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Ultrafast dynamics in heme and in heme-CO adduct of Tf-trHb

Ligand photodissociation with ultrashort laser pulses in hemoproteins-CO adducts is widely employed to unravel the elementary dynamical processes related to the functional properties of these proteins [1]. The characterization of the rebinding processes following CO photolysis allows us to obtain information about the active site organization. If the ligand remains near the metal, the geminate recombination takes place ; instead, if the ligand easily diffuses away, the chance of ligand-metal encounters decreases. Here we present the results of a study on truncated hemoglobin from Thermobifida fusca (Tf-trHb) investigated with femtosecond time-resolved UVvisible absorption spectroscopy. A fast geminate recombination following CO photolysis has been observed, suggesting that the ligand is confined within the heme pocket. The distal single TrpG8Phe and triple TyrB10Phe, TyrCD1Phe, TrpG8Phe mutants have been also investigated in order to understand the role of specific amino acids in the distal pocket. The photon energy in excess to dissociation is deposited initially into the heme, triggering a cascade of dynamical processes. The nature of the short lived intermediates generated instantaneously after photolysis is a topic of current interest in hemoprotein dynamics [2]. The time-resolved behavior of the Tf-trHb is discussed on the basis of electronic/vibrational results on heme [3] and porphyrin macrocycle [4]. References [1] J.-L. Martin, M. H. Vos, Annu. Rev. Biophys. Biomol. Struct. 21, 199-222 (1992). [2] X. Ye, A. Demidov, F. Rosca, W. Wang, A. Kumar, D. Ionascu, L. Zhu, D. Barrick, D. Wharton, P. M. Champion, J. Phys. Chem. A 107, 8156-8165 (2003). [3] J. R. Challa, T. C. Gunaratne, M. C. Simpson, J. Phys. Chem. B 110, 19956-19965 (2006). [4] A. Marcelli, P. Foggi, L. Moroni, C. Gellini, P. R. Salvi, J. Phys. Chem. A 112, 1864-1872 (2008).

truncated hemoglobin; Ligand photodissociation; time-resolved absorption spectroscopy

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