Low-molecular-weight gels from amino acid and peptide derivatives for controlled release and delivery

Low-molecular-weight (LMW) gelators are a versatile class of compounds able to self-assemble and to form supramolecular materials, such as gels. The use of LMW peptides to produce these gels shows many advantages, because of their wide structure tunability, the low-cost and effective synthesis, and the in vivo biocompatibility and biodegradability, which makes them optimal candidates for release and delivery applications. In addition, in these materials, the binding of the hosts may occur through a variety of noncovalent interactions, which are also the main factors responsible for the self-assembly of the gelators, and through specific interactions with the fibers or the pores of the gel matrix. This review aims to report LMW gels based on amino acid and peptide derivatives used for the release of many different species (drugs, fragrances, dyes, proteins, and cells) with a focus on the possible strategies to incorporate the cargo in these materials, and to demonstrate how versatile these self-assembled materials are in several applications.

A supramolecular approach towards the photorelease of encapsulated caged acids in water: 7-diethylaminothio-4-coumarinyl molecules as triggers

Herein, we establish the release of aliphatic acids in water upon excitation of 7-diethylaminothio-4-coumarinyl derivatives encapsulated within the organic host octa acid (OA). The 7-diethylaminothio-4-coumarinyl skeleton, employed here as the trigger, photoreleases caged molecules from the excited triplet state, in contrast to its carbonyl analogue, where the same reaction is known to occur from the excited singlet state. Encapsulation in OA solubilizes molecules in water that are otherwise water-insoluble, and retains the used trigger within itself following the release of the aliphatic acid. Such supramolecular characteristics usher in new features to the photorelease methodology. The thiocarbonyl chromophore extends the absorption of coumarinyl trigger to visible range while enhancing the intersystem crossing (ISC) to the triplet state, making it the reactive state. Despite the non-polar environment within the OA capsules the photocleavage occurs in a heterolytic fashion to release the conjugate base and the used trigger as triplet carbocation in an adiabatic process. Interestingly, the triplet carbocation crosses to the ground singlet surface (closed shell singlet carbocation) with the help of water molecules, possibly aided by C = S chromophore. Utilizing the known excited state dynamics of related thiocoumarinyl and coumarinyl systems, we have identified a few of the important mechanistic features of the photorelease process of 7-diethylaminothio-4-coumarinyl derivatives. Ultrafast excited state dynamic studies and quantum chemical calculations planned should help us better understand the photorelease process so as to effectively exploit the proposed system for potential applications.

Switchable Synthesis of Tritylone Alcohols and 2-Benzoylbenzoate Esters from Spiroindane-1,3-diones

A solvent-controlled regioselective rearrangement reaction of spiroindane-1,3-diones with a leaving group has been developed. In acetonitrile solvent, the spiroindane-1,3-diones 3 were rearranged to provide tritylone alcohols, while 2-benzoylbenzoate ester derivatives were obtained if the reactions were performed in alcohols.

Photocaging of Carboxylic Function Bearing Biomolecules by New Thiazole Derived Fluorophore

The design and synthesis of a new fluorophore containing an arylidene thiazole scaffold resulted in a compound with good photophysical characteristics. Furthermore, the thiazole C5-methyl group was easily modified into specific functional groups (CH2 Br and CH2 OH) for the formation of a series of photocourier molecules containing model compounds (benzoic acids), as well as prodrugs, including salicylic acid, caffeic acid, and chlorambucil via a "benzyl" linker. Spectral characteristics (1 H, 13 C NMR, and high-resolution mass spectra) corresponded to the proposed structures. The photocourier molecules demonstrated absorption with high values of coefficient of molar extinction, exhibited contrasting green emission, and showed good dark stability. The mechanism of the photorelease was investigated through spectral analysis, HPLC-HRMS, and supported by TD-DFT calculations. The photoheterolysis and elimination of carboxylic acids were proved to occur in the excited state, yielding a carbocation as an intermediate moiety. The fluorophore structure provided stability to the carbocation through the delocalization of the positive charge via resonance structures. Viability assessment of Vero cells using the MTT-test confirmed the weak cytotoxicity of prodrugs without irradiation and it increase upon UV-light.

