-helical hydrogen bonding clusters in the buried regions of transmembrane proteins can be potential elements in intra-protein signal transduction.

Inductive vs solvation effects in primary alkyl amines: determination of the standard potentials.

The determination of the standard potential of alkyl primary amines is reported for the first time using the nanosecond equilibrium method. The versatility and accuracy of the method demonstrates that it is not only an alternative to the classical and modern electrochemical methods, but also a powerful tool for quantifying inductive and/or solvation effects in a related family of compounds. Two different trends were observed depending on alkyl chain length. For "short-chain" alkyl primary amines, where the solvation around the amino group is expected to be the same, the standard potential value appears to follow a linear relationship with the number of carbon atoms, which indicates that the methylene group (-CH2-) causes an inductive effect that is responsible for the stabilization of the amine cation radical. Meanwhile, the E(o) rises slightly to a constant potential value 1.500 V for "long-chain" unbranched alkyl primary amines. This interesting result can be explained by a steric inhibition of solvation around the amino group due to a fold of the long alkyl chain following a solvent exclusion mechanism.

The role of proline residues in the dynamics of transmembrane helices: the case of bacteriorhodopsin.

Proline residues in transmembrane helices have been found to have important roles in the functioning of membrane proteins. Moreover, Pro residues occur with high frequency in transmembrane alpha-helices, as compared to alpha-helices for soluble proteins. Here, we report several properties of the bacteriorhodopsin mutants P50A (helix B), P91A (helix C) and P186A (helix F). Compared to wild type, strongly perturbed behaviour has been found for these mutants. In the resting state, increased hydroxylamine accessibility and altered Asp-85 pKa and light-dark adaptation were observed. On light activation, hydroxylamine accessibility was increased and proton transport activity, M formation kinetics and FTIR difference spectra of M and N intermediates showed clear distortions. On the basis of these alterations and the near identity of the crystalline structures of mutants with that of wild type, we conclude that the transmembrane proline residues of bacteriorhodopsin fulfil a dynamic role in both the resting and the light-activated states. Our results are consistent with the notion that mutation of Pro to Ala allows the helix to increase its flexibility towards the direction originally hindered by the steric clash between the ring Cgamma and the carbonyl O of the i-4 residue, at the same time decreasing the mobility towards the opposite direction. Due to their properties, transmembrane Pro residues may serve as transmission elements of conformational changes during the transport process. We propose that these concepts can be extended to other transmembrane proteins.

A photocatalytic acid- and base-free Meerwein-Ponndorf-Verley-type reduction using a [Ru(bpy)3]2+/viologen couple.

A photocatalytic system to effect the Meerwein-Ponndorf-Verley reduction of carbonylic compounds to alcohols has been developed. The system comprises [Ru(bpy)3]2+ as a photosensitizer, triethanolamine as a sacrificial electron donor, viologen as an electron acceptor, and the carbonyl compound and iPrOH as Meerwein-Ponndorf-Verley reagents. The photocatalytic reaction can be performed in neat iPrOH or in 1-butyl-3-methylimidazolium ionic liquid. Mass spectrometric detection of the viologen hydride derivative VH+ confirms that this species is the reducing agent responsible for the carbonyl compound reduction. The reaction intermediates involved in the photocatalytic system have also been characterized by laser flash photolysis.

Anionic organic guests incorporated in zeolites: adsorption and reactivity of a Meisenheimer complex in faujasites.

Zeolites are suitable microporous hosts for positively charged organic species, but it is believed that they cannot adsorb organic anions. Pure Meisenheimer complex, derived from reduction of 2,4-dinitroaniline with NaBH4, was adsorbed inside faujasite cavities. Evidence for the internal incorporation of this negatively charged reaction intermediate comes from 1) XPS elemental analysis as a function of the depth of penetration into the particle, 2) the remarkable blue shift in lambda(max) of the Meisenheimer complex adsorbed on zeolite (ca. 470 nm) as compared to that in acetonitrile (580 nm) and 3) from the lack of reactivity with size-excluded hydride-acceptor reagents. Evidence is provided in support of an adsorption mechanism in which a neutral ion pair (alkali metal ion + Meisenheimer anion) is the actual species being adsorbed. In fact it appears that there is remarkable increase in the association constant for the ion-pair complex within the zeolite cavities as compared to DMF solution. Although this mechanism of adsorption as an ion-pair complex has precedents in the adsorption of some inorganic salts, what is novel is the notable increase in the stability and persistence of the Meisenheimer anion (a anionic reaction intermediate) as a result of zeolite inclusion. Adsorbed Meisenheimer complex exhibits much lower reactivity towards electron acceptors, oxygen, and water. Cyclic voltammetry of zeolite-modified electrodes reveals for the Meisenheimer complex adsorbed on LiY a reversible redox peak that is not observed in solution and has been interpreted as arising from site isolation and stabilisation of the electrochemically generated species.

Thr-90 plays a vital role in the structure and function of bacteriorhodopsin.

The role of Thr-90 in the bacteriorhodopsin structure and function was investigated by its replacement with Ala and Val. The mutant D115A was also studied because Asp-115 in helix D forms a hydrogen bond with Thr-90 in helix C. Differential scanning calorimetry showed a decreased thermal stability of all three mutants, with T90A being the least stable. Light-dark adaptation of T90A was found to be abnormal and salt-dependent. Proton transport monitored using pyranine signals was approximately 10% of wild type for T90A, 20% for T90V, and 50% for D115A. At neutral or alkaline pH, the M rise of these mutants was faster than that of wild type, whereas M decay was slower in T90A. Overall, Fourier transform infrared (FTIR) difference spectra of T90A were strongly pH-dependent. Spectra recorded on films adjusted at the same pH at 243 or 277 K, dry or wet, showed similar features. The D115A and T90V FTIR spectra were closer to WT, showing minor structural differences. The band at 1734 cm(-1) of the deconvoluted FTIR spectrum, corresponding to the carboxylate of Asp-115, was absent in all mutants. In conclusion, Thr-90 plays a critical role in maintaining the operative location and structure of helix C through three complementary interactions, namely an interhelical hydrogen bond with Asp-115, an intrahelical hydrogen bond with the peptide carbonyl oxygen of Trp-86, and a steric contact with the retinal. The interactions established by Thr-90 emerge as a general feature of archaeal rhodopsin proteins.