Considerations about the inhibition of monophenolase and diphenolase activities of tyrosinase. Characterization of the inhibitor concentration which generates 50 % of inhibition, type and inhibition constants. A review.

Tyrosinase is a copper oxidase enzyme which catalyzes the first two steps in the melanogenesis pathway, L-tyrosine to L-dopa conversion and, then, to o-dopaquinone and dopachrome. Hypopigmentation and, above all, hyperpigmentation issues can be originated depending on their activity. This enzyme also promotes the browning of fruits and vegetables. Therefore, control of their activity by regulators is research topic of great relevance. In this work, we consider the use of inhibitors of monophenolase and diphenolase activities of the enzyme in order to accomplish such control. An experimental design and data analysis which allow the accurate calculation of the degree of inhibition of monophenolase activity (iM) and diphenolase activity (iD) are proposed. The IC50 values (amount of inhibitor that causes 50 % inhibition at a fixed substrate concentration) can be calculated for the two activities and from the values of IC50M (monophenolase) and IC50D(diphenolase). Additionally, the strength and type of inhibition can be deduced from these values. The data analysis from these IC50D values allows to obtain the values of [Formula: see text] or [Formula: see text] , or and [Formula: see text] from the values of IC50M. In all cases, the values of the different must satisfy their relationship with IC50M and IC50D.

Synthesis of 'Impossible' Rotaxanes.

'Impossible' rotaxanes, which are constituted by interlocked components without obvious binding motifs, have attracted the interest of the mechanically interlocked molecules (MIMs) community. Within the synthetic efforts reported in the last decades towards the preparation of MIMs, some innovative protocols for accessing 'impossible' rotaxanes have been developed. This short review highlights different selected synthetic examples of 'impossible' rotaxanes, as well as suggests some future directions of this research area.

Exploring the Chemistry of the Mechanical Bond: Synthesis of a [2]Rotaxane through Multicomponent Reactions.

The synthesis of a [2]rotaxane through three- or five-component coupling reactions has been adapted to an organic chemistry experiment for upper-division students. The experimental procedure addresses the search for the most favorable reaction conditions for the synthesis of the interlocked compound, which is obtained in a yield of up to 71%. Moreover, the interlocked nature of the rotaxane is proven by NMR spectroscopy. The content of the sessions has been designed on the basis of a proactive methodology whereby upper-division undergraduate students have a dynamic role. The laboratory experience not only introduces students to the chemistry of the mechanical bond but also reinforces their previous knowledge of basic organic laboratory procedures and their skills with structural elucidation techniques such as NMR and FT-IR spectroscopies. The experiment has been designed in such a customizable way that both experimental procedures and laboratory material can be adapted to a wide range of undergraduate course curricula.

Light-responsive rotaxane-based materials: inducing motion in the solid state.

Light-responsive rotaxane-based solid-state materials are ideal scaffolds in order to develop smart materials due to the properties provided by the mechanical bond, such as control over the dynamics of the components upon application of external stimuli. This perspective aims to highlight the relevance of these materials, by pointing out recent examples of photoresponsive materials prepared from a rotaxanated architecture in which motion of the counterparts and/or macroscopic motion of the interlocked materials are achieved. Although further development is needed, these materials are envisioned as privileged scaffolds which will be used for different advanced applications in the area of molecular machinery.

Control of the assembly of a cyclic hetero[4]pseudorotaxane from a self-complementary [2]rotaxane.

The synthesis of a ditopic interlocked building-block and its self-assembly into a cyclic dimer is reported herein. Starting from a thread with two recognition sites, a three-component clipping reaction was carried out to construct a bistable [2]rotaxane. A subsequent Suzuki cross-coupling reaction allowed the connection of a second ring to that of the rotaxane, affording a self-complementary ditopic system. NMR studies were carried out to identify a cyclic hetero[4]pseudorotaxane as the main supramolecular structure in solution. Its assembly is the result of a positive cooperativity operating in the hydrogen-bonding-driven assembly of this mechanically interlocked supramolecule, as revealed by computational studies. The increase of the polarity of the solvent allows the disruption of the intercomponent interactions and the disassembly of the hetero[4]pseudorotaxane into the two interlocked units. The disassembly of the cyclic dimer was also achieved through a Diels-Alder reaction over the fumaramide binding site of the thread, triggering the translational motion of the entwined macrocycle to an adjacent glycylglycine-based station and precluding the supramolecular dimerization. The competitive molecular recognition of a guest molecule by one of the self-templating counterparts of the dimer also led to the controlled disassembly of the hetero[4]pseudorotaxane.

