Phase-fluorometry study on dielectric relaxation of acrylodan-labeled human serum albumin

Elucidation of active site dynamics of papain and the effect of encapsulation within cationic and anionic reverse micelles

In this study, steady state, solvation dynamics and rotational dynamics experiments have been carried out on a system of DACIA-tagged papain in bulk water and inside the water pool of cationic (cetyltrimethylammonium bromide, CTAB) and anionic (sodium bis(2-ethylhexyl)sulfosuccinate, AOT) reverse micelles with varying water contents (W₀ = 20 to 50). While the absorption and emission maxima and the excited state lifetime did not show any noticeable change with the variation of the size of the reverse micelle, the change in solvation time, Stokes shift, rotational correlation time and residual anisotropy with the change in reverse micellar size were quite revealing. The average solvation time and Stokes shift of papain in bulk water are 0.22 ns and 125 cm^-1 respectively, which increase to 0.96 ns and 718 cm^-1 while inside CTAB reverse micelle of W₀ = 20. The solvation time and Stokes shift values decrease with the increase in the size of reverse micelle, approaching the corresponding values in bulk water when W₀ = 50. The solvation time and Stokes shift of the DACIA-tagged papain was found to be high while inside AOT reverse micelle also (0.47 ns and 438 cm^-1 respectively when W₀ = 20), but there was no monotonous variation with the change in size of micellar size as in the case with CTAB reverse micelle. From the anisotropy studies, it was seen that inside CTAB and AOT reverse micelles, there is a significant amount of residual anisotropy, which is absent in the case of DACIA-tagged papain in bulk water. The rotational correlation times were also found to be higher inside the reverse micelles than those in bulk water. Both residual anisotropy and rotational correlation time were found to be more in the case with AOT reverse micelle than with CTAB reverse micelle. These behaviours could be explained based on the electrostatic forces acting between the papain having a positive surface charge and the reverse micelles of cationic CTAB and anionic AOT.

Resolution of Cys and Lys labeling of alpha-crystallin with site-sensitive fluorescent 3-hydroxyflavone dye

Ratiometric fluorescent probes based on 3-hydroxyflavone (3HF) are highly sensitive tools for studying polarity, hydration, electronic polarizability, and electrostatics in different microheterogeneous systems, including protein molecules. In the present work, a reactive derivative of 3HF, 6-bromomethyl-4'-diethylamino-3-hydroxyflavone, recently synthesized in our group, was applied to label covalently bovine lens alpha-crystallin. The labeling of SH and NH(2) groups are clearly distinguished by spectroscopic criteria. We observe that the NH(2) labeling creates the positive charge in the proximity to fluorophore, which results in strong internal Stark effect producing the shift in excitation spectrum by ca. 15 nm. Analysis of excitation-dependent fluorescence spectra allows separation of the emission profiles of these SH- and NH(2)-labeled species. Applying recently developed multiparametric analysis of the obtained emission spectra, we described the physicochemical properties of the sites of SH and NH(2) labeling in alpha-crystallin. The site of SH labeling has medium-low polarity (dielectric constant, epsilon = 4.9 +/- 0.9) is protic, and does not contain proximal aromatic residues (according to the obtained refractive index, n = 1.41 +/- 0.14). The site of NH(2) labeling is also of medium-low polarity. The novel label due to its two-wavelength ratiometric response and high sensitivity to the type of labeling may offer new possibilities in the studies of structure, dynamics, and interactions of proteins by probing their SH- and NH(2)-labeling sites.

Ultrafast hydration dynamics in protein unfolding: human serum albumin

We report studies of unfolding and ultrafast hydration dynamics of the protein human serum albumin. Unique in this study is our ability to examine different domains of the same protein and the intermediate on the way to the unfolded state. With femtosecond resolution and site-selective labeling, we isolate the dynamics of domains I and II of the native protein, domain I of the intermediate at 2 M guanidine hydrochloride, and the unfolded state at 6 M of the denaturant. For studies of unfolding, we used the fluorophores, acrylodan (covalently bound to Cys-34 in domain I) and the intrinsic tryptophan (domain II), whereas for hydration dynamics, we probed acrylodan and prodan; the latter is bound to domain II. From the time-dependent spectra and the correlation functions, we obtained the time scale of dynamically ordered water: 57 ps for the more stable domain I and 32 ps for the less stable domain II, in contrast to approximately 0.8 ps for labile, bulk-type water. This trend suggests an increased hydrophilic residues-water interaction of domain I, contrary to some packing models. In the intermediate state, which is characterized by essentially intact domain I and unfolded domain II, the dynamics of ordered water around domain I is nearly the same (61 ps) as that of native state (57 ps), whereas that in the unfolded protein is much shorter (13 ps). We discuss the role of this fluidity in the correlation between stability and function of the protein.

On the hydrodynamics and temperature dependence of the solution conformation of human serum albumin from viscometry approach

The paper presents the results of viscosity determinations on aqueous solutions of human serum albumin (HSA) at a wide range of concentrations and at temperatures ranging from 5 to 45 degrees C. On the basis of a modified Arrhenius formula and Mooney's equation, the viscosity-temperature and viscosity-concentration dependence of the solutions are discussed. The effective specific volume, the activation energy and entropy of viscous flow for hydrated HSA were calculated. Different models of HSA molecule are discussed and the best one-from the hydrodynamic point of view-was established. At low concentration limit, such rheological quantities as the intrinsic viscosity and Huggins coefficient were obtained. Using the dimensionless parameter [eta]c, the existence of three characteristic ranges of concentrations: diluted, semi-diluted and concentrated, was shown.