The influence of urea and trimethylamine-N-oxide (TMAO) on the structure of water and secondary structure of hen egg white lysozyme (HEWL) has been investigated. The hydration of these osmolytes was studied in aqueous solutions by means of FTIR spectra of HDO isotopically diluted in H(2)O. The difference spectra procedure was applied to remove the contribution of bulk water and thus to separate the spectra of solute-affected HDO. The structural-energetic characteristic of these solute-affected water molecules shows that, on average, water affected by TMAO forms stronger H-bonds and is more ordered than pure water. In the case of urea, the H-bonds are very similar to those in pure water. To facilitate the interpretation of the obtained spectral results, calorimetric measurements, DFT calculations, and molecular dynamics (MD) simulations of aqueous osmolyte clusters were performed. All of these results confirmed that the interactions of TMAO with water molecules are much stronger than those of urea with water. Additional ATR FTIR measurements were performed to characterize the influence of the examined osmolytes on the secondary structure of HEW lysozyme. The type of interactions (direct or indirect) was determined, based on the second derivatives of ATR protein spectra record during an increase in the osmolyte concentration. The changes in the amide I band shape caused by urea or TMAO were found to correlate quite well with changes in the water structure around these osmolytes.

Results concerning the thermostability of hen egg white lysozyme in aqueous solutions with stabilizing osmolytes, trimethylamine-N-oxide (TMAO), glycine (Gly), and its N-methyl derivatives, N-methylglycine (NMG), N,N-dimethylglycine (DMG), and N,N,N-trimethylglycine (betaine, TMG), have been presented. The combination of spectroscopic (IR) and calorimetric (DSC) data allowed us to establish a link between osmolytes' influence on water structure and their ability to thermally stabilize protein molecule. Structural and energetic characteristics of stabilizing osmolytes' and lysozyme's hydration water appear to be very similar. The osmolytes increase lysozyme stabilization in the order bulk water < TMAO < TMG < Gly < DMG < NMG, which is consistent with the order corresponding to the value of the most probable oxygen-oxygen distance of water molecules affected by osmolytes in their surrounding. Obtained results verified the hypothesis concerning the role of water molecules in protein stabilization, explained the osmophobic effect, and finally helped to bring us nearer to the exact mechanism of protein stabilization by osmolytes.

The stability of proteins in an aqueous solution can be modified by the presence of osmolytes. The hydration sphere of stabilizing osmolytes is strikingly similar to the enhanced hydration sphere of a protein. This similarity leads to an increase in the protein stability. Moreover, the hydration sphere of destabilizing osmolytes is significantly different. These solutes generate in their surroundings so-called "structurally different water". The addition of such osmolytes causes "dissolution" of the specific protein hydration sphere and destabilizes its folded form. No relationship is seen between the stabilizing/destabilizing properties of osmolytes and their structure-making/-breaking influence on water. Furthermore, their accumulation at the protein surface or their exclusion does not determine the osmolytes' effect on protein stability. An explanation to the osmolytes' stabilizing/destabilizing influence originates in the similarity of water properties in osmolytes and protein solutions. The spectral infrared characteristic of water in an osmolyte solution allowed us to develop practical criteria for classifying solutes as stabilizing or destabilizing agents.

Dr fimbriae of uropathogenic Eschericha coli strains are an example of surface-located adhesive structures assembled via the chaperone-usher pathway. These structures are crucial for specific attachment of bacteria to host receptors. Dr fimbriae are linear associates of DraE proteins, the structure of which is determined by a donor strand complementation between the consecutive subunits. The biogenesis of these structures is dependent on a function of the specific periplasmic chaperone and outer membrane usher proteins. In a consequence of these structural and assembly properties the potential unfolding of a single subunit in a linear associate would cause a destruction of fimbrial adhesion function. This correlates with the observed high resistance of fimbrial structures for denaturation. In this paper we show that the mechanism of thermal denaturation of DraE-sc protein is well described by an irreversible two-state model which is the reduced form of a Lumry-Eyring protein denaturation model. In theory of this model the observed stability of DraE-sc protein is determined by the high activation barrier for the unfolding stage N-->U. The microcalorimetry experiments permit to determine kinetic parameters of the DraE-sc unfolding process: energy of activation of 463.5 +/- 20.8 kJ.mol(-1) and rate constant of order 10(-17) s(-1). This corresponds to the dissociation/unfolding half-life of Dr fimbriae of 10(8) years at 25 degrees C. The FT-IR experiments show that the high stability of DraE is determined by the cooperative rigid protein core. The presented mechanism of kinetic stability of Dr fimbriae is probably universal to adhesive structures of the chaperone-usher type.