Cooperative charge fluctuations by migrating protons in globular proteins

Investigation of Antibody Tolerance in Methanol for Analytical Purposes: Methanol Effect Patterns and Molecular Mechanisms

The extraction of targets from biological samples for immunoassays using organic solvents, such as methanol, is often necessary. However, high concentrations of organic solvents in extracts invariably lead to instability of the employed antibody, resulting in poor performance of the immunoassay. Evaluating the tolerance ability and exploring the molecular mechanisms of antibody tolerance in organic solvents are essential for the development of robust immunoassays. In this work, 25 monoclonal antibodies and methanol are utilized as models to address these questions. A novel protocol is initially established to precisely and rapidly determine antibody tolerance in methanol, identifying two distinct methanol effect patterns. Through a detailed investigation of the structural basis, a novel hypothesis regarding methanol effect patterns is proposed, termed "folding-aggregation," which is subsequently validated through molecular dynamics simulations. Furthermore, the investigation of sequence basis reveals significant differences in residue types within the complementarity-determining regions and ligand-binding residues, distinguishing the two antibody methanol effect patterns. Moreover, the methanol effect patterns of the antibodies are defined by germline antibodies. This work represents the first exploration of antibody methanol effect patterns and associated molecular mechanisms, with potential implications for the discovery and engineering of tolerant antibodies for the development of robust immunoassays.

Rational and random mutagenesis of GDEst-95 carboxylesterase: New functionality insights

Lipolytic enzymes are important contributors in industrial processes from lipid hydrolysis to biofuel production or even polyester biodegradation. While these enzymes can be used in numerous applications, the genotype-phenotype space of certain promising enzymes is still poorly explored. This limits the effective application of such biocatalysts. In this work the genotype space of a 55 kDa carboxylesterase GDEst-95 from Geobacillus sp. 95 was explored using site-directed mutagenesis and directed evolution methods. In this study four site-directed mutants (Gly108Arg, Ala410Arg, Leu226Arg, Leu411Ala) were created based on previous analysis of GDEst-95 carboxylesterase. Error-prone PCR resulted three mutants: two of them with distal mutations: GDEst-RM1 (Arg75Gln), GDEst-RM2 (Gly20Ser Arg75Gln) and the third, GDEst-RM3, with a distal (Ser210Gly) and Tyr317Ala (amino acid position near to the active site) mutation. Mutants with Ala substitution displayed approximately twofold higher specific activity. Arg mutations lead a reduced specific activity, retaining 2.86 % (Gly108Arg), 10.95 % (Ala410Arg), and 44.23 % (Leu226Arg) of lipolytic activity. All three random mutants displayed increased specific activity as well as improved catalytic properties. This research provides the first deeper insights into the functionality of understudied Geobacillus spp. carboxylesterases with 55 kDa in size.

Incoherent Neutron Scattering and Terahertz Time-Domain Spectroscopy on Protein and Hydration Water

Incoherent inelastic and quasi-elastic neutron scattering (INS) and terahertz time-domain spectroscopy (THz-TDS) are spectroscopy methods that directly detect molecular dynamics, with an overlap in the measured energy regions of each method. Due to the different characteristics of their probes (i.e., neutron and light), the information obtained and the sample conditions suitable for each method differ. In this review, we introduce the differences in the quantum beam properties of the two methods and their associated advantages and disadvantages in molecular spectroscopy. Neutrons are scattered via interaction with nuclei; one characteristic of neutron scattering is a large incoherent scattering cross-section of a hydrogen atom. INS records the auto-correlation functions of atomic positions. By using the difference in neutron scattering cross-sections of isotopes in multi-component systems, some molecules can be selectively observed. In contrast, THz-TDS observes the cross-correlation function of dipole moments. In water-containing biomolecular samples, the absorption of water molecules is particularly large. While INS requires large-scale experimental facilities, such as accelerators and nuclear reactors, THz-TDS can be performed at the laboratory level. In the analysis of water molecule dynamics, INS is primarily sensitive to translational diffusion motion, while THz-TDS observes rotational motion in the spectrum. The two techniques are complementary in many respects, and a combination of the two is very useful in analyzing the dynamics of biomolecules and hydration water.

