Quaternary relaxations in sol-gel encapsulated hemoglobin studied via NIR and UV spectroscopy

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In this work, we study the kinetics of the R f T transition in hemoglobin using a combinationof near-infrared and near-ultraviolet spectroscopy. We use a sol-gel encapsulation protocol to deceleratethe conformational transitions and to avoid spectral perturbations arising from ligand migration andrecombination. We monitor two spectroscopic markers: band III in the near-IR, which is a fine probe ofthe heme pocket conformation, and the tryptophan band in the near-UV, which probes the formation ofthe Trpâ37-AspR94 hydrogen bond, characteristic of the T structure, at the critical R1â2 subunit interface.The time evolution of these two bands is monitored after deoxygenation of encapsulated oxyhemoglobin,obtained by diffusion of a reducing agent into the porous silica matrix. Characteristic spectral shifts areobserved: comparison with myoglobin enables us to assign them to quaternary structure relaxations. BandIII spectral relaxation is clearly nonexponential, and analysis with the Maximum Entropy Method enablesus to identify three processes. On the other hand, near-UV spectral relaxation follows an exponentialdecay with a time constant closely corresponding to the second process observed in the near IR. Veryinterestingly, the rates of all processes markedly depend on the viscosity of the co-encapsulated solvent,following a power law. Our results reveal correlations between heme pocket relaxations, induced by theR f T transition, and structural event(s) occurring at the R1â2 interface and highlight their solventdependence. The power law viscosity dependence of relaxation rates suggests that the observed proteinrelaxations are “slaved” to the co-encapsulated solvent. The stepwise character of the quaternary transitionis also evidenced
Lingua originaleEnglish
pagine (da-a)11568-11576
Numero di pagine9
Stato di pubblicazionePublished - 2007

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