Ultrafast Electronic Relaxation of Excited State of Biomimetic Metalloporphyrins in the Gas Phase

Abstract

Biomimetic molecules are systems that provide a direct and simpler reproduction of a molecular process essential to the living world, i.e. they mimic a function or a structure of a biomolecular edifice. The idea is to isolate this process and allow a selective study of a specific function by physicochemical methods, in a reductionist way. In that respect, gas phase studies of biomolecules are ideal, since they allow the application of a variety of highly sensitive and selective tools to the study of these model systems: mass spectrometry, photoelectron spectroscopy, for example, provided that the molecular system has been vaporised. In addition, the system is studied in absence of solvent, which greatly simplifies the interpretation of the fundamental effects and consequently allows to infer the effects of the medium since, in the gas phase solvents can be introduced in a gradual manner via clusters. Also gas phase measurements allow a direct comparison with results from quantum chemistry calculations. Great progresses have been fuelled in this direction by the developments of mass spectrometry. The studies in the gas phase have been mostly dedicated to the elucidation of the basic structure of the building blocks of proteins, i.e. small peptides (Simons, 2004). In turn, other biological functions have been less studied by the gas phase biomimetic approach. Hematoproteins hosting hemoporphyrins (the heme, see fig 1) are ideal systems to examine in this way, since they are at the heart of the oxygen and small molecules transport within blood and that their active site is localised very precisely on the metal atom embedded in their central porphyrin. Thus, if one studies the active porphyrinic site of myoglobin at short time scales (in the subpicosecond time domain), the interaction with the environing protein (globin myoglobin) can be minimised. In hemoproteins, the attachment of oxygen to the iron of the heme is a complex process involving a potential barrier. This barrier results from the crossing of two surfaces of