Biochemical characterization of an alkaline and detergent-stable Lipase from Fusarium annulatum Bugnicourt strain CBS associated with olive tree dieback

This work describes a novel extracellular lipolytic carboxylester hydrolase named FAL, with lipase and phospholipase A1 (PLA1) activity, from a newly isolated filamentous fungus Ascomycota CBS strain, identified as Fusarium annulatum Bunigcourt. FAL was purified to about 62-fold using ammonium sulphate precipitation, Superdex® 200 Increase gel filtration and Q-Sepharose Fast Flow columns, with a total yield of 21%. The specific activity of FAL was found to be 3500 U/mg at pH 9 and 40°C and 5000 U/mg at pH 11 and 45°C, on emulsions of triocanoin and egg yolk phosphatidylcholine, respectively. SDS-PAGE and zymography analysis estimated the molecular weight of FAL to be 33 kDa. FAL was shown to be a PLA1 with a regioselectivity to the sn-1 position of surface-coated phospholipids esterified with α-eleostearic acid. FAL is a serine enzyme since its activity on triglycerides and phospholipids was completely inhibited by the lipase inhibitor Orlistat (40 μM). Interestingly, compared to Fusarium graminearum lipase (GZEL) and the Thermomyces lanuginosus lipase (Lipolase®), this novel fungal (phospho)lipase showed extreme tolerance to the presence of non-polar organic solvents, non-ionic and anionic surfactants, and oxidants, in addition to significant compatibility and stability with some available laundry detergents. The analysis of washing performance showed that it has the capability to efficiently eliminate oil-stains. Overall, FAL could be an ideal choice for application in detergents.

Amphiphilic Nanointerface: Inducing the Interfacial Activation for Lipase

Graphene-based materials are widely used in the field of immobilized enzymes due to their easily tunable interfacial properties. We designed amphiphilic nanobiological interfaces between graphene oxide (GO) and lipase TL (Thermomyces lanuginosus) with tunable reduction degrees through molecular dynamics simulations and a facile chemical modulation, thus revealing the optimal interface for the interfacial activation of lipase TL and addressing the weakness of lipase TL, which exhibits weak catalytic activity due to an inconspicuous active site lid. It was demonstrated that the reduced graphene oxide (rGO) after 4 h of ascorbic acid reduction could boost the relative enzyme activity of lipase TL to reach 208%, which was 48% higher than the pristine GO and 120% higher than the rGO after 48 h of reduction. Moreover, TL-GO-4 h's tolerance against heat, organic solvent, and long-term storage environment was higher than that of free TL. The drawbacks of strong hydrophobic nanomaterials on lipase production were explored in depth with the help of molecular dynamics simulations, which explained the mechanism of enzyme activity enhancement. We demonstrated that nanomaterials with certain hydrophilicity could facilitate the lipase to undergo interfacial activation and improve its stability and protein loading rate, displaying the potential of the extensive application.

Hydration and its Hydrogen Bonding State on a Protein Surface in the Crystalline State as Revealed by Molecular Dynamics Simulation

Protein hydration is crucial for the stability and molecular recognition of a protein. Water molecules form a hydration water network on a protein surface via hydrogen bonds. This study examined the hydration structure and hydrogen bonding state of a protein, staphylococcal nuclease, at various hydration levels in its crystalline state by all-atom molecular dynamics (MD) simulation. Hydrophilic residues were more hydrated than hydrophobic residues. As the water content increases, both types of residues were uniformly more hydrated. The number of hydrogen bonds per single water asymptotically approaches 4, the same as bulk water. The distances and angles of hydrogen bonds in hydration water in the protein crystal were almost the same as those in the tetrahedral structure of bulk water regardless of the hydration level. The hydrogen bond structure of hydration water observed by MD simulations of the protein crystalline state was compared to the Hydrogen and Hydration Database for Biomolecule from experimental protein crystals.