Visible-Light-Initiated Palladium-Catalyzed Cross-coupling by PPh3 Uncaging from an Azobenzene Ruthenium-Arene Complex

Photo-release of triphenylphosphine from a sulfonamide azobenzene ruthenium-arene complex was exploited to activate PdII Cl2 into Pd0 catalyst, for the photo-initiation of Sonogashira cross-coupling. The transformation was initiated on demand - by using simple white LED strip lights - with a high temporal response and the ability to control reaction rate by changing the irradiation time. Various substrates were successfully applied to this photo-initiated cross-coupling, thus illustrating the wide functional-group tolerance of our photo-caged catalyst activator, without any need for sophisticated photochemistry apparatus.

Synthesis of Cyclic Fragrances via Transformations of Alkenes, Alkynes and Enynes: Strategies and Recent Progress

With increasing demand for customized commodities and the greater insight and understanding of olfaction, the synthesis of fragrances with diverse structures and odor characters has become a core task. Recent progress in organic synthesis and catalysis enables the rapid construction of carbocycles and heterocycles from readily available unsaturated molecular building blocks, with increased selectivity, atom economy, sustainability and product diversity. In this review, synthetic methods for creating cyclic fragrances, including both natural and synthetic ones, will be discussed, with a focus on the key transformations of alkenes, alkynes, dienes and enynes. Several strategies will be discussed, including cycloaddition, catalytic cyclization, ring-closing metathesis, intramolecular addition, and rearrangement reactions. Representative examples and the featured olfactory investigations will be highlighted, along with some perspectives on future developments in this area.

Photoactivatable o-Hydroxycinnamic Platforms for Bioimaging and Therapeutic Release

Photoactivatable or photoremovable protecting groups (PPGs) have become a powerful material and gained enormous interest in the field of biomedical applications. PPGs have been utilized for noninvasive, on-demand, spatio-temporal controlled release of biological effectors by irradiation with light to induce biochemical function. Over the past few years, o-hydroxycinnamate (oHC)-based PPGs have received considerable attention for the release of molecules of interest by either UV (one-photon) or near-IR (two-photon) irradiation. In this miniperspective, we have summarized the development of oHC PPGs for bioimaging and the controlled release of therapeutics, bioactive volatiles and other payloads with real-time monitoring. In addition, several future perspectives of oHC systems have been highlighted at the end of this miniperspective.

Mechanically Interlocked Profragrances for the Controlled Release of Scents

The synthesis of a series of interlocked profragrances and the study of the controlled release of the corresponding scents are reported. The structures of the profragrances are based on a [2]pseudorotaxane scaffold with a fumaramate thread derived from perfumery alcohols and a tetrabenzylamido ring. The delivery of the scents was accomplished by sequential thermal dethreading and further enzymatic hydrolysis. Alternatively, the dethreading can be achieved by increasing the polarity of the solvent or photochemical isomerization. The temperature of dethreading can be modulated by the steric demand of the ends of the thread, which allows the selection of different precursor structures depending on the desired rates of delivery. The inputs and outputs for the controlled release of the interlocked profragrances correspond to those of YES or AND logic gates.

