SEARCH · PubMed Health

Results for “THERMODYNAMICS”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,603 records · Page 89Linked to original sources

PubMed1990

Effect of the thermodynamics of an infant plethysmograph on the measurement of thoracic gas volume.

Adult plethysmographs have frequency responses that are essentially flat over the range of frequencies encountered in the measurement of thoracic gas volume (TGV). An infant plethysmograph is necessarily much smaller than an adult model. This means that there is a smaller mean distance over which heat diffusion must occur between the air in the plethysmograph and its walls. This in turn leads to a much reduced thermal time constant. We examined the effects of thermal time constant of a 60 L infant plethysmograph on measurements of TGV in infants. The thermal time constant was measured by rapidly injecting 20 mL of air into the plethysmograph, and found to be 0.16 +/- 0.09s when the box was empty. We calculated from this time constant that measurements of TGV should be quite dependent on the frequency at which the associated panting maneuvers are performed. TGV was measured in 5 infants less than 6 months old in the recovery phase following acute viral bronchiolitis. When we performed a digital correction of the measurements, to compensate for the thermal time constant of the plethysmograph, the TGV values decreased by a mean of 12%. Agitating the air in the plethysmograph with a fan decreased the thermal time constant of the box and reduced measured TGV by a mean of 8.4%. These results indicate that thermodynamics of infant plethysmographs can be an important source of error in TGV measurements.

Algorithms↗

PubMed1992

Thermodynamics of melittin tetramerization determined by circular dichroism and implications for protein folding.

The tetramerization of melittin, a 26-amino acid peptide from Apis mellifera bee venom, has been studied as a model for protein folding. Melittin converts from a monomeric random coil to an alpha-helical tetramer as the pH is raised from 4.0 to 9.5, as ionic strength is increased, as temperature is raised or lowered from about 37 degrees C, or as phosphate is added. The thermodynamics of this tetramerization (termed "folding") are explored using circular dichroism. The melittin tetramer has two pKa values of 7.5 and 8.5 corresponding to protonation of the N-terminus and Lys 23, respectively. pKa values calculated with the program DelPhi (Gilson, M.K., Sharp, K.A., & Honig, B.H., 1987, J. Comp. Chem. 9, 327-335; Gilson, M.K. & Honig, B.H., 1988a, Proteins 3, 32-52; Gilson, M.K. & Honig, B.H., 1988b, Proteins 4, 7-18) agree with experimental titration data. Greater electrostatic repulsion of these protonated groups destabilizes the tetramer by 3.6 kcal/mol at pH 4.0 compared to pH 9.5. Increasing the concentration of NaCl in the solution from 0 to 0.5 M stabilizes the tetramer by 5-6 kcal/mol at pH 4.0. The effect of NaCl is modeled with a ligand-binding approach. The melittin tetramer is found to have a temperature of maximum stability ranging from 35.5 to 43 degrees C depending on the pH, unfolding above and below that temperature. delta Cp0 for folding ranges from -0.085 to -0.102 cal g-1 K-1, comparable to that of other small globular proteins (Privalov, P.L., 1979, Adv. Protein Chem. 33, 167-241). delta H0 and delta S0 are found to decrease with temperature, presumably due to the hydrophobic effect (Kauzmann, W., 1959, Adv. Protein Chem. 14, 1-63). Phosphate is found to perturb the equilibrium substantially with a maximal effect at 150 mM, stabilizing the tetramer at pH 7.4 and 25 degrees C by 4.6 kcal/mol. The enthalpy change due to addition of phosphate (-7.5 kcal/mol at 25 degrees C) can be accounted for by simple dielectric screening. Both circular dichroism and crystallographic results suggest that phosphate may bind Lys 23 at the ends of the elongated tetramer. These detailed measurements give insight into the relative importance of various forces for the stability of melittin in the folded form and may provide an experimental standard for future tests of computational energetics on this simple protein system.

Amino Acid Sequence↗

PubMed1993

Thermodynamics of unfolding for turkey ovomucoid third domain: thermal and chemical denaturation.