The Role of Hydrogen Bonding in the Folding/Unfolding Process of Hydrated Lysozyme: A Review of Recent NMR and FTIR Results

The biological activity of proteins depends on their three-dimensional structure, known as the native state. The main force driving the correct folding mechanism is the hydrophobic effect and when this folding kinetics is altered, aggregation phenomena intervene causing the occurrence of illnesses such as Alzheimer and Parkinson’s diseases. The other important effect is performed by water molecules and by their ability to form a complex network of hydrogen bonds whose dynamics influence the mobility of protein amino acids. In this work, we review the recent results obtained by means of spectroscopic techniques, such as Fourier Transform Infrared (FTIR) and Nuclear Magnetic Resonance (NMR) spectroscopies, on hydrated lysozyme. In particular, we explore the Energy Landscape from the thermal region of configurational stability up to that of the irreversible denaturation. The importance of the coupling between the solute and the solvent will be highlighted as well as the different behaviors of hydrophilic and hydrophobic moieties of protein amino acid residues.

A novel organic solvent- and detergent-stable serine alkaline protease from Trametes cingulata strain CTM10101

A protease-producing fungus was isolated from an alkaline wastewater of chemical industries and identified as Trametes cingulata strain CTM10101 on the basis of the ITS rDNA gene-sequencing. It was observed that the fungus strongly produce extracellular protease grown at 30°C in potato-dextrose-broth (PDB) optimized media (13500U/ml). The pure serine protease isolated by Trametes cingulata (designated SPTC) was purified by ammonium sulfate precipitation-dialysis followed by heat-treatment and UNO S-1 FPLC cation-exchange chromatography. The chemical characterization carried on include phisico-chemical determination and spectroscopie analysis. The MALDI-TOF/MS analysis revealed that the purified enzyme was a monomer with a molecular mass of 31405.16-Da. The enzyme had an NH2-terminal sequence of ALTTQTEAPWALGTVSHKGQAST, thus sharing high homology with those of fungal-proteases. The optimum pH and temperature values of its proteolytic activity were pH 9 and 60°C, respectively, and its half-life times at 60 and 70°C were 9 and 5-h, respectively. It was completely inhibited by PMSF and DFP, which strongly suggested its belonging to the serine protease family. Compared to Flavourzyme(®)500L from Aspergillus oryzae and Thermolysin typeX from Geobacillus stearothermophilus, SPTC displayed higher levels of hydrolysis, substrate specificity, and catalytic efficiency as well as elevated organic solvent tolerance and considerable detergent stability. Finally, SPTC could potentially be used in peptide synthesis and detergent formulations.

The role of N-glycosylation sites in the activity, stability, and expression of the recombinant elastase expressed by Pichia pastoris

The Pseudomonas aeruginosa elastase (PAE), produced by Pseudomonas aeruginosa (P. aeruginosa), is a promising biocatalyst for peptide synthesis in organic solvents. As P. aeruginosa is an opportunistic pathogen, the enzyme has been heterologously over-expressed in the safe and efficient host, Pichia pastoris (P. pastoris) for its industrial application. The recombinant elastase (rPAE) contains three potential N-glycosylation sites (Asn-Xaa-Ser/Thr consensus sequences), and is heterogeneously N-glycosylated. To investigate the role of N-glycosylation in the activity, stability, and expression of rPAE, these potential N-glycosylation sites (N43, N212, and N280) were mutated using site-directed mutagenesis. Specifically the asparagine (Asn, N) residues were converted to glutamine (Gln, Q). The enzymatic activity and stability of non-glycosylated and glycosylated rPAE were then compared. The results indicated that the influence of N-glycosylation on its activity was insignificant. The non- and glycosylated isoforms of rPAE displayed similar kinetic parameters for hydrolyzing casein in aqueous medium, and when catalyzing bipeptide synthesis in 50% (v/v) DMSO, they exhibited identical substrate specificity and activity, and produced similar yields. However, N-glycosylation improved rPAE stability both in aqueous medium and in 50% (v/v) organic solvents. The half-lives of the glycosylated and non-glycosylated forms of rPAE at 70°C were 32.2 and 23.1 min, respectively. Mutation of any potential N-glycosylation site was detrimental to its expression in P. pastoris. There was a 23.9% decrease in expression of the N43Q mutant, 63.6% of the N212Q mutant, and 63.7% of the N280Q mutant compared with the wild type. Furthermore, combined mutation of these sites resulted in an additional decrease in the caseinolytic activities of the mutants. These results indicated that all of the N-glycosylation sites were necessary for high-level expression of rPAE.

