Mechanism of Osmolyte Stabilization-Destabilization of Proteins: Experimental Evidence.

In this work, we investigated the influence of stabilizing (N,N,N-trimethylglycine) and destabilizing (urea) osmolytes on the hydration spheres of biomacromolecules in folded forms (trpzip-1 peptide and hen egg white lysozyme─hewl) and unfolded protein models (glycine─GLY and N-methylglycine─NMG) by means of infrared spectroscopy. GLY and NMG were clearly limited as minimal models for unfolded proteins and should be treated with caution. We isolated the spectral share of water changed simultaneously by the biomacromolecule/model molecule and the osmolyte, which allowed us to provide unambiguous experimental arguments for the mechanism of stabilization/destabilization of proteins by osmolytes. In the case of both types of osmolytes, the decisive factor determining the equilibrium folded/unfolded state of protein was the enthalpy effect exerted on the hydration spheres of proteins in both forms. In the case of stabilizing osmolytes, enthalpy was also favored by entropy, as the unfolded state of a protein was more entropically destabilized than the folded state.

Interactions in Ternary Aqueous Solutions of NMA and Osmolytes-PARAFAC Decomposition of FTIR Spectra Series.

Intermolecular interactions in aqueous solutions are crucial for virtually all processes in living cells. ATR-FTIR spectroscopy is a technique that allows changes caused by many types of such interactions to be registered; however, binary solutions are sometimes difficult to solve in these terms, while ternary solutions are even more difficult. Here, we present a method of data pretreatment that facilitates the use of the Parallel Factor Analysis (PARAFAC) decomposition of ternary solution spectra into parts that are easier to analyze. Systems of the NMA-water-osmolyte-type were used to test the method and to elucidate information on the interactions between N-Methylacetamide (NMA, a simple peptide model) with stabilizing (trimethylamine N-oxide, glycine, glycine betaine) and destabilizing osmolytes (n-butylurea and tetramethylurea). Systems that contain stabilizers change their vibrational structure to a lesser extent than those with denaturants. Changes in the latter are strong and can be related to the formation of direct NMA-destabilizer interactions.

Hydration of Simple Model Peptides in Aqueous Osmolyte Solutions.

The biology and chemistry of proteins and peptides are inextricably linked with water as the solvent. The reason for the high stability of some proteins or uncontrolled aggregation of others may be hidden in the properties of their hydration water. In this study, we investigated the effect of stabilizing osmolyte-TMAO (trimethylamine N-oxide) and destabilizing osmolyte-urea on hydration shells of two short peptides, NAGMA (N-acetyl-glycine-methylamide) and diglycine, by means of FTIR spectroscopy and molecular dynamics simulations. We isolated the spectroscopic share of water molecules that are simultaneously under the influence of peptide and osmolyte and determined the structural and energetic properties of these water molecules. Our experimental and computational results revealed that the changes in the structure of water around peptides, caused by the presence of stabilizing or destabilizing osmolyte, are significantly different for both NAGMA and diglycine. The main factor determining the influence of osmolytes on peptides is the structural-energetic similarity of their hydration spheres. We showed that the chosen peptides can serve as models for various fragments of the protein surface: NAGMA for the protein backbone and diglycine for the protein surface with polar side chains.

Preparation and characterization of dummy-template molecularly imprinted polymers as potential sorbents for the recognition of selected polybrominated diphenyl ethers.

The main aim of this work was to conduct the preliminary/basic research concerning the preparation process of a new dummy molecularly imprinted polymer (DMIP) materials. Developed DMIPs were proposed as a sorption material in solid-phase extraction (SPE) technique for recognition of selected low mass polybrominated diphenyl ethers (PBDEs) - PBDE-47 and PBDE-99. Four new DMIPs were synthesized employing bulky polymerization technique by application of structural analogue of low mass PBDEs - 4,4'-Dihydroxydiphenyl ether, as a dummy template. The DMIPs and corresponding non-imprinted polymers were prepared using different functional monomers: methacrylic acid; methyl methacrylate and different porogen agents: acetonitrile and tetrahydrofuran. The polymerization reaction was thermally initiated with 1,1'-azobis (cyclohexanecarbonitryle). Ethylene glycol dimethacrylate was applied as a cross-linker. To optimize geometries and to calculate energies of the respective template-monomer complexes, the computational molecular modeling method was employed. The particles morphology and physicochemical characteristics of developed DMIPs and their equivalent NIPs were performed using nitrogen sorption porosimetry, scanning electron microscopy, and Fourier transform infrared spectroscopy. The sorption capacities of prepared DMIPs and corresponding NIPs were studied using standard binding test. The adsorption capability studies give a possibility to assess the imprinting factor (IF) values, which were in the range from 1.1 to 4.0, depending on the DMIP type. The recovery values of PBDE-47 and PBDE-99 from prepared organic solutions were in the range from 43 to 92%, depending on the studied DMIP. Performed basic laboratory studies give a possibility to select the optimal DMIP material which might be applied in the environmental samples preparation process as a potential sorbent for the recognition of low mass PBDEs.