Stereoselective synthesis of ß-lactams: recent examples.

The synthesis of ß-lactam derivatives is a research topic of great interest due to the biological activity of these molecules. Indeed, there are several antibiotics which include a ß-lactam core in their structures, such as penicilins, monobactams, carbacephems and cephamycins. The development of stereoselective approaches to access these molecular architectures turns out to be necessary in order to take advantage of the distinct properties provided by the different stereoisomers. This review covers recent advances towards the stereoselective synthesis of ß-lactams, including Staudinger syntheses, cascade reactions, metal-catalyzed syntheses and base-promoted cyclizations. Within these methods, some particularly novel synthetic approaches are highlighted, such as the induction of chirality through bimetallic synergistic catalysis or the transfer of chirality between components in mechanically interlocked molecules. Additionally, a critical opinion on the state of the art of this research field is offered, remarking key points on which the future research should be focused on.

Recent Advances in the Preparation of Delivery Systems for the Controlled Release of Scents.

Scents are volatile compounds highly employed in a wide range of manufactured items, such as fine perfumery, household products, and functional foods. One of the main directions of the research in this area aims to enhance the longevity of scents by designing efficient delivery systems to control the release rate of these volatile molecules and also increase their stability. Several approaches to release scents in a controlled manner have been developed in recent years. Thus, different controlled release systems have been prepared, including polymers, metal-organic frameworks and mechanically interlocked systems, among others. This review is focused on the preparation of different scaffolds to accomplish a slow release of scents, by pointing out examples reported in the last five years. In addition to discuss selected examples, a critical perspective on the state of the art of this research field is provided, comparing the different types of scent delivery systems.

Ring-to-Thread Chirality Transfer in [2]Rotaxanes for the Synthesis of Enantioenriched Lactams.

The synthesis of chiral mechanically interlocked molecules has attracted a lot of attention in the last few years, with applications in different fields, such as asymmetric catalysis or sensing. Herein we describe the synthesis of orientational mechanostereoisomers, which include a benzylic amide macrocycle with a stereogenic center, and nonsymmetric N-(arylmethyl)fumaramides as the axis. The base-promoted cyclization of the initial fumaramide thread allows enantioenriched value-added compounds, such as lactams of different ring sizes and amino acids, to be obtained. The chiral information is effectively transmitted across the mechanical bond from the encircling ring to the interlocked lactam. High levels of enantioselectivity and full control of the regioselectivity of the final cyclic compounds are attained.

Photoresponsive Metal-Organic Frameworks as Adjustable Scaffolds in Reticular Chemistry.

The easy and remote switching of light makes this stimulus an ideal candidate for a large number of applications, among which the preparation of photoresponsive materials stands out. The interest of several scientists in this area in order to achieve improved functionalities has increase parallel to the growth of the structural complexity of these materials. Thus, metal-organic frameworks (MOFs) turned out to be ideal scaffolds for light-responsive ligands. This review is focused on the integration of photoresponsive organic ligands inside MOF crystalline arrays to prepare enhanced functional materials. Besides the summary of the preparation, properties and applications of these materials, an overview of the future outlook of this research area is provided.

Mechanically interlocked molecules in metal-organic frameworks.

Mechanically interlocked molecules (MIMs) have great potential in the development of molecular machinery due to their intercomponent dynamics. The incorporation of these molecules in a condensed phase makes it possible to take advantage of the control of the motion of the components at the macroscopic level. Metal-organic frameworks (MOFs) are postulated as ideal supports for intertwined molecules. This review covers the chemistry of the mechanical bond incorporated into metal-organic frameworks from the seminal studies to the latest published advances. We first describe some fundamental concepts of MIMs and MOFs. Next, we summarize the advances in the incorporation of rotaxanes and catenanes inside MOF matrices. Finally, we conclude by showing the study of the rotaxane dynamics in MOFs and the operation of some stimuli-responsive MIMs within MOFs. In addition to emphasising some selected examples, we offer a critical opinion on the state of the art of this research field, remarking the key points on which the future of these systems should be focused.