Protein dynamics by neutron scattering

Neutron scattering techniques represent a powerful tool for characterizing both the structure and dynamical properties of bio-systems, for example, proteins and membranes interacting with their solvents. In this paper, Elastic Neutron Scattering (ENS) data collected at the Institut Laue-Langevin (Grenoble, France) on dry and D2O hydrated lysozyme by varying hydration level are presented, and compared with previously published data on the same protein system, also with the addition of bio-protectants. The data have been collected with three different spectrometers, i.e. IN13, IN10 and IN4. This set of ENS data gives direct access to the temperature behavior of both (i) the Mean Square Displacement (MSD) and (ii) the characteristic system relaxation time. As a result, an explicative hypothesis on the relationship between the so-called "protein dynamical transition" (PDT) and the "fragile-to-strong dynamical crossover" (FSC) is formulated. Furthermore, by taking into proper account the effect of the finite instrumental energy resolution of the used spectrometers, the vibrational MSD of dry and hydrated lysozyme is calculated. The vibrational MSD of the lysozyme in the dry state resulted to be higher than the one in the hydrated state; the latter reaches the former at a temperature value of T=220K that corresponds to the temperature at which the FSC occurs. As a result, a cage effect resulting from the hydration water on the protein surface is hypothesized and subsequently linked to the FSC.

Simplicity within the complexity: bilateral impact of DMSO on the functional and unfolding patterns of α-chymotrypsin

New understanding of the fundamental links between protein stability, conformational flexibility and function, can be gained through synergic studies on their catalytic and folding/unfolding properties under the influence of stabilizing/destabilizing additives. We explored an impact of dimethyl sulfoxide (DMSO), the moderate effector of multilateral action, on the kinetic (functional) and thermodynamic (thermal unfolding) patterns of a hydrolytic enzyme, α-chymotrypsin (α-CT), over a wide range of additive concentrations, 0-70% (v/v). Both the calorimetric and kinetic data exhibited rich behavior pointing to the complex interplay of global/local stability (and flexibility) patterns. The complex action of DMSO is explained through the negative and positive preferential solvation motifs that prevail for the extreme opposite, native-like and unfolded states, respectively, implying essential stabilization of compact domains by enhancement of interfacial water networks and destabilization of a flexible active site by direct binding of DMSO to the unoccupied specific positions intended for elongated polypeptide substrates.

Dynamic behavior of oligomeric inorganic pyrophosphatase explored by quasielastic neutron scattering

The purpose of this investigation is to determine whether a large oligomeric protein, inorganic pyrophosphatase (IPPase) from Thermococcus thioreducens with quaternary structural complexity, would have distinguishable dynamic characteristics compared to those of the small simple monomeric model protein, lysozyme. In this study, the β-relaxational dynamics of the two proteins, IPPase and lysozyme, are compared in the 10 ps to 0.5 ns time interval using quasi-elastic neutron scattering (QENS). Both of the protein dynamics show a characteristic logarithmic-like decay in the intermediate scattering function (ISF) of the hydrogen atoms. Distinguishable dynamical behavior found between two proteins reveals local flexibility and conformational substates unique to oligomeric structures. Moreover, the temperature dependence of the mean square displacement (MSD) of the hydrogen atoms in protein molecules, which is a traditional way to determine the "softness" of the protein molecule, is measured and shows no difference for the two proteins.