Design of Stimuli-Responsive Dynamic Covalent Delivery Systems for Volatile Compounds (Part 2): Fragrance-Releasing Cleavable Surfactants in Functional Perfumery Applications

Amphiphilic imines prepared by condensation of a hydrophobic fragrance aldehyde with a hydrophilic amine derived from a poly(propylene oxide) and poly(ethylene oxide) diblock copolymer were investigated as cleavable surfactant profragrances in applications of functional perfumery. In water, the cleavable surfactants assemble into micelles that allow solubilization of perfume molecules that are not covalently attached to the surfactant. Dynamic headspace analysis on a glass surface showed that solubilized perfume molecules evaporated in a similar manner in the presence of the cleavable surfactant as compared with a non-cleavable reference surfactant. Under application conditions, the cleavable surfactant imine hydrolysed to release the covalently linked fragrance aldehyde. The profragrances were stable during storage in aqueous media, and upon dilution showed a blooming effect for the hydrolytical fragrance release and a more balanced performance of a solubilized perfume by retaining the more volatile fragrances and boosting the evaporation of the less volatile fragrances.

Design of Stimuli-Responsive Dynamic Covalent Delivery Systems for Volatile Compounds (Part 1): Controlled Hydrolysis of Micellar Amphiphilic Imines in Water

Despite their intrinsic hydrolysable character, imine bonds can become remarkably stable in water when self-assembled in amphiphilic micellar structures. In this work, we systematically studied some of these structures and the influence of various parameters that can be used to take control of their hydrolysis, including pH, concentration, the position of the imine function in the amphiphilic structure, relative lengths of the linked hydrophilic and hydrophobic moieties. Thermodynamic and kinetic data led us to the rational design of stable imines in water, partly based on the location of the imine function within the hydrophobic part of the amphiphile and on a predictable quantitative term that we define as the total hydrophilic-lipophilic balance (HLB). In addition, we show that such stable systems are also stimuli-responsive and therefore, of potential interest in trapping and releasing micellar components on demand.

Efficient implementation of isotropic cubic response functions for two-photon absorption cross sections within the self-consistent field approximation

Within the self-consistent field approximation, computationally tractable expressions for the isotropic second-order hyperpolarizability have been derived and implemented for the calculation of two-photon absorption cross sections. The novel tensor average formulation presented in this work allows for the evaluation of isotropic damped cubic response functions using only ∼3.3% (one-photon off-resonance regions) and ∼10% (one-photon resonance regions) of the number of auxiliary Fock matrices required when explicitly calculating all the needed individual tensor components. Numerical examples of the two-photon absorption cross section in the one-photon off-resonance and resonance regions are provided for alanine-tryptophan and 2,5-dibromo-1,4-bis(2-(4-diphenylaminophenyl)vinyl)-benzene. Furthermore, a benchmark set of 22 additional small- and medium-sized organic molecules is considered. In all these calculations, a quantitative assessment is made of the reduced and approximate forms of the cubic response function in the one-photon off-resonance regions and results demonstrate a relative error of less than ∼5% when using the reduced expression as compared to the full form of the isotropic cubic response function.

Visible-to-NIR-Light Activated Release: From Small Molecules to Nanomaterials

Photoactivatable (alternatively, photoremovable, photoreleasable, or photocleavable) protecting groups (PPGs), also known as caged or photocaged compounds, are used to enable non-invasive spatiotemporal photochemical control over the release of species of interest. Recent years have seen the development of PPGs activatable by biologically and chemically benign visible and near-infrared (NIR) light. These long-wavelength-absorbing moieties expand the applicability of this powerful method and its accessibility to non-specialist users. This review comprehensively covers organic and transition metal-containing photoactivatable compounds (complexes) that absorb in the visible- and NIR-range to release various leaving groups and gasotransmitters (carbon monoxide, nitric oxide, and hydrogen sulfide). The text also covers visible- and NIR-light-induced photosensitized release using molecular sensitizers, quantum dots, and upconversion and second-harmonic nanoparticles, as well as release via photodynamic (photooxygenation by singlet oxygen) and photothermal effects. Release from photoactivatable polymers, micelles, vesicles, and photoswitches, along with the related emerging field of photopharmacology, is discussed at the end of the review.