Maximizing the [c2]daisy chain to lasso ratio through competitive self-templating clipping reactions.

Self-templating two-component coupling reactions allowed the isolation of two threaded products with different molecular sizes: a lasso-type [1]rotaxane and a [c2]daisy chain rotaxane. Their distribution in the final reaction mixture varies as a factor of the concentration of the reactants. Through this methodology we obtained a large 84-membered cyclic multistation [2]rotaxane.

Effective Encapsulation of C60 by Metal-Organic Frameworks with Polyamide Macrocyclic Linkers.

A flexible benzylic amide macrocycle, functionalized with two carboxylic acid groups, was employed as the organic ligand for the preparation of robust copper(II)- and zinc(II)-based metal-organic frameworks. These polymers crystallized in the C2/m space group of the monoclinic crystal system, creating non-interpenetrated channels in one direction with an extraordinary solvent-accessible volume of 46 %. Unlike metal-organic rotaxane frameworks having benzylic amide macrocycles as linkers, the absence of the thread in these novel reticular materials causes a decrease of dimensionality and an improvement of pore size and dynamic guest adaptability. We studied the incorporation of fullerene C60 inside the adjustable pocket generated between two macrocycles connected to the same dinuclear clusters, occupying a remarkable 98 % of the cavities inside the network. The use of these materials as hosts for the selective recognition of different fullerenes was evaluated, mainly encapsulating the smaller size fullerene derivative in several mixtures of C60 and C70 .

Copper-Linked Rotaxanes for the Building of Photoresponsive Metal Organic Frameworks with Controlled Cargo Delivery.

We have prepared a photoresponsive metal-organic framework by using an amide-based [2]rotaxane as linker and copper(II) ions as metal nodes. The interlocked linker was obtained by the hydrogen bond-directed approach employing a fumaramide thread as template of the macrocyclic component, this latter incorporating two carboxyl groups. Single crystal X-ray diffraction analysis of the metal-organic framework, prepared under solvothermal conditions, showed the formation of stacked 2D rhombohedral grids forming channels decorated with the interlocked alkenyl threads. A series of metal-organic frameworks differing in the E/Z olefin ratio were prepared either by the previous isomerization of the linker or by postirradiation of the reticulated materials. By dynamic solid state 2H NMR measurements, using deuterium-labeled materials, we proved that the geometry of the olefinic axis of the interlocked struts determined the obtention of materials with different independent local dynamics as a result of the strength of the intercomponent noncovalent interactions. Moreover, the usefulness of these novel copper-rotaxane materials as molecular dosing containers has also been assayed by the diffusion and photorelease of p-benzoquinone, evaluated in different solvents and temperatures.

Charge transfer in graphene quantum dots coupled with tetrathiafulvalenes.

Water-soluble fluorescent graphene quantum dots have been successfully prepared through a top-down approach, that is, starting with graphite, and covalently functionalizing it with π-extended tetrathiafulvalene. Charge transfer investigations reveal noticeably slower charge recombination when compared with exTTF nanoconjugates featuring carbon nanodots, for which a larger presence of trap states is observed.

Light-driven exchange between extended and contracted lasso-like isomers of a bistable [1]rotaxane.

The synthesis of a set of benzylic amide [1]rotaxanes via a self-templating clipping approach is described. This methodology supposes the 1 + 1 coupling of isophthaloyl dichloride with an acyclic diamine precursor incorporating a templating arm. The structure of the threaded compounds was determined both in solution and in the solid state. The conversion into the corresponding unthreaded isomers, also obtained by deslipping of [2]rotaxane models, was evaluated in competitive and non-competitive hydrogen-bonding solvents. The switch of the extended and contracted lasso-like isomers of a bistable [1]rotaxane by an olefin isomerization promoted by UV light irradiation was also accomplished and their ring positional integrity was examined.

Catalysis and inhibition of tyrosinase in the presence of cinnamic acid and some of its derivatives.