The catalytic activity enhancement and biodegradation potential of free laccase and novel sol-gel laccase in non-conventional solvents

The catalytic activity of free laccase and a novel sol-gel laccase (SOLAC) in ionic liquids and organic solvents was demonstrated by using 2,6-dimethoxyphenol (2,6-DMP) as a substrate. The enhancement of the catalytic activity of the SOLAC was observed and compared to the free laccase in both media. The oxidative biodegradation of o-chlorophenol as a model of phenolic environmental pollutants in organic media shows that the degradation was observed only when using water pre-saturated organic solvents or reverse micelle system. The SOLAC gave higher biodegradation rate in either aqueous or organic solvents, in which the optimum temperature was observed at 40 °C for the reverse micelle system as a reaction medium. All results demonstrated the potential use of the SOLAC for biodegradation of phenolic environmental pollutants in non-conventional media.

Localized hydration in lyophilized myoglobin by hydrogen-deuterium exchange mass spectrometry. 1. Exchange mapping

The local effects of hydration on myoglobin (Mb) in solid matrices containing mannitol or sucrose (1:1 w/w, protein:additive) were mapped using hydrogen-deuterium exchange with mass spectrometric analysis (HDX-MS) at 5 °C and compared to solution controls. Solid powders were exposed to D₂O(g) at controlled activity (a(w)) followed by reconstitution and analysis of the intact protein and peptides produced by pepsin digestion. HDX varied with matrix type, a(w), and position along the protein backbone. HDX was less in sucrose matrices than in mannitol matrices at all a(w) while the difference in solution was negligible. Differences in HDX in the two matrices were detectable despite similarities in their bulk water content. The extent of exchange in solids is proposed as a measure of the hydration of exchangeable amide groups, as well as protein conformation and dynamics; pepsin digestion allows these effects to be mapped with peptide-level resolution.

Lewis-inspired representation of dissociable water in clusters and Grotthuss chains

Proton transfer to and from water is critical to the function of water in many settings. However, it has been challenging to model. Here, we present proof-of-principle for an efficient yet robust model based on Lewis-inspired submolecular particles with interactions that deviate from Coulombic at short distances to take quantum effects into account. This "LEWIS" model provides excellent correspondence with experimental structures for water molecules and water clusters in their neutral, protonated and deprotonated forms; reasonable values for the proton affinities of water and hydroxide; a good value for the strength of the hydrogen bond in the water dimer; the correct order of magnitude for the stretch and bend force constants of water; and the expected time course for Grotthuss transport in water chains.

Bio-protective effects of homologous disaccharides on biological macromolecules

In this contribution the effects of the homologous disaccharides trehalose and sucrose on both water and hydrated lysozyme dynamics are considered by determining the mean square displacement (MSD) from elastic incoherent neutron scattering (EINS) experiments. The self-distribution function (SDF) procedure is applied to the data collected, by use of IN13 and IN10 spectrometers (Institute Laue Langevin, France), on trehalose and sucrose aqueous mixtures (at a concentration corresponding to 19 water molecules per disaccharide molecule), and on dry and hydrated (H(2)O and D(2)O) lysozyme also in the presence of the disaccharides. As a result, above the glass transition temperature of water, the MSD of the water-trehalose system is lower than that of the water-sucrose system. This result suggests that the hydrogen-bond network of the water-trehalose system is stronger than that of the water-sucrose system. Furthermore, by taking into account instrumental resolution effects it was found that the system relaxation time of the water-trehalose system is longer than that of the water-sucrose system, and the system relaxation time of the protein in a hydrated environment in the presence of disaccharides increases sensitively. These results explain the higher bioprotectant effectiveness of trehalose. Finally, the partial MSDs of sucrose/water and trehalose/water have been evaluated. It clearly emerges from the analysis that these are almost equivalent in the low-Q domain (0-1.7 Å(-1)) but differ substantially in the high-Q range (1.7-4 Å(-1)). These findings reveal that the lower structural sensitivity of trehalose to thermal changes is connected with the local spatial scale.

