M. Consuelo Jiménez

Rotaxanes as new architectures for photoinduced electron transfer and molecular motions

Rotaxanes are molecular architectures ideally suited for building integrated, multicomponent modular systems, displaying novel chemical and physical properties. In this new field of functional rotaxanes, those incorporating transition metals, which are incidentally used as synthetic templates, are particularly attractive for their photophysical and electronic properties as well as their dynamic behaviour. It is believed that they will provide, in the future, the basic elements for constructing nanoscale machines and motors.

Photochemistry of naproxen in the presence of β-cyclodextrin

Abstract Irradiation of naproxen ( 1 ) in the presence of β -cyclodextrin (CD) leads to photodecarboxylation products with ethyl ( 2 ), 1-hydroxyethyl ( 3 ) and acetyl ( 4 ) side chains. The presence of CD does not protect against photodegradation, but rather results in a more rapid disappearance of the drug. The most important change associated with CD is the marked predominance of alcohol 3 over ketone 4 . Since 3 is more cytotoxic than 4 , the reduced photohaemolytic activity of 1 in the presence of CD must be attributed to the sequestering and stabilization of the radical intermediates by complexation, rather than to the nature of the stable photoproducts.

A joint experimental/theoretical study of the ultrafast excited state deactivation of deoxyadenosine and 9-methyladenine in water and acetonitrile.

The excited states of deoxyadenosine (dA) and 9-methyladenine (9Me-Ade) were studied in water and acetonitrile by a combination of steady-state and time-resolved spectroscopy and quantum chemical calculations. Femtosecond fluorescence upconversion experiments show that the decays of dA and 9Me-Ade after excitation at 267 nm are very similar, confirming that 9Me-Ade is a valid model for the calculations. The fluorescence decays can be described by an ultrafast component (<100 fs) and a slower one (≈ 300-500 fs); they are slightly slower in acetonitrile than in water. Time-dependent DFT calculations on 9Me-Ade, using PBE0 and M052X functionals and including both bulk and specific solvent effects, provide absorption and emission spectra in good agreement with experiments, giving a comprehensive description of the decay mechanism. It is shown that, in the Franck-Condon region, the lowest in energy state is the optically bright La state, with the Lb state situated about 2000 cm(-1) higher. Both states are populated when excited at 267 nm, but the Lb state undergoes an ultrafast Lb → La decay, too fast for our time-resolution (≈ 80 fs). This is confirmed by the experimentally observed fluorescence anisotropies, attaining values lower than 0.4 already at time zero. Consequently, the ensuing excited state relaxation mechanism can be described as the evolution along an almost barrierless path from the Franck-Condon region of the La potential energy surface towards a conical intersection with the ground state. This internal conversion mechanism proceeds without any significant involvement of any near-lying nπ* state.

Characterisation of the lowest singlet and triplet excited states of S-flurbiprofen.

The photophysical properties of S-flurbiprofen [S-2-fluoro-alpha-methyl-4-biphenylacetic acid], a nonsteroidal anti-inflammatory drug, have been examined using steady-state and time-resolved spectroscopic techniques. The energy of its first singlet excited state is 99 kcal mol(-1). The fluorescence quantum yields and lifetimes (at 300 nm) have been determined in acetonitrile, methanol, hexane and PBS; they are in the range 0.15<phi(F)< 0.33 and 0.7<tau(F)<2.0 ns. The intersystem crossing quantum yields are between 0.45 and 0.71; the lambda(max) of the T-T absorption is 360 nm, and the triplets live from 15 to 106 micros. Steady state photolysis in aqueous medium leads to S-2-hydroxy-alpha-methyl-4-biphenylacetic acid via photonucleophilic aromatic substitution, in addition to the photodecarboxylation products observed in organic solvents.

Use of triplet excited States for the study of drug binding to human and bovine serum albumins.

The triplet excited states of (S)‐ and (R)‐flurbiprofen (FBP) have been used as reporters for the microenvironments experienced within the binding sites of human and bovine serum albumins. Regression analysis of triplet decay provides valuable information on the degree of protection that these excited states are afforded from attack by a second FBP molecule, oxygen, or other reagents. The multiexponential fitting of these decays can be satisfactorily correlated with the distribution of the drug among the two binding sites and its presence as the noncomplexed form in the bulk solution. This assignment has been confirmed by using (S)‐ibuprofen or capric acid as selective site II replacement probes. Triplet lifetimes and site occupancy are sensitive to the type of serum albumin employed (human versus bovine). Finally, the binding behaviour of (S)‐ and (R)‐FBP exhibits little stereoselectivity.

Experimental and theoretical studies on the radical-cation-mediated imino-Diels-Alder reaction.