General Mechanism of Osmolytes' Influence on Protein Stability Irrespective of the Type of Osmolyte Cosolvent.

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.

Protein thermal stabilization in aqueous solutions of osmolytes.

Proteins' thermal stabilization is a significant problem in various biomedical, biotechnological, and technological applications. We investigated thermal stability of hen egg white lysozyme in aqueous solutions of the following stabilizing osmolytes: Glycine (GLY), N-methylglycine (NMG), N,N-dimethylglycine (DMG), N,N,N-trimethylglycine (TMG), and trimethyl-N-oxide (TMAO). Results of CD-UV spectroscopic investigation were compared with FTIR hydration studies' results. Selected osmolytes increased lysozyme's thermal stability in the following order: Gly>NMG>TMAO≈DMG>TMG. Theoretical calculations (DFT) showed clearly that osmolytes' amino group protons and water molecules interacting with them played a distinctive role in protein thermal stabilization. The results brought us a step closer to the exact mechanism of protein stabilization by osmolytes.

Hydration of amino acids: FTIR spectra and molecular dynamics studies.

The hydration of selected amino acids, alanine, glycine, proline, valine, isoleucine and phenylalanine, has been studied in aqueous solutions by means of FTIR spectra of HDO isotopically diluted in H2O. The difference spectra procedure and the chemometric method have been applied to remove the contribution of bulk water and thus to separate the spectra of solute-affected HDO. To support interpretation of obtained spectral results, molecular dynamics simulations of amino acids were performed. The structural-energetic characteristic of these solute-affected water molecules shows that, on average, water affected by amino acids forms stronger and shorter H-bonds than those in pure water. Differences in the influence of amino acids on water structure have been noticed. The effect of the hydrophobic side chain of an amino acid on the solvent interactions seems to be enhanced because of the specific cooperative coupling of water strong H-bond chain, connecting the carboxyl and amino groups, with the clathrate-like H-bond network surrounding the hydrocarbon side chain. The parameter derived from the spectral data, which corresponds to the contributions of the population of weak hydrogen bonds of water molecules which have been substituted by the stronger ones in the hydration sphere of amino acids, correlated well with the amino acid hydrophobicity indexes.

Influence of osmolytes on protein and water structure: a step to understanding the mechanism of protein stabilization.

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.

Characteristics of hydration water around hen egg lysozyme as the protein model in aqueous solution. FTIR spectroscopy and molecular dynamics simulation.

In this paper, the hydration of a model protein--hen egg white lysozyme in aqueous solution has been presented. The leading method used was FTIR spectroscopy with an application of a technique of semi-heavy water (HDO) isotope dilution. Analysis of spectra of HDO isotopically diluted in water solution of lysozyme allowed us to isolate HDO spectra affected by lysozyme, and thus to characterise the energetic state of water molecules and their arrangement around protein molecules. The number of water molecules and the shape of the affected HDO spectrum were obtained using a classical and a chemometric method. This shape showed that the HDO spectrum affected by lysozyme may be presented as a superposition of two spectra corresponding to HDO affected by N-methylacetamide and the carboxylate anion (of the formic acid). Moreover, based on the difference in intermolecular distances distribution of water molecules (obtained from spectral data), we demonstrated that the lysozyme molecule causes a decrease in population of weak hydrogen bonds, and concurrently increases the probability of an occurrence of short hydrogen bonds in water affected by lysozyme. This conclusion was also confirmed by the molecular dynamics (MD) simulation.

Fourier transform infrared spectroscopic and theoretical study of water interactions with glycine and its N-methylated derivatives.

In this study we attempt to explain the molecular aspects of amino acids' hydration. Glycine and its N-methylated derivatives: N-methylglycine, N,N-dimethylglycine, and N,N,N-trimethylglycine were used as model solutes in aqueous solution, applying FT-IR spectroscopy as the experimental method. The quantitative version of the difference spectra method enabled us to obtain the solute-affected HDO spectra as probes of influenced water. The spectral results were confronted with density functional theory calculated structures of small hydration complexes of the solutes using the polarizable continuum model. It appears that the hydration of amino acids in the zwitterionic form can be understood allowing a synchronized fluctuation of hydrogen bonding between the solute and the water molecules. This effect is caused by a noncooperative interaction of water molecules with electrophilic groups of amino acid and by intramolecular hydrogen bond, allowing proton transfer from the carboxylic to the amine group, accomplishing by the chain of two to four water molecules. As a result, an instantaneous water-induced asymmetry of the carboxylate and the amino group of amino acid molecule is observed and recorded as HDO band splitting. Water molecules interacting with the carboxylate group give component bands at 2543 ± 11 and 2467 ± 15 cm(-1), whereas water molecules interacting with protons of the amine group give rise to the bands at 2611 ± 15 and 2413 ± 12 cm(-1). These hydration effects have not been recognized before and there are reasons to expect their validity for other amino acids.