Glass transition and dynamics in BSA-water mixtures over wide ranges of composition studied by thermal and dielectric techniques

Protein-water dynamics in mixtures of water and a globular protein, bovine serum albumin (BSA), was studied over wide ranges of composition, in the form of solutions or hydrated solid pellets, by differential scanning calorimetry (DSC), thermally stimulated depolarization current technique (TSDC) and dielectric relaxation spectroscopy (DRS). Additionally, water equilibrium sorption isotherm (ESI) measurements were performed at room temperature. The crystallization and melting events were studied by DSC and the amount of uncrystallized water was calculated by the enthalpy of melting during heating. The glass transition of the system was detected by DSC for water contents higher than the critical water content corresponding to the formation of the first sorption layer of water molecules directly bound to primary hydration sites, namely 0.073 (grams of water per grams of dry protein), estimated by ESI. A strong plasticization of the T(g) was observed by DSC for hydration levels lower than those necessary for crystallization of water during cooling, i.e. lower than about 0.3 (grams of water per grams of hydrated protein) followed by a stabilization of T(g) at about -80°C for higher water contents. The α relaxation associated with the glass transition was also observed in dielectric measurements. In TSDC a microphase separation could be detected resulting in double T(g) for some hydration levels. A dielectric relaxation of small polar groups of the protein plasticized by water, overlapped by relaxations of uncrystallized water molecules, and a separate relaxation of water in the crystallized water phase (bulk ice crystals) were also recorded.

Puzzle of protein dynamical transition

Despite recent extensive efforts, the nature of the dynamics of biological macromolecules still remains unclear. In particular, contradicting models have been proposed for explaining the temperature behavior of the mean square displacement, MSD, and of the system relaxation time, τ. To solve this puzzle, different neutron scattering experiments with different instrumental energy resolutions were performed on dry and hydrated lysozyme. The obtained results show that the so called dynamical transition: (i) is a finite instrumental energy resolution effect, and more specifically, it appears when the characteristic system relaxation time intersects the resolution time, (ii) it does not imply any transition in the dynamical properties of the systems, (iii) it is not due to the fragile-to-strong dynamical crossover (FSC) in the temperature behavior of the system relaxation time, differently to what S. H. Chen et al. proposed [Proc. Natl. Acad. Sci. U.S.A.2006, 103, 9012]. Furthermore, the obtained results confirm the change in the τ-temperature dependence at T = 220 K of S. H. Chen et al., and show that it is not due to finite instrumental energy resolution effects and it is not connected to numerical errors in the data analysis protocol, differently to what W. Doster et al. proposed [Phys. Rev. Lett.2010, 104, 098101].

Mean square displacements from elastic incoherent neutron scattering evaluated by spectrometers working with different energy resolution on dry and hydrated (H2O and D2O) lysozyme

The main aim of the present paper is the evaluation of the effects of the instrumental energy resolution on the mean square displacement (MSD) obtained by elastic incoherent neutron scattering (EINS). In particular, this study is performed in the time domain, through the time-Fourier transform of the elastically scattered neutron intensity, and is mainly focused on the connection between the system MSD and the measured MSD. It is shown how in the case of EINS, the instrumental energy resolution gives rise to the time integration of the time-dependent system MSD function weighted in time by the resolution function. The formulated approach is applied to the data collected on dry and hydrated (H(2)O and D(2)O with h = 0.4) lysozyme samples by two spectrometers working with a different instrumental resolution (the IN10 and IN13 spectrometers of the Institute Laue-Langevin). As a result, the procedure furnishes an excellent agreement for the system MSD evaluated in the low temperature range up to T = 40 K.

Quantum behavior of water protons in protein hydration shell

Quantum effects on the water proton dynamics over the surface of a hydrated protein are measured by means of broadband dielectric spectroscopy and deep inelastic neutron scattering. Dielectric spectroscopy indicates a reduced energy barrier for a hydrogenated protein sample compared to a deuterated one, along with a large and temperature-dependent isotopic ratio, in good agreement with theoretical studies. Recent deep inelastic neutron scattering data have been reanalyzed, and now show that the momentum distribution of water protons reflects a characteristic delocalization at ambient temperatures. These experimental findings might have far-reaching implications for enzymatic catalysis involving proton transfer processes, as in the case of the lysozyme protein studied in this report.