The feasibility of an electron transfer imino-Diels-Alder reaction between N-benzylideneaniline and arylalkenes in the presence of a pyrylium salt as a photosensitizer has been demonstrated by a combination of product studies, laser flash photolysis (LFP), and DFT theoretical calculations. A stepwise mechanism involving two intermediates and two transition states is proposed.

Enhanced Photostability of the Anthracene Chromophore in Aqueous Medium upon Protein Encapsulation

In the present work, 9-anthraceneacetic acid (1) has been selected as a simple, water-compatible derivative of the anthracene chromophore to investigate the photophysical and photochemical behavior upon binding to human and bovine serum albumins (HSA and BSA) and alpha-acid glycoproteins (HAAG and BAAG). The UV-vis absorption spectrum of 1 exhibited the typical four maxima between 320 and 400 nm, which were slightly red-shifted in the presence of proteins. These minor changes suggested the formation of 1@protein complexes; their stoichiometry (1:1) was determined by means of the corresponding Job plots. As expected, the fluorescence spectrum of 1 in phosphate-buffered saline (PBS) consisted of a structured emission with maxima between 390 and 470 nm. The addition of increasing amounts of HSA resulted in a decrease in the emission intensity. In the presence of BSA, HAAG, or BAAG, the same trend was observed, although the changes were less pronounced. The determination of binding constants was achieved from fluorescence titration, considering one (AAGs) or two (SAs) binding sites. The binding constants (K(B)) were found to be 2.3 x 10(6) M(-1) (HAAG), 2.4 x 10(6) M(-1) (BAAG), 4.57 x 10(4)/1.45 x 10(6) M(-1) (HSA), and 1.44 x 10(4)/1.20 x 10(6) M(-1) (BSA). Binding within two different sites of SAs was confirmed by displacement experiments using warfarin and ibuprofen as site I and site II probes, respectively. Laser flash photolysis of 1 at lambda(exc) = 355 nm in PBS/air gave rise to several transient species; by contrast, in the presence of 1 equiv of proteins, only the triplet excited state was detected. Moreover, the triplet lifetime (tau(T)) monitored at 420 nm lengthened considerably (up to 50-fold) in the protein media. This can be attributed to a slower deactivation of the species inside the protein binding pockets, where an exceptional microenvironment provides protection from attack by a second molecule of 1, oxygen, or other reagents. In agreement with the results from fluorescence titration, the presence of two binding sites in SAs was revealed by two different triplet lifetimes; by contrast, only one tau(T) value was found for HAAG and BAAG. The major, longer-lived component under nonsaturating conditions was assigned to (1@SA)(II), while the minor component was assigned to (1@SA)(I). Irradiation of 1 at 350 nm in PBS/air led to anthraquinone as a major product. In the presence of proteins, the degree of conversion was markedly lower than in PBS, as revealed by the photodegradation kinetics monitored through the absorbance changes at 367 nm. Thus, a dramatic protection from photooxidation is provided within the protein microenvironment.

Determination of Enantiomeric Compositions by Transient Absorption Spectroscopy using Proteins as Chiral Selectors

Determination of the enantiomeric composition is a relevant issue in different areas, including the synthesis of enantiopure chemicals and biologically active substances. In pharmacology, the enantiomers exhibit different behavior in terms of activity, side-effects, toxicity, metabolism, or transport mechanism. Therefore, the development of analytical methods to assess the enantiomeric composition of chiral drugs is of great interest. Direct determination of the enantiomeric excess (ee) can be achieved by polarimetry; however, this technique presents some practical limitations, mainly related to sensitivity and low tolerance to impurities. Other commonly used analytical methods are based on GC and HPLC on chiral stationary phases; their main disadvantages are that they are time-consuming and require serial analysis, which limits the number of samples that can be studied. Other methods have been developed in the last decades, based on the determination of different properties; they include MS, UV/ Vis absorption spectroscopy, IR thermography, circular dichroism, capillary electrophoresis, NMR spectroscopy, fluorescence spectroscopy, biochemical assays, and so forth. In spite of this effort, quantification of stereoisomer levels continues to be an important problem, and therefore further research is still required to develop new analytical methodologies. In principle, discrimination between enantiomers is possible by making use of supramolecular host–guest interactions. In this context, useful information has been obtained by chemometric analysis of the UV/Vis absorption or fluorescence spectra in the presence of cyclodextrins. Proteins are another important class of chiral selectors. Particularly, serum albumins (SA) have been widely used as stationary phases for the chromatographic resolution of enantiomeric mixtures, a concept based on the possible stereoselectivity of the binding process. Recently, we have explored the suitability of triplet excited states as reporters for the binding of drugs to transport proteins, as the properties of these states are very sensitive to the experienced microenvironment. Thus, laser flash photolysis (LFP) measurements have been performed on (S)and (R)-flurbiprofen methyl esters (FBPMe) in the presence of human serum albumin (HSA). Actually, FBPMe is a prodrug of the nonsteroidal antiinflammatory drug flurbiprofen (FBP). In the absence of protein, after LFP at 266 nm (2.5 10 m, in phosphate-buffered saline (PBS), air), (S)-FBPMe exhibits a characteristic transient triplet– triplet absorption spectrum centered at 360 nm, with a lifetime (tT) of 1.5 ms. However, in the presence of 2.5 10 m HSA, two tT values are observed (31.5 and 4.1 ms); [15a] they have been assigned to (S)-FBPMe inside the two known HSA binding sites (named site I and site II by Sudlow). From the relative contributions of the different tT values at various (S)-FBPMe/HSA ratios, it has been possible to determine the drug distribution among the bulk solution and the protein binding sites. The same trend, but with remarkably different triplet lifetimes (157.6 and 16.6 ms), has been observed for (R)-FBPMe/HSA systems. For both (S)and (R)-FBPMe, the major (longer-lived) component under non-saturating conditions was assigned to FBPMe within site I, and the minor (shorter-lived) component corresponded to site II-bound FBPMe. [a] Dr. M. C. Jim nez, Prof. M. A. Miranda Departamento de Qu mica Instituto de Tecnolog a Qu mica UPV-CSIC Universidad Polit cnica de Valencia, Camino de Vera s/n Apdo 46022, Valencia (Spain) Fax: (+34)963877344 E-mail : mcjimene@qim.upv.es mmiranda@qim.upv.es [b] Dr. I. Vay , C. J. Bueno, Dr. M. C. Jim nez, Prof. M. A. Miranda Departamento de Qu mica Instituto de Tecnolog a Qu mica UPV-CSIC Universidad Polit cnica de Valencia, Camino de Vera s/n Apdo 46022, Valencia (Spain) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.200801657.