We have used thermal and chemical denaturation to characterize the thermodynamics of unfolding for turkey ovomucoid third domain (OMTKY3). Thermal denaturation was monitored spectroscopically at a number of wave-lengths and data were subjected to van't Hoff analysis; at pH 2.0, the midpoint of denaturation (Tm) occurs at 58.6 +/- 0.4 degrees C and the enthalpy of unfolding at this temperature (delta Hm) is 40.8 +/- 0.3 kcal/mol. When Tm was perturbed by varying pH and denaturant concentration, the resulting plots of delta Hm versus Tm yield a mean value of 590 +/- 120 cal/(mol.K) for the change in heat capacity upon unfolding (delta Cp). A global fit of the same data to an equation that includes the temperature dependence for the enthalpy of unfolding yielded a value of 640 +/- 110 cal/(mol.K). We also performed a variation of the linear extrapolation method described by Pace and Laurents, which is an independent method for determining delta Cp (Pace, C.N. & Laurents, D., 1989, Biochemistry 28, 2520-2525). First, OMTKY3 was thermally denatured in the presence of a variety of denaturant concentrations. Linear extrapolations were then made from isothermal slices through the transition region of the denaturation curves. When extrapolated free energies of unfolding (delta Gu) were plotted versus temperature, the resulting curve appeared linear; therefore, delta Cp could not be determined. However, the data for delta Gu versus denaturant concentration are linear over an extraordinarily wide range of concentrations. Moreover, extrapolated values of delta Gu in urea are identical to values measured directly.

Animals↗

PubMed1994

Thermodynamic characterization of an artificially designed amphiphilic alpha-helical peptide containing periodic prolines: observations of high thermal stability and cold denaturation.

To investigate the structural stability of proteins, we analyzed the thermodynamics of an artificially designed 30-residue peptide. The designed peptide, NH2-EELLPLAEALAPLLEALLPLAEALAPLLKK-COOH (PERI COIL-1), with prolines at i + 7 positions, forms a pentameric alpha-helical structure in aqueous solution. The thermal denaturation curves of the CD at 222 nm (pH 7.5) show an unusual cold denaturation occurring well above 0 degrees C and no thermal denaturation is observable under 90 degrees C. This conformational change is reversible and depends on peptide concentration. A 2-state model between the monomeric denatured state (5D) and the pentameric helical state (H5) was sufficient to analyze 5 thermal denaturation curves of PERI COIL-1 with concentrations between 23 and 286 microM. The analysis was carried out by a nonlinear least-squares method using 3 fitting parameters: the midpoint temperature, Tm, the enthalpy change, delta H(Tm), and the heat capacity change, delta Cp. The association number (n = 5) was determined by sedimentation equilibrium and was not used as a fitting parameter. The heat capacity change suggests that the hydrophobic residues are buried in the helical state and exposed in the denatured one, as it occurs normally for natural globular proteins. On the other hand, the enthalpy and the entropy changes have values close to those found for coiled-coils and are quite distinct from typical values reported for natural globular proteins. In particular, the enthalpy change extrapolated at 110 degrees C is about 3 kJ/mol per amino acid residue, i.e., half of the value found for globular proteins.(ABSTRACT TRUNCATED AT 250 WORDS)

Amino Acid Sequence↗

PubMed1994

Thermodynamics of staphylococcal nuclease denaturation. I. The acid-denatured state.

Using high-sensitivity differential scanning calorimetry, we reexamined the thermodynamics of denaturation of staphylococcal nuclease. The denaturational changes in enthalpy and heat capacity were found to be functions of both temperature and pH. The denatured state of staphylococcal nuclease at pH 8.0 and high temperature has a heat capacity consistent with a fully unfolded protein completely exposed to solvent. At lower pH values, however, the heat capacity of the denatured state is lower, resulting in a lower delta Cp and delta H for the denaturation reaction. The acid-denatured protein can thus be distinguished from a completely unfolded protein by a defined difference in enthalpy and heat capacity. Comparison of circular dichroism spectra suggests that the low heat capacity of the acid-denatured protein does not result from residual helical secondary structure. The enthalpy and heat capacity changes of denaturation of a less stable mutant nuclease support the observed dependence of delta H on pH.

Calorimetry, Differential Scanning↗

PubMed1994

Thermodynamics of staphylococcal nuclease denaturation. II. The A-state.

