Cecilia Paris

A Blocked Diketo Form of Avobenzone: Photostability, Photosensitizing Properties and Triplet Quenching by a Triazine‐derived UVB‐filter

Novel sunscreens are required providing active protection in the UVA and UVB regions. On the other hand, there is an increasing concern about the photosafety of UV filters, as some of them are not sufficiently photostable. Avobenzone is one of the most frequently employed sunscreen ingredients, but it has been reported to partially decompose after irradiation. In the present work, photophysical and photochemical studies on a methylated avobenzone‐derivative have shown that the diketo form is responsible for photodegradation. A transient absorption was observed at 380 nm after laser flash photolysis excitation at 308 nm. It was assigned to the triplet excited state of the diketo form, as inferred from quenching by oxygen and β‐carotene. This transient also interacted with key building blocks of biomolecules by triplet–triplet energy transfer (in the case of thymidine) or electron transfer processes (for 2′‐deoxyguanosine, tryptophan and tyrosine). Irradiation of the avobenzone derivative in the presence of a triazine UV‐B filter (E‐35852) diminished the undesirable effects of the compound by an efficient quenching of the triplet excited state. Thus, sunscreen formulations including triplet quenchers could provide effective protection from the potential phototoxic and photoallergic effects derived from poor photostability of avobenzone.

“Ab initio” synthesis of zeolites for preestablished catalytic reactions

Start with the transition state Zeolites catalyze a wide range of industrial reactions, but identifying the reactions that they catalyze well tends to be done by trial and error. Gallego et al. propose a way to rationalize the process by using structure-directing agents that act as scaffolds for creating the zeolite pores and channels (see the Perspective by Millini). They picked molecules that mimic the transition states for reactions such as the isomerization of ethylbenzene into xylene, and they successfully identified catalysts with improved reactant conversion and product selectivity. Science, this issue p. 1051; see also p. 1028 Organic molecules for directing structure formation in zeolites mimic the transition states of a desired catalytic reaction. Unlike homogeneous catalysts that are often designed for particular reactions, zeolites are heterogeneous catalysts that are explored and optimized in a heuristic fashion. We present a methodology for synthesizing active and selective zeolites by using organic structure-directing agents that mimic the transition state (TS) of preestablished reactions to be catalyzed. In these zeolites, the pores and cavities could be generated approaching a molecular-recognition pattern. For disproportionation of toluene and isomerization of ethylbenzene into xylenes, the TSs are larger than the reaction products. Zeolite ITQ-27 showed high disproportionation activity, and ITQ-64 showed high selectivity for the desired para and ortho isomers. For the case of a product and TS of similar size, we synthesized a catalyst, MIT-1, for the isomerization of endo-dicyclopentane into adamantane.

Metal-containing zeolites as efficient catalysts for the transformation of highly valuable chiral biomass-derived products

Metal-containing zeolites, especially Sn-Beta, perform as very efficient heterogeneous catalysts in the selective oxidation of levoglucosenone, which is considered as a platform chemical for the production of highly-valuable chemicals, towards the synthesis of the optically pure γ-lactone (S)-γ-hydroxymethyl-α,β-butenolide (HBO) using H2O2 as an oxidizing agent. Using Sn-Beta as a catalyst, yields up to 75% of (S)-γ-hydroxymethyl-α,β-butenolide are achieved in a “one-pot” cascade reaction. When Sn-Beta is combined with an acid resin, such as Amberlyst-15, the “two-step” process allows yields up to 90%.

Model studies on a carprofen derivative as dual photosensitizer for thymine dimerization and (6-4) photoproduct repair

Cyclobutane pyrimidine dimers (CPD) and (6–4) photoproducts are among the main UV‐induced DNA lesions. Both types of damage are mostly repaired in prokaryotes by photolyase enzymes. The repair mechanism of (6–4) photolyases has still not been fully elucidated, but it is assumed that back rearrangement to the oxetane occurs prior to repair. In this work, a non‐steroidal anti‐inflammatory drug derivative corresponding to the dechlorinated methyl ester of carprofen (namely methyl 2‐(carbazol‐2‐yl)propanoate, PPMe) has been used to achieve the photosensitized cycloreversion of model oxetanes (formally resulting from photocycloaddition between benzophenone and 1,3‐dimethylthymine or 2′‐deoxyuridine), by employing fluorescence spectroscopy, laser flash photolysis, HPLC and NMR. Although PPMe is able to photoinduce the cycloreversion of both oxetanes, the fluorescence quenching of PPMe is faster for the 2′‐deoxyribose‐containing oxetane; this underlines the importance of the structure in such studies. Moreover, PPMe was shown to photoinduce the formation of thymidine cyclobutane dimers through a triplet–triplet energy transfer from a vibrationally excited state, as suggested by the enhanced PPMe triplet quenching by thymidine with increasing temperature. These results reveal a dual role of PPMe in DNA photosensitization, in that it photoinduces either damage or repair.