The kinetic action of tyrosinase on l-tyrosine and l-Dopa as substrates in the presence of cinnamic acid and some of its derivatives has been characterized. Cinnamic acid, 2-hydroxycinnamic, 2,3 and 4-methoxycinnamic acids were seen to be inhibitors of tyrosinase being determined the type of inhibition and inhibition constants of all of them. However, 3-hydroxycinnamic, 4-hydroxycinnamic and 3,4-dihydroxycinnamic acids were seen to be substrates of tyrosinase at the same time. The kinetic constants of the catalysis of these substrates were determined and found to be perfectly correlated with the chemical shifts of the carbon with the phenolic hydroxyl group revealed by NMR. Docking studies of 2-hydroxycinnamic and 3-hydroxycinnamic acids showed that tyrosinase is able to hydroxylate 3-hydroxycinnamic acid but is unable to hydroxylate 2-hydroxycinnamic acid.

Action of tyrosinase on caffeic acid and its n-nonyl ester. Catalysis and suicide inactivation.

Different mechanisms for inhibiting tyrosinase can be designed to avoid postharvest quality losses of fruits and vegetables. The action of tyrosinase on caffeic acid and its n-nonyl ester (n-nonyl caffeate) was characterized kinetically in this work. The results lead us to propose that both compounds are suicide substrates of tyrosinase, for which we establish the catalytic and inactivation efficiencies. The ester is more potent as inactivator than the caffeic acid and the number of turnovers made by one molecule of the enzyme before its inactivation (r) is lower for the ester. We proposed that the anti-browning and antibacterial properties may be due to suicide inactivation processes.

Structural and kinetic considerations on the catalysis of deoxyarbutin by tyrosinase.

Deoxyarbutin, a potent inhibitor of tyrosinase, could act as substrate of the enzyme. Oxytyrosinase is able to hydroxylate deoxyarbutin and finishes the catalytic cycle by oxidizing the formed o-diphenol to quinone, while the enzyme becomes deoxytyrosinase, which evolves to oxytyrosinase in the presence of oxygen. This compound is the only one described that does not release o-diphenol after the hydroxylation step. Oxytyrosinase hydroxylates the deoxyarbutin in ortho position of the phenolic hydroxyl group by means of an aromatic electrophilic substitution. As the oxygen orbitals and the copper atoms are not coplanar, but in axial/equatorial position, the concerted oxidation/reduction cannot occur and the release of a copper atom to bind again in coplanar position, enabling the oxidation/reduction or release of the o-diphenol from the active site to the medium. In the case of deoxyarbutin, the o-diphenol formed is repulsed by the water due to its hydrophobicity, and so can bind correctly and be oxidized to a quinone before being released. Deoxyarbutin has been characterized with: [Formula: see text] = 1.95 ± 0.06 s-1 and [Formula: see text] = 33 ± 4 µM. Computational simulations of the interaction of ß-arbutin, deoxyarbutin and their o-diphenol products with tyrosinase show how these ligands bind at the copper centre of tyrosinase. The presence of an energy barrier in the release of the o-diphenol product of deoxyarbutin, which is not present in the case of ß-arbutin, together with the differences in polarity and, consequently differences in their interaction with water help understand the differences in the kinetic behaviour of both compounds. Therefore, it is proposed that the release of the o-diphenol product of deoxyarbutin from the active site might be slower than in the case of ß-arbutin, contributing to its oxidation to a quinone before being released from the protein into the water phase.

Effects on Rotational Dynamics of Azo and Hydrazodicarboxamide-Based Rotaxanes.

The synthesis of novel hydrogen-bonded [2]rotaxanes having two pyridine rings in the macrocycle and azo- and hydrazodicarboxamide-based templates decorated with four cyclohexyl groups is described. The different affinity of the binding sites for the benzylic amide macrocycle and the formation of programmed non-covalent interactions between the interlocked components have an important effect on the dynamic behavior of these compounds. Having this in mind, the chemical interconversion between the azo and hydrazo forms of the [2]rotaxane was investigated to provide a chemically-driven interlocked system enable to switch its circumrotation rate as a function of the oxidation level of the binding site. Different structural modifications were carried out to further functionalize the nitrogen of the pyridine rings, including oxidation, alkylation or protonation reactions, affording interlocked azo-derivatives whose rotation dynamics were also analyzed.