Staphylococcal nuclease, at low pH and in the presence of high salt concentrations, has previously been proposed to exist in a partially folded or molten globule form called the "A-state" (Fink et al., 1993, Protein Sci 2:1155-1160). We have found that the A-state of nuclease at pH 2.1 in the presence of moderate to high salt concentrations and at low temperature exists in a substantially folded form structurally more similar to a native state. The A-state has the far-UV circular dichroism spectra characteristic of the native protein, which indicates that it has a large degree of secondary structure. Upon heating, the A-state denatures with a sigmoidal change in far-UV ellipticity and an observable peak in a differential scanning calorimeter trace, indicating that it is thermodynamically distinct from the denatured state. Three different mutations in a residue normally buried in the protein's core stabilize or destabilize the A-state in the same way as they affect the denaturation of the native state. The A-state must, therefore, contain at least some tertiary packing of side chains. Unlike the native state, which shows cold denaturation at low temperatures, the A-state is most stable at temperatures below 0 degrees C.

Calorimetry, Differential Scanning↗

PubMed1995

Investigation of a side-chain-side-chain hydrogen bond by mutagenesis, thermodynamics, and NMR spectroscopy.

Anomalous NMR behavior of the hydroxyl proton resonance for Ser 31 has been reported for histidine-containing protein (HPr) from two microorganisms: Escherichia coli and Staphylococcus aureus. The unusual slow exchange and chemical shift exhibited by the resonance led to the proposal that the hydroxyl group is involved in a strong hydrogen bond. To test this hypothesis and to characterize the importance of such an interaction, a mutant in which Ser 31 is replaced by an alanine was generated in HPr from Escherichia coli. The activity, stability, and structure of the mutant HPr were assessed using a reconstituted assay system, analysis of solvent denaturation curves, and NMR, respectively. Substitution of Ser 31 yields a fully functional protein that is only slightly less stable (delta delta G(folding) = 0.46 +/- 0.15 kcal mol-1) than the wild type. The NMR results confirm the identity of the hydrogen bond acceptor as Asp 69 and reveal that it exists as the gauche- conformer in wild-type HPr in solution but exhibits conformational averaging in the mutant protein. The side chain of Asp 69 interacts with two main-chain amide proteins in addition to its interaction with the side chain of Ser 31 in the wild-type protein. These results indicate that removal of the serine has led to the loss of all three hydrogen bond interactions involving Asp 69, suggesting a cooperative network of interactions. A complete analysis of the thermodynamics was performed in which differences in side-chain hydrophobicity and conformational entropy between the two proteins are accounted for.(ABSTRACT TRUNCATED AT 250 WORDS)

Bacterial Proteins↗

PubMed1995

Thermodynamics of the temperature-induced unfolding of globular proteins.

The heat capacity, enthalpy, entropy, and Gibbs energy changes for the temperature-induced unfolding of 11 globular proteins of known three-dimensional structure have been obtained by microcalorimetric measurements. Their experimental values are compared to those we calculate from the change in solvent-accessible surface area between the native proteins and the extended polypeptide chain. We use proportionality coefficients for the transfer (hydration) of aliphatic, aromatic, and polar groups from gas phase to aqueous solution, we estimate vibrational effects, and we discuss the temperature dependence of each constituent of the thermodynamic functions. At 25 degrees C, stabilization of the native state of a globular protein is largely due to two favorable terms: the entropy of non-polar group hydration and the enthalpy of interactions within the protein. They compensate the unfavorable entropy change associated with these interactions (conformational entropy) and with vibrational effects. Due to the large heat capacity of nonpolar group hydration, its stabilizing contribution decreases quickly at higher temperatures, and the two unfavorable entropy terms take over, leading to temperature-induced unfolding.

Calorimetry↗

PubMed1995

Volume changes of the molten globule transitions of horse heart ferricytochrome c: a thermodynamic cycle.

Volume changes among the unfolded (U), native (N), and molten globule (MG) conformations of horse heart ferricytochrome c have been measured. U to N (pH 2 to pH 7) was determined in the absence of added salt to be -136 +/- 5 mL/mol protein. U to MG (pH 2, no added salt to pH 2, 0.5 M KCl) yielded + 100 +/- 6 mL/mol. MG to N was broken into two steps, N to NClx at pH 7 by addition of buffered KCl to buffered protein lacking added salt (NClx = N interacting with an unknown number, X, of chloride ions), and MG to NClx by jumping MG at pH 2 in 0.5 M KCl to pH7 at the same salt concentration. The delta V of N to NClx was -30.9 +/- 1.4 mL/mol protein, whereas MG to NClx entailed a delta V of -235 +/- 6 mL/mol. Within experimental error, the results add up to zero for a complete thermodynamic cycle. We believe this to be the first volumetric cycle to have been measured for the conformational transitions of a protein. The results are discussed in terms of hydration contributions from deprotonation of the protein, other hydration effects, and the formation and/or enlargement of packing defects in the protein's tertiary structure during the steps of folding.