Understanding the Role of Aromaticity and Conformational Changes in Bond Dissociation Processes of Photo-Protecting Groups

Photoremovable protecting groups (PPGs) provide spatial and temporal control over the release of various chemicals. Using surface hopping studies with multireference electronic structure methods we have unravelled the nuclear and the electronic events at play. Furthermore, the electronic changes along the reaction path were probed using excited state aromaticity quantifiers and orbital analysis. We find that upon irradiation with light of appropriate wavelength on the substituted coumarin system a π-π* electronic excitation occurs which is followed by an electron loss from the aromatic ring on gaining proper alignment between the π* and the C-LG (LG = leaving group) σ*. This alignment is brought about by a critical dihedral angle change in the molecule, which subsequently triggers C-LG bond cleavage. The sequence of events is indicative of an intramolecular electron catalyzed process which is established through investigations of changes in aromaticity of the phenyl ring which acts as an electron reservoir.

Photocontrollable Release with Coumarin-Based Profragrances

The achievement of controllable and lasting scent on a targeted surface is a long-term goal in the field of flavors and fragrances. Herein, we design a novel series of phototriggered coumarin-based profragrances conjugated with volatile carboxylic fragrances via activatable chemical bridge of ester group, thereby achieving the controllable release of volatile fragrances under ambient conditions. Upon exposure to light, the fragile ester group of profragrances allows the slow release of fragrance molecules, building up a new light-sensitive fragrance delivery system. The incorporated coumarin unit of CM-OH as phototrigger is killing two birds with one stone, that is, precise photocontrollable release of fragrance molecules, and unprecedented fluorescence intensity to monitor the releasing process of fragrance molecules with linear relationship (R² > 0.95). In comparison, the light-induced releasing amount from profragrances of CM-O-EA, CM-O-PEA, CM-O-PA, and CM-O-CA is much lower than corresponding free fragrances by 33-, 8.5-, 13-, and 983-fold, respectively. As demonstrated, the coumarin-based profragrances provide a phototriggered platform to realize the controllable release of volatile fragrances, resulting in a long-lasting headspace concentration on the targeted surface of wallpaper.

Steric Demand and Rate-determining Step for Photoenolization of Di-ortho-substituted Acetophenone Derivatives

Laser flash photolysis of ketone 1 in argon-saturated methanol yields triplet biradical 1BR (τ = 63 ns) that intersystem crosses to form photoenols Z-1P (λ_max = 350 nm, τ ~ 10 μs) and E-1P (λ_max = 350 nm, τ > 6 ms). The activation barrier for Z-1P re-forming ketone 1 through a 1,5-H shift was determined as 7.7 ± 0.3 kcal mol^-1 . In contrast, for ketone 2, which has a less sterically hindered carbonyl moiety, laser flash photolysis in argon-saturated methanol revealed the formation of biradical 2BR (λ_max = 330 nm, τ ~ 303 ns) that intersystem crosses to form photoenol E-2P (λ_max = 350 nm, τ > 42 μs), but photoenol Z-2P was not detected. However, in more viscous basic H-bond acceptor (BHA) solvent, such as hexamethylphosphoramide, triplet 2BR intersystem crosses to form both Z-2P (λ_max = 370 nm, τ ~ 1.5 μs) and E-2P. Thus, laser flash photolysis of ketone 2 in methanol reveals that intersystem crossing from 2BR to form Z-2P is slower than the 1,5-H shift of Z-2P, whereas in viscous BHA solvents, the 1,5-H shift becomes slower than the intersystem crossing from 2BR to Z-2P. Density functional theory and coupled cluster calculations were performed to support the reaction mechanisms for photoenolization of ketones 1 and 2.