Proton momentum distribution in a protein hydration shell

The momentum distribution of protons in the hydration shell of a globular protein has been measured through deep inelastic neutron scattering at 180 and 290 K, below and above the crossover temperature Tc=1.23Tg, where Tg=219 K is the glass transition temperature. It is found that the mean kinetic energy of the water hydrogens shows no temperature dependence, but the measurements are accurate enough to indicate a sensible change of momentum distribution and effective potential felt by protons, compatible with the transition from a single to a double potential well. This could support the presence of tunneling effects even at room temperature, playing an important role in biological function.

Protein folding is slaved to solvent motions

Proteins, the workhorses of living systems, are constructed from chains of amino acids, which are synthesized in the cell based on the instructions of the genetic code and then folded into working proteins. The time for folding varies from microseconds to hours. What controls the folding rate is hotly debated. We postulate here that folding has the same temperature dependence as the alpha-fluctuations in the bulk solvent but is much slower. We call this behavior slaving. Slaving has been observed in folded proteins: Large-scale protein motions follow the solvent fluctuations with rate coefficient k(alpha) but can be slower by a large factor. Slowing occurs because large-scale motions proceed in many small steps, each determined by k(alpha). If conformational motions of folded proteins are slaved, so a fortiori must be the motions during folding. The unfolded protein makes a Brownian walk in the conformational space to the folded structure, with each step controlled by k(alpha). Because the number of conformational substates in the unfolded protein is extremely large, the folding rate coefficient, k(f), is much smaller than k(alpha). The slaving model implies that the activation enthalpy of folding is dominated by the solvent, whereas the number of steps n(f) = k(alpha)/k(f) is controlled by the number of accessible substates in the unfolded protein and the solvent. Proteins, however, undergo not only alpha- but also beta-fluctuations. These additional fluctuations are local protein motions that are essentially independent of the bulk solvent fluctuations and may be relevant at late stages of folding.

Relation between solvent and protein dynamics as studied by dielectric spectroscopy

We present results obtained by dielectric spectroscopy in wide frequency (10(-2)-10(9) Hz) and temperature ranges on human hemoglobin in the three different solvents water, glycerol, and methanol, at a solvent level of 0.8 g of solvent/g of protein. In this broad frequency region, there are motions on several time-scales in the measured temperature range (110-370 K for water, 170-410 K for glycerol, and 110-310 K for methanol). For all samples, the dielectric data shows at least four relaxation processes, with frequency dependences that are well described by the Havriliak-Negami or Cole-Cole functions. The fastest and most pronounced process in the dielectric spectra of hemoglobin in glycerol and methanol solutions is similar to the alpha-relaxation of the corresponding bulk solvent (but shifted to slower dynamics due to surface interactions). For water solutions, however, this process corresponds to earlier results obtained for water confined in various systems and it is most likely due to a local beta-relaxation. The slowing down of the glycerol and methanol relaxations and the good agreement with earlier results on confined water show that this process is affected by the interaction with the protein surface. The second fastest process is attributed to motions of polar side groups on the protein, with a possible contribution from tightly bound solvent molecules. This process is shifted to slower dynamics with increasing solvent viscosity, and it shows a crossover in its temperature dependence from Arrhenius behavior at low temperatures to non-Arrhenius behavior at higher temperatures where there seems to be an onset of cooperativity effects. The origins of the two slowest relaxation processes (visible at high temperatures and low frequencies), which show saddlelike temperature dependences for the solvents water and methanol, are most likely due to motions of the polypeptide backbone and an even more global motion in the protein molecule.

Observation of fragile-to-strong dynamic crossover in protein hydration water

At low temperatures, proteins exist in a glassy state, a state that has no conformational flexibility and shows no biological functions. In a hydrated protein, at temperatures greater-- similar 220 K, this flexibility is restored, and the protein is able to sample more conformational substates, thus becoming biologically functional. This "dynamical" transition of protein is believed to be triggered by its strong coupling with the hydration water, which also shows a similar dynamic transition. Here we demonstrate experimentally that this sudden switch in dynamic behavior of the hydration water on lysozyme occurs precisely at 220 K and can be described as a fragile-to-strong dynamic crossover. At the fragile-to-strong dynamic crossover, the structure of hydration water makes a transition from predominantly high-density (more fluid state) to low-density (less fluid state) forms derived from the existence of the second critical point at an elevated pressure.