The low-frequency dynamics of copper azurin has been studied at different temperatures for a dry and deuterium hydrated sample by incoherent neutron scattering and the experimental results have been compared with molecular dynamics (MD) simulations carried out in the same temperature range. Experimental Debye-Waller factors are consistent with a dynamical transition at approximately 200 K which appears partially suppressed in the dry sample. Inelastic and quasielastic scattering indicate that hydration water modulates both vibrational and diffusive motions. The low-temperature experimental dynamical structure factor of the hydrated protein shows an excess of inelastic scattering peaking at about 3 meV and whose position is slightly shifted downwards in the dry sample. Such an excess is reminiscent of the "boson peak" observed in glass-like materials. This vibrational peak is quite well reproduced by MD simulations, although at a lower energy. The experimental quasielastic scattering of the two samples at 300 K shows a two-step relaxation behaviour with similar characteristic times, while the corresponding intensities differ only by a scale factor. Also, MD simulations confirm the two-step diffusive trend, but the slow process seems to be characterized by a decay faster than the experimental one. Comparison with incoherent neutron scattering studies carried out on proteins having different structure indicates that globular proteins display common elastic, quasielastic and inelastic features, with an almost similar hydration dependence, irrespective of their secondary and tertiary structure.
Azurin is a blue single copper protein involved in the respiratory chain of denitrifying bacteria. The structural gene for azurin from Pseudomonas aeruginosa was cloned in an Escherichia coli recombinant strain. The protein overexpressed in the bacterial periplasmic space was subsequently purified. Two strategies were followed to anchor azurin to gold surfaces. First, the protein was immobilised on bare gold. Azurin adsorbs on gold via its disulfide group. Scanning tunnelling microscopy (STM) inspection of the azurin-Au(111) interface revealed the formation of a closely packed protein monolayer and allowed individual azurin molecules to be resolved. In order to uncouple the protein layer from the metal, the gold surfaces were then covered with self-assembled monolayers of 11-mercaptoundecanoic acid. The changes in the sample morphology due to the protein adsorption have been investigated by atomic force microscopy (AFM). A fairly uniform distribution of protein molecules covers the surface. Owing to the tip broadening effect, an average protein diameter of about 20 nm was measured. An upper limit of 1 nN for the non-disruptive imaging force in the contact mode was found
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