Azobenzene-benzoylphenylureas as photoswitchable chitin synthesis inhibitors

Benzoylphenylureas (BPUs) are used as synthetic insect growth regulators for inhibiting chitin synthesis. Merging insecticidal BPUs with photoswitchable azobenzene generated photoresponsive chitin synthesis inhibitors. A prepared azobenzene-benzoylphenylurea can be activated upon irradiation with UV light, and shows 6-fold and 2-fold activity difference to armyworm (Mythimna separata) and German cockroach (Blattella germanica) sulfonylurea receptors, respectively. This is the first example of a photoswitchable BPU insecticide. The generation of such a photoresponsive BPU insecticide allows for modulation of the insecticidal activity by light, and may facilitate the spatiotemporal control over the sulfonylurea receptor and the mechanistic study of this kind of insecticide.

Quinoline-Derived Two-Photon-Sensitive Octupolar Probes

A systematic study on quinoline‐derived light sensitive probes, having third‐order rotational symmetry is presented. The electronically linked octupolar structures show considerably improved linear and nonlinear photophysical properties under one‐ and two‐photon irradiation conditions compared to the corresponding monomers. Photolysis of the three acetate derivatives shows strong structure dependency: whereas irradiation of the 6‐ and 7‐aminoquinoline derivatives resulted in fast intramolecular cyclization and only trace amounts of fragmentation products, the 8‐aminoquinoline derivative afforded clean and selective photolysis, with a sequential release of their acetate groups (δ_u^[730]=0.67 GM).

Photolysis of a peptide with N-peptidyl-7-nitroindoline units using two-photon absorption

N-acyl-7-nitroindolines have been used as caged compounds to photorelease active molecules by a one- or two-photon excitation mechanism in biological systems. Here, we report the photolysis of a polypeptide that contains 7-nitroindoline units as linker moieties in its peptide backbone for potential materials engineering applications. Upon two-photon excitation with femtosecond laser light at 710 nm the photoreactive amide bond in N-peptidyl-7-nitroindolines is cleaved rendering short peptide fragments. Thus, this photochemical process changes the molecular composition at the laser focal volume. Gel modifications of this peptide can potentially be used for three-dimensional microstructure fabrication.

Photolabile acetals as profragrances: the effect of structural modifications on the light-induced release of volatile aldehydes on cotton

Dynamic headspace analysis above a cotton surface treated with a fabric softening formulation showed that, depending on the substitution pattern, the photoirradiation of 4,4-diphenyl-4H-benzo[d][1,3]dioxine profragrances released perfumery aldehydes in quantities spanning several orders of magnitude.

Photocages for protection and controlled release of bioactive compounds

A photoactivatable ketoprofen–avobenzone dyad was designed for the protection and controlled release of phototoxic drugs.

Trigger chemistries for better industrial formulations

In recent years, innovations and consumer demands have led to increasingly complex liquid formulations. These growing complexities have provided industrial players and their customers access to new markets through product differentiation, improved performance, and compatibility/stability with other products. One strategy for enabling more complex formulations is the use of active encapsulation. When encapsulation is employed, strategies are required to effect the release of the active at the desired location and time of action. One particular route that has received significant academic research effort is the employment of triggers to induce active release upon a specific stimulus, though little has translated for industrial use to date. To address emerging industrial formulation needs, in this review, we discuss areas of trigger release chemistries and their applications specifically as relevant to industrial use. We focus the discussion on the use of heat, light, shear, and pH triggers as applied in several model polymeric systems for inducing active release. The goal is that through this review trends will emerge for how technologies can be better developed to maximize their value through industrial adaptation.

Controlled release of encapsulated bioactive volatiles by rupture of the capsule wall through the light-induced generation of a gas

The encapsulation of photolabile 2-oxoacetates in core-shell microcapsules allows the light-induced, controlled release of bioactive compounds. On irradiation with UVA light these compounds degrade to generate an overpressure of gas inside the capsules, which expands or breaks the capsule wall. Headspace measurements confirmed the light-induced formation of CO and CO2 and the successful release of the bioactive compound, while optical microscopy demonstrated the formation of gas bubbles, the cleavage of the capsule wall, and the leakage of the oil phase out of the capsule. The efficiency of the delivery system depends on the structure of the 2-oxoacetate, the quantity used with respect to the thickness of the capsule wall, and the intensity of the irradiating UVA light.