Effects of urea and trimethylamine-N-oxide on the properties of water and the secondary structure of hen egg white lysozyme.

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.

Hydration of carboxylate anions: infrared spectroscopy of aqueous solutions.

Hydration of carboxylate ions was studied in aqueous solutions of sodium salts by means of FTIR spectroscopy using the HDO molecule as a probe. The quantitative version of the difference spectra method has been applied to determine the solute-affected water spectra. They display two-component bands of affected HDO at ca. 2550 and 2420 cm(-1). These bands are attributed to the -COO(-) group of the R-COO(-) ion (R = H, CH(3), C(2)H(5)), because water molecules surrounding the substituent R behave roughly as molecules in the bulk phase. For the studied carboxylates the net water structure making effect is observed, which increases with electron-donor ability of R, by means of changing the relative intensity of solute-affected HDO component bands. The observed splitting of the carboxylate-ion-affected HDO band is unique for these anions. The experimental results were confronted with DFT-calculated structures of small gas-phase and polarizable continuum model (PCM) solvated aqueous clusters to establish the structural and energetic states of carboxylate ions hydrates. This was achieved by comparison of the calculated optimal geometries with the interatomic distances derived from HDO band positions. Different possibilities have been considered to explain the peculiar spectral results. The plausible explanation assumes symmetry breaking of the carboxylate ion induced by interaction with water solvent: C-O bond lengths of RCOO(-) and electric charge localization become unequal. It is demonstrated by nonequivalent interaction of oxygen atoms of the RCOO(-) anion with water molecules. Taking into account only the energetic effect, the phenomenon is explained by the anticooperative H-bond formation of the carboxylate group with water molecules, which increases with the electron-donor ability of the substituent R. In this interaction two water molecules play an important part, as appears from the calculated clusters. They interact with oxygen atoms of the RCOO(-) ion, forming a cooperative system, within which solvent molecules are nonequivalent with respect to H-bond formation with both proton-accepting sites of the solute. This additionally enhances solvent-induced symmetry breaking of carboxylate anion. Strongly hydrogen-bonded solvent is more effective in inducing symmetry breaking; thus, increasing the temperature decreases the splitting of the carboxylate-ion-affected water, as experimentally observed.

Hydration of simple amides. FTIR spectra of HDO and theoretical studies.

The hydration of formamide (F), N-methylformamide (NMF), N,N-dimethylformamide (DMF), acetamide (A), N-methylacetamide (NMA), and N,N-dimethylacetamide (DMA) has been studied in aqueous solutions by means of FTIR spectra of HDO isotopically diluted in H2O. The difference spectra procedure has been applied to remove the contribution of bulk water and thus to separate the spectra of solute-affected HDO. To facilitate the interpretation of obtained spectral results, DFT calculations of aqueous amide clusters were performed. Molecular dynamics (MD) simulation for the cis and trans forms of NMA was also carried out for the SPC model of water. Infrared spectra reveal that only two to three water molecules from the surrounding of the amides are statistically affected, from among ca. 30 molecules present in the first hydration sphere. The structural-energetic characteristic of these solute-affected water molecules differs only slightly from that in the bulk and corresponds to the clathrate-like hydrogen-bonded cage typical for hydrophobic hydration, with the possible exception of F. MD simulations confirm such organization of water molecules in the first hydration sphere of NMA and indicate a practical lack of orientation and energetic effects beyond this sphere. The geometry of hydrogen-bonded water molecules in the first hydration sphere is very similar to that in the bulk phase, but MD simulations have affirmed subtle differences recognized by the spectral method and enabled their understanding. The spectral data and simulations results are highly compatible. In the case of F, NMF, and A, there is a visible spectral effect of water interactions with N-H groups, which have destabilizing influence on the amides hydration shell. There is no spectral sign of such interaction for NMA as the solute. The energetic stability of water H-bonds in the amide hydration sphere and in the bulk fulfills the order: NMA > DMA > A > NMF > bulk > DMF > F. Microscopic parameters of water organization around the amides obtained from the spectra, which have been used in the hydration model based on volumetric data, confirm the more hydrophobic character of the first three amides in this sequence. The increased stability of the hydration sphere of NMA relative to DMA and of NMF relative to DMF seems to have its origin in different geometries, and so the stability, of water cages containing the amides.