Biradical vs singlet oxygen photogeneration in suprofen-cholesterol systems.

Cholesterol (Ch) is an important lipidic building block and a target for oxidative degradation, which can be induced via free radicals or singlet oxygen ((1)O2). Suprofen (SP) is a nonsteroidal anti-inflammatory drug that contains the 2-benzoylthiophene (BZT) chromophore and has a π,π* lowest triplet excited state. In the present work, dyads (S)- and (R)-SP-α-Ch (1 and 2), as well as (S)-SP-ß-Ch (3) have been prepared from ß- or α-Ch and SP to investigate the possible competition between photogeneration of biradicals and (1)O2, the key mechanistic steps in Ch photooxidation. Steady-state irradiation of 1 and 2 was performed in dichloromethane, under nitrogen, through Pyrex, using a 400 W medium pressure mercury lamp. The spectral analysis of the separated fractions revealed formation of two photoproducts 4 and 5, respectively. By contrast, under the same conditions, 3 did not give rise to any isolable Ch-derived product. These results point to an intramolecular hydrogen abstraction in 1 and 2 from the C7 position of Ch and subsequent C-C coupling of the generated biradicals. Interestingly, 2 was significantly more photoreactive than 1 indicating a clear stereodifferentiation in the photochemical behavior. Transient absorption spectra obtained for 1-3 were very similar and matched that described for the SP triplet excited state (typical bands with maxima at ca. 350 nm and 600 nm). Direct kinetic analysis of the decay traces at 620 nm led to determination of triplet lifetimes that were ca. 4.1 µs for 1 and 2 and 5.8 µs for 3. From these data, the intramolecular quenching rate constants in 1 and 2 were determined as 0.78 × 10(5) s(-1). The capability of dyads 1-3 to photosensitize the production of singlet oxygen was assessed by time-resolved near infrared emission studies in dichloromethane using perinaphthenone as standard. The quantum yields (ΦΔ) were 0.52 for 1 and 2 and 0.56 for 3. In conclusion, SP-α-Ch dyads are unique in the sense that they can be used to photogenerate both biradicals and singlet oxygen, thus being able to initiate Ch oxidation from their triplet excited states following either of the two competing mechanistic pathways.

Metodología innovadora en la universidad. Sus efectos sobre los procesos de aprendizaje de los estudiantes universitarios / Innovative methodology at the university. Its effects on learning processes of university students

Se pretendía evaluar el impacto de metodología centrada en el aprendizaje sobre las estrategias de aprendizaje y los enfoques de aprendizaje de un grupo de alumnos de Química, de 1º curso de Ingeniería, de la Universitat Politècnica de València. Se utilizó un diseño preexperimental, con medidas de pretest y postest, tomadas mediante el cuestionario CEVEAPEU y el CPE. La muestra estaba constituida por 20 alumnos. Los métodos docentes utilizados integraban metodología expositiva, preguntas, resolución de problemas, elaboración de una monografía, exposiciones orales, prácticas de laboratorio, trabajo en grupo, tutorías en el aula y un sistema de evaluación que hacía uso de procedimientos formativos que devolvían feedback a los estudiantes (dos pruebas diagnósticas, valoración de problemas resueltos, autoevaluación mediante el uso de la plataforma de elearning de la universidad, evaluación de las exposiciones orales en clase, pruebas escritas, etc.). Se encontraron mejoras significativas en las estrategias de aprendizaje en el postest y en las puntuaciones de enfoque profundo

We wanted to assess the impact of learning-centered methodology on learning strategies and learning approaches of a students‟ group of Chemistry. They were first year engineering students from the Polytechnic University of Valencia. We used a pre-experimental design with pretest and posttest measures by means of the CEVEAPEU and CPE questionnaires. The sample consisted of 20 students. The teaching methods included expositive methodology, questions, problem solving, development of a monograph, presentations, laboratory practices, team work, tutoring in the classroom and an evaluation system that made use of training procedures that returned feedback to students (two diagnostic tests, assessment of solved problems, self-assessment using the e-learning platform of the university, oral assessment in class, written tests, etc..). Significant improvements in learning strategies scores in the posttest and increasing in deep approach were found

Steric shielding vs. σ-π orbital interactions in triplet-triplet energy transfer.

The influence of non-covalent σ-π orbital interactions on triplet-triplet energy transfer (TTET) through tuning of the donor excitation energy remains basically unexplored. In the present work, we have investigated intermolecular TTET using donor moieties covalently linked to a rigid cholesterol (Ch) scaffold. For this purpose, diaryl ketones of π,π* electronic configuration tethered to α- or ß-Ch were prepared from tiaprofenic acid (TPA) and suprofen (SUP). The obtained systems TPA-α-Ch, TPA-ß-Ch, SUP-α-Ch and SUP-ß-Ch were submitted to photophysical studies (laser flash photolysis and phosphorescence), in order to delineate the influence of steric shielding and σ-π orbital interactions on the rate of TTET to a series of energy acceptors. As a matter of fact, fine tuning of the donor triplet energy significantly modifies the rate constants of TTET in the absence of diffusion control. The experimental results are rationalized by means of theoretical calculations using first principles methods based on DFT as well as molecular dynamics.