Photodecar☐ylation of 2-phenylpropionic acid in solution and included within β-cyclodextrin

Abstract Photolysis of 2-phenylpropionic acid ( 1 ) in acetonitrile, methanol or benzene leads to ethylbenzene ( 2 ), 2,3-diphenylbutane ( 3 d,l and meso ), 1-(2-ethylphenyl)-1-phenylethane ( 4 ), 1-(4-ethylphenyl)-1-phenylethane ( 5 ) and acetophenone ( 6 ). In cyclohexane or carbon tetrachloride, solvent derived products are formed. These results involve homolytic cleavage of the C C bond α to the car☐y group, which affords 1-phenylethyl radical (PER) as key intermediate. The α,α coupling of PER in solution to give 3 is nonstereoselective; by contrast, formation of the meso isomer is preferred upon inclusion of 1 within β-cyclodextrin. This is attributed to the coupling of two long-lived PER-CD units

Enhanced photosafety of cinacalcet upon complexation with serum albumin.

Cinacalcet (CIN) is a calcimimetic drug, which contains a naphthalene chromophore and binds almost quantitatively to human serum albumin (HSA). In the present work, the excited states of CIN have been characterized in order to obtain relevant information about complexation of CIN with HSA. The fluorescence spectrum in acetonitrile, at λ(exc) = 290 nm, displayed two bands with maxima at 332 and 439 nm, assigned to the monomer and exciplex emission. Upon protonation of the amino group, the exciplex band disappeared, with a concomitant increase of the monomer emission intensity. Time-resolved fluorescence evidenced an intramolecular dynamic quenching, attributed to exciplex formation and/or photoinduced electron transfer, in agreement with the favorable thermodynamics predicted by the Rehm-Weller equations. Diffusion controlled dynamic quenching of CINH(+) fluorescence by oxygen was observed. The emission properties in PBS were similar to those obtained for CINH(+) in acetonitrile. Laser flash photolysis (LFP) of CIN and CINH(+) in acetonitrile/N(2), at λ(exc) = 308 nm, gave rise to the naphthalene-like triplet excited states, with maxima at 420 nm and lifetimes of 4 and 7 μs; they were efficiently quenched by oxygen. No significant singlet excited state interaction was observed in CINH(+)/HSA complexes, as revealed by the emission spectra, which were roughly explained taking into account the relative contributions of drug and protein in the absorption spectra. Upon LFP of the complexes, triplet excited states were generated; the decays monitored at 420 nm were satisfactorily fitted using a function containing two monoexponential terms, corresponding to a short-lived (τ(1) = 8 μs) and a long-lived (τ(2) = 37 μs) component. This indicates that the drug is incorporated into two different binding sites of HSA. Despite the long triplet lifetimes of the CINH(+)/HSA complexes, the rate constant of quenching by oxygen was found to be 2 orders of magnitude lower than that determined in acetonitrile, which can be attributed to the relative slower diffusion rates in this microheterogeneous system. Therefore, the protein microenvironment protects cinacalcet from attack by oxygen; this prevents the phototoxic effects caused by formation of singlet oxygen and results in an enhanced photosafety of the drug.