Joint spectroscopic and theoretical investigation of cationic cyanine dye Astrazon Orange-R: solvent viscosity controlled relaxation of excited states

In this paper, the first study of cationic cyanine dye Astrazon Orange-R by combined spectroscopic and theoretical investigation is presented.

“Old” chemistry in a new context: photocleavable 2-oxoacetate-containing latex dispersions and core–shell microcapsules for the controlled release of volatile compounds

2-Oxoacetates polymerised into nanoparticles or encapsulated in microcapsules are stable against hydrolysis and release fragrances on UVA irradiation.

Functionalized cellulose nanocrystals as nanocarriers for sustained fragrance release

A new family of nanocarrier-based pro-fragrances with high affinity to cotton releasing their payload under everyday life conditions is reported. The delivery systems were prepared by decorating cellulose nanocrystals (CNCs) with β-damascone, which is slowly released via a retro 1,4-Michael-type reaction in applications of functional perfumery.

Dynamic combinatorial/covalent chemistry: a tool to read, generate and modulate the bioactivity of compounds and compound mixtures

Reversible covalent bond formation under thermodynamic control adds reactivity to self-assembled supramolecular systems, and is therefore an ideal tool to assess complexity of chemical and biological systems. Dynamic combinatorial/covalent chemistry (DCC) has been used to read structural information by selectively assembling receptors with the optimum molecular fit around a given template from a mixture of reversibly reacting building blocks. This technique allows access to efficient sensing devices and the generation of new biomolecules, such as small molecule receptor binders for drug discovery, but also larger biomimetic polymers and macromolecules with particular three-dimensional structural architectures. Adding a kinetic factor to a thermodynamically controlled equilibrium results in dynamic resolution and in self-sorting and self-replicating systems, all of which are of major importance in biological systems. Furthermore, the temporary modification of bioactive compounds by reversible combinatorial/covalent derivatisation allows control of their release and facilitates their transport across amphiphilic self-assembled systems such as artificial membranes or cell walls. The goal of this review is to give a conceptual overview of how the impact of DCC on supramolecular assemblies at different levels can allow us to understand, predict and modulate the complexity of biological systems.

Tailoring of organic dyes with oxidoreductive compounds to obtain photocyclic radical generator systems exhibiting photocatalytic behavior

The combination of a dye which absorbs the photon, an electron acceptor and an electron donor leading to energy conversion through electron transfer, was the basis of the so called three-component systems. In this paper, an experimental work combining Rose bengal dye with a triazine derivative as electron acceptor and ethyl 4-(dimethylamino)benzoate as electron donor, will underline the benefit of the photocyclic behavior of three-component systems leading to the dye regeneration. A thermodynamic approach of the photocycle is presented, followed by a mechanistic and computational study of ideal photocycles, in order to outline the specific kinetics occuring in so called photocatalytic systems. The simple kinetic model used is enough to outline the benefit of the cyclic system and to give the basic requirements in term of chemical combination needed to be fulfilled in order to obtain a photocatalytic behavior.

Quaternary ammonium salts of phenylglyoxylic acid as photobase generators for thiol-promoted epoxide photopolymerization

A novel series of photobase generators (PBGs), which are quaternary ammonium salts of phenylglyoxylic acid (PA) having the ability to generate strong bases such as 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), 1,5-diazabicyclo[4.3.0]non-5-ene (DBN) or 1,1,3,3-tetramethylguanidine (TMG), was synthesized.

Controlled fragrance release from galactose-based pro-fragrances

Conjugating α,β-unsaturated ketonesviaa thioether linkage to carbohydrates is an efficient way to modulate the polarity of the precursors and thus to influence the release of damascones in functional perfumery.