Photogeneration of 2-deoxyribonolactone in benzophenone-purine dyads. Formation of ketyl-C1' biradicals.

Photolysis of the title dyads under aerobic conditions leads to a 2-deoxyribonolactone derivative. Laser flash photolysis reveals that the process occurs from the short-lived benzophenone-like triplet excited state. A mechanism involving intramolecular electron transfer with the purine bases (adenine, guanine, or 8-oxoadenine) as donors is proposed.

Photochemical and photophysical properties of dibenzoylmethane derivatives within protein

In the present work, three dibenzoylmethane derivatives in their β-diketo form have been selected to investigate their photophysical and photochemical behavior upon interaction with human serum albumin (HSA). In organic solvents, absorption and phosphorescence emission spectra of the α-bromo derivative of avobenzone (BrAB) were similar to those of the α-methylated and α-propyl analogs (MeAB and PrAB). However, laser flash photolysis experiments revealed a different transient species centered at 350 nm, assigned to the radical obtained from a singlet excited state dehalogenation process. Interestingly, the transient absorption spectrum of BrAB within HSA showed the typical features of the β-diketone triplet excited state. In the case of MeAB and PrAB derivatives, binding to HSA was associated with a significant increase of their triplet lifetimes as compared to acetonitrile. Finally, the Norrish type II process has been considered as a model to evaluate the influence of the protein microenvironment on the photoreactivity. In this context, photodegradation of PrAB in aerated solutions, to give avobenzone (AB), has been monitored by UV spectroscopy. Interestingly, the quantum yields of AB formation were markedly dependent on the reaction medium (1.4 × 10(-2) in acetonitrile and 3.9 × 10(-2) within albumin medium); by contrast, chemical yields of ca. 50% were obtained in both cases.

Synthesis of Al-MTW with low Si/Al ratios by combining organic and inorganic structure directing agents

A rationalized combination of alkali cations and bulky dicationic organic structure directing agents (OSDAs) has allowed the synthesis of the Al-rich MTW zeolites with Si/Al ratios of ∼12 and large pore accessibility. 27Al MAS NMR spectroscopy indicates that most of the aluminum atoms are in tetrahedral coordination in framework positions, and in situ infrared pyridine adsorption/desorption spectroscopy reveals strong Bronsted acidity after cationic exchange for the Al-rich MTW. In addition, another MTW material with a Si/Al ratio of 30 has been synthesized under alkali-free conditions using a bulky dicationic molecule such as OSDA, the lowest Si/Al ratio being achieved for a MTW zeolite synthesized in the absence of alkali-cations in the synthesis media. The catalytic activity of these MTW materials has been tested for the n-decane cracking reaction, achieving higher catalytic activities and olefin yields than other related large pore zeolites.

Role of Supramolecular Chemistry During Templating Phenomenon in Zeolite Synthesis

In the last years, there is an increasing interest in the use of organic molecules with the appropriate functionalities to interact with other organic molecules and/or inorganic cations through non-covalent supramolecular interactions, as very specific organic structure-directing agents (OSDAs) for zeolite synthesis. These assembled molecular subunits allow directing the crystallization of zeolite structures with particular physico-chemical properties, such as novel framework topologies, crystal size, chemical compositions, acid-base properties, or metal incorporation, which otherwise would not be achieved using “classical” amine or ammonium-based OSDA molecules. Along the present chapter, different zeolite synthesis strategies employing assembled molecular subunits will be presented, including the use of crown ether-based supramolecular templates, metal-organic complexes, aromatic molecules able to interact through π–π interactions, or supramolecular assembled amphiphilic molecules, among others. The most relevant results described in the literature using these supramolecular-based templating routes will be discussed, together with the current challenges and perspectives.

Synthesis of highly stable metal-containing extra-large-pore molecular sieves.

The isomorphic substitution of two different metals (Mg and Co) within the framework of the ITQ-51 zeotype (IFO structure) using bulky aromatic proton sponges as organic structure-directing agents (OSDAs) has allowed the synthesis of different stable metal-containing extra-large-pore zeotypes with high pore accessibility and acidity. These metal-containing extra-large-pore zeolites, named MgITQ-51 and CoITQ-51, have been characterized by different techniques, such as powder X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectrometry, UV–Vis spectroscopy, temperature programmed desorption of ammonia and Fourier transform infrared spectroscopy, to study their physico-chemical properties. The characterization confirms the preferential insertion of Mg and Co atoms within the crystalline structure of the ITQ-51 zeotype, providing high Brønsted acidity, and allowing their use as efficient heterogeneous acid catalysts in industrially relevant reactions involving bulky organic molecules.

Photocages for protection and controlled release of bioactive compounds

Using a sunscreen-based photocage, we have demonstrated that it is possible to prevent photodegradation of a bioactive compound and to achieve its controlled photorelease. The concept has been proven linking avobenzone, one of the most important UVA blockers, to ketoprofen, which is a representative example of a photosensitive drug.