SUMMARY

	The use of supramolecular photochemistry has been largely exploited to control the reactivity, selectivity and progress of chemical reactions. In this context, advantadge has been taken of the host tridimensional structure to channel the conformation of the substrate excited states involved in the reaction. A number of photoreactions catalyzed by supramolecules such as zeolites, cyclodextrins, micelles, etc. have been reported in the literature. However, examples involving biomolecules are still scarce.
	With this background, the main objective of this thesis has been to study classical photochemical reactions within biomolecules as they have generally demonstrated to present some advantages in the supramolecular interactions in comparison with abiotic hosts. Thus, biomolecules may provide a close interaction with the substrate, which could be reflected in an important increase of the reaction selectivity and quantum yield. In principle, the stereoselectivity can also be optimized as a result of the chiral character of binding sites. These characteristics are difficult to find in abiotic supramolecular compounds due to the high complexity of their design.
	Taking advantage of our group experience, serum albumins have been selected as biotic hosts. This choice has been based on two key properties of albumins: (i) they are transport proteins and act as vehicles of a large variety of endogenous or exogenous substances, (ii) they possess generally two different binding sites called site I and site II, where the substrates bind mainly through hydrophobic and H-bond/electrostatic interactions, respectively.
	Photo-Fries rearrangement was the first model reaction considered. Specifically, a special attention has been devoted to the influence that the binding site and the species of serum albumin (from human, bovine or rabbit source) should have on the reaction chemical and quantum yields. With this aim, 4-methoxy-1-naphthyl hydrogen succinate or glutarate (1a or 1b) and 4-methoxynaphthyl acetate (1c) have been selected as substrates due to their potential interaction in site II and I, respectively. Preliminary kinetic studies demonstrated that an intramolecularly catalyzed hydrolysis renders compound 1a unstable in phosphate buffer. Although 1a suffered a weak stabilization in the presence of protein, this compound was ruled out and replaced by 1b. A 1:1 stoichiometry was determined for the substrate@protein complexes for combination of substrates with each one of the three albumins. Thus, in the case of HSA and BSA, experiments using fluorescence probe displacement from HSA and BSA binding sites, confirmed that 1b and 1c mainly interact in site II and I, respectively. However, a marked difference exists in the case of RSA, where both substrates displaced the specific probes from both sites. Finally, UV-Vis spectroscopy and HPLC analysis demonstrated that quantum yields of photoproucts formation (2b and 2c) are dependent not only on the binding site but also on the albumin species used. Thus, the reaction was more efficient in site II (substrate 1b) than in site I (substrate 1c) and in BSA more than in HSA. In the special case of RSA, same quantum yields were determined for both compounds pointing to the presence of only one common binding site or of two sites with similar binding properties, as it is proposed in the literature.
	In a second part, derivatives of avobenzone (AB, a largely used solar filter) substituted in the ? position of the carbonyl moieties with a bromide (BrAB), a methyl (MeAB) or a propyl group (PrAB) were considered. Since no data were available concerning the photophysical and photochemical properties of BrAB, a complete spectroscopic characterization was performed. Absorption and phosphorescence together with NMR spectra demonstrated that BrAB exists only as its ?-diketo form, like the other derivatives MeAB and PrAB. No triplet-triplet transition was detected by laser flash photolysis in solution at 295 K, but it was observed in low temperature experiments at 77 K. However, at room temperature, formation of AB radical arising from a photodehalogenation process was demonstrated. Steady-state photolysis showed the formation of a complex mixture of photoproducts, including AB. Then, the influence of protein encapsulation on the three AB derivatives excited states was investigated. Remarkably, the transient absorption corresponding to BrAB triplet excited state was detected in the presence of HSA at 295 K; an increase of MeAB and PrAB triplet excited state lifetimes was also observed. These results illustrate not only the protecting effect of the protein cavity towards oxygen attack and substrate selfquenching but also the confined environment provided by the albumin, which for BrAB could manage to mimic the rigid matrix at low temperature. Next, albumin effect on the Norrish type II fragmentation of PrAB leading to AB formation was considered. It was determined that the reaction quantum yield is 2.8 times lower in the presence of protein (?R= 1.4 ? 10-2 and 3.9 ? 10-2 for HSA and acetonitrile, respectively). These data are in agreement with the increase of triplet excited state lifetime of encapsulated PrAB. Thus, the conformation required for the intramolecular ? hydrogen abstraction by the carbonyl group (Norrish II reaction) appears to be disfavored inside the protein cavity.
	Finally, [6¹] photoelectrocyclization of N-methyldiphenylamine (A) was studied. This class of reactions is of paramount importance for applications such as natural product synthesis, photochromism, molecular devices and actinometry. The reaction, in the special case of A, takes place through a complex mechanism that involves its triplet excited state (3A). At this point, an adiabatic ring closure occurs to form N-methyldihydrocarbazole in its triplet excited state (3B), which then decays to its ground state (B). The latter is subsequently oxidized by oxygen to give almost quantitatively N-methylcarbazole (C). Thus, oxygen plays a dual role: on the one hand it disfavors the reaction by quenching the amine triplet excited state (3A), and, on the other hand, it is required as an oxidant in the last step. Kinetics of reaction, determined by UV-Vis absorption and fluorescence emission, demonstrated that quantum yields are higher in the presence of albumin. Reaction intermediates monitored by laser flash photolysis exhibited a longer lifetime in the presence of albumin, which supports the matrix effect provided by the biomolecule cavity. The attention was especially centered on the signal corresponding to the ground state of B, because it is able to provide information not only on the substrate@protein interaction (complex stoichiometry, binding site) but also on the efficiency of the cyclization process. In this way, it was shown that photocyclization can be optimized in the microenvironment provided by site I of HSA and BSA. Determination of quantum yields, as well as detection of transient species, demonstrated that protein encapsulation contributes to a subtle control of the oxygen levels, which allowed us reaching the upper limit imposed by the intersystem crossing quantum yield to the amine triplet excited state 3A.