Probing lipid peroxidation by using linoleic acid and benzophenone.

A thorough mechanistic study has been performed on the reaction between benzophenone (BZP) and a series of 1,4-dienes, including 1,4-cyclohexadiene (CHD), 1,4-dihydro-2-methylbenzoic acid (MBA), 1,4-dihydro-1,2-dimethylbenzoic acid (DMBA) and linoleic acid (LA). A combination of steady-state photolysis, laser flash photolysis (LFP), and photochemically induced dynamic nuclear polarization (photo-CIDNP) have been used. Irradiation of BZP and CHD led to a cross-coupled sensitizer-diene product, together with 6, 7, and 8. With MBA and DMBA as hydrogen donors, photoproducts arising from cross-coupling of sensitizer and diene radicals were found; compound 7 was also obtained, but 6 and o-toluic acid were only isolated in the irradiation of BZP with MBA. Triplet lifetimes were determined in the absence and in the presence of several diene concentrations. All three model compounds showed similar reactivity (k(q) ≈10(8) M(-1) s(-1)) towards triplet excited BZP. Partly reversible hydrogen abstraction of the allylic hydrogen atoms of CHD, MBA, and DMBA was also detected by photo-CIDNP on different timescales. Polarizations of the diamagnetic products were in full agreement with the results derived from LFP. Finally, LA also underwent partly reversible hydrogen abstraction during photoreaction with BZP. Subsequent hydrogen transfer between primary radicals led to conjugated derivatives of LA. The unpaired electron spin population in linoleyl radical (LA(.)) was predominantly found on H(1-5) protons. To date, LA-related radicals were only reported upon hydrogen transfer from highly substituted model compounds by steady-state EPR spectroscopy. Herein, we have experimentally established the formation of LA(.) and shown that it converts into two dominating conjugated isomers on the millisecond timescale. Such processes are at the basis of alterations of membrane structures caused by oxidative stress.

Solvent effects in hydrogen abstraction from cholesterol by benzophenone triplet excited state.

Hydrogen abstraction from the C-7 position of cholesterol (Ch) by triplet excited benzophenone (BZP) exhibits remarkable solvent-dependence in product studies. Kinetic measurements on the intramolecular version of the process in dyads containing covalently linked Ch and BZP units reveal important solvent effects and significant stereodifferentiation.

Cholesterol-diaryl ketone stereoisomeric dyads as models for "clean" type I and type II photooxygenation mechanisms.

Cholesterol (Ch) is a major target for oxidative degradation in cell membranes, a process which can occur by two mechanisms: Type I (via free radicals) and Type II (mediated by 1O2). In the present work, several dyads have been synthesized from beta- and alpha-Ch and ketoprofen (KP) or tiaprofenic acid (TPA). Upon irradiation under anaerobic conditions, KP-alpha-Ch dyads were efficiently photolyzed, via intramolecular hydrogen abstraction from C-7. By contrast, KP-beta-Ch, TPA-alpha-Ch, and TPA-beta-Ch remained unchanged after prolonged irradiation. The transient absorption spectra of KP-alpha-Ch were assigned to the short-lived biradicals resulting from intramolecular hydrogen abstraction. Interestingly, the spectra and lifetimes obtained for the TPA-derived dyads were very similar to those of the TPA triplet excited state. For the KP-alpha-Ch dyads, generation of singlet oxygen was expectedly negligible. Conversely, for TPA-alpha-Ch a Phi delta) value as high as 0.5 was determined. Thus, KP-based dyads are appropriate models for clean type I Ch oxidation, whereas the TPA derivatives are suitable systems for investigation of the purely type II process.

Efficient and selective photogeneration of cholesterol-derived radicals by intramolecular hydrogen abstraction in model dyads.

Three model dyads have been synthesized by esterification of beta- and alpha-cholesterol (Ch) with (S)- and/or (R)-ketoprofen (Kp). The alpha-dyads are efficient photogenerators of the 7-allyl Ch radicals by intramolecular H abstraction. Subsequent cyclization via C-C coupling occurs in a stereoselective way.

Stereoselective intramolecular hydrogen abstraction by a chiral benzophenone derivative.

An unprecedented stereoselective photoreduction of a chiral BZP is observed in steady state as well as in time-resolved studies.