Animals↗

PubMed1997

Thermodynamics of maltose binding protein unfolding.

The maltose binding protein (MBP or MalE) of Escherichia coli is the periplasmic component of the transport system for malto-oligosaccharides. It is used widely as a carrier protein for the production of recombinant fusion proteins. The melting of recombinant MBP was studied by differential scanning and titration calorimetry and fluorescence spectroscopy under different solvent conditions. MBP exhibits a single peak of heat absorption with a delta(Hcal)/delta(HvH) ratio in the range of 1.3-1.5, suggesting that the protein comprises two strongly interacting thermodynamic domains. Binding of maltose resulted in elevation of the Tm by 8-15 degrees C, depending of pH. The presence of ligand at neutral pH, in addition to shifting the melting process to higher temperature, caused it to become more cooperative. The delta(Hcal)/delta(HvH) ratio decreased to unity, indicating that the two domains melt together in a single two-state transition. This ligand-induced merging of the two domains appears to occur only at neutral pH, because at low pH maltose simply stabilized MBP and did not cause a decrease of the delta(Hcal)/delta(HvH) ratio. Binding of maltose to MBP is characterized by very low enthalpy changes, approximately -1 kcal/mol. The melting of MBP is accompanied by an exceptionally large change in heat capacity. 0.16 cal/K-g, which is consistent with the high amount of nonpolar surface--0.72 A2/g--that becomes accessible to solvent in the unfolded state. The high value of delta Cp determines a very steep delta G versus T profile for this protein and predicts that cold denaturation should occur above freezing temperatures. Evidence for this was provided by changes in fluorescence intensity upon cooling the protein. A sigmoidal cooperative transition with a midpoint near 5 degrees C was observed when MBP was cooled at low pH. Analysis of the melting of several fusion proteins containing MBP illustrated the feasibility of assessing the folding integrity of recombinant products prior to separating them from the MBP carrier protein.

ATP-Binding Cassette Transporters↗

PubMed1997

Temperature dependence of the preferential interactions of ribonuclease A in aqueous co-solvent systems: thermodynamic analysis.

The temperature dependence of preferential solvent interactions with ribonuclease A in aqueous solutions of 30% sorbitol, 0.6 M MgCl2, and 0.6 M MgSO4 at low pH (1.5 and 2.0) and high pH (5.5) has been investigated. This protein was stabilized by all three co-solvents, more so at low pH than high pH (expect 0.6 M MgCl2 at pH 5.5). The preferential hydration of protein in all three co-solvents was high at temperatures below 30 degrees C and decreased with a further increase in temperature (for 0.6 M MgCl2 at pH 5.5, this was not significant), indicating a greater thermodynamic instability at low temperature than at high temperature. The preferential hydration of denatured protein (low pH, high temperature) was always greater than that of native protein (high pH, high temperature). In 30% sorbitol, the interaction passed to preferential binding at 45% for native ribonuclease A and at 55 degrees C for the denatured protein. Availability of the temperature dependence of the variation with sorbitol concentration of the chemical potential of the protein, (delta mu(2)/delta m3)T,p,m2, permitted calculation of the corresponding enthalpy and entropy parameters. Combination with available data on sorbitol concentration dependence of this interaction parameter gave (approximate) values of the transfer enthalpy, delta H2,tr, and transfer entropy delta S2,tr. Transfer of ribonuclease A from water into 30% sorbitol is characterized by positive values of the transfer free energy, transfer enthalpy, transfer entropy, and transfer heat capacity. On denaturation, the transfer enthalpy becomes more positive. This increment, however, is small relative to both the enthalpy of unfolding in water and to the transfer enthalpy of the native protein from water a 30% sorbitol solution.

Hydrogen-Ion Concentration↗

PubMed1997

Thermodynamic analysis of the effect of selective monodeamidation at asparagine 67 in ribonuclease A.

Selective deamidation of proteins and peptides is a reaction of great interest, both because it has a physiological role and because it can cause alteration in the biological activity, local folding, and overall stability of the protein. In order to evaluate the thermodynamic effects of this reaction in proteins, we investigated the temperature-induced denaturation of ribonuclease A derivatives in which asparagine 67 was selectively replaced by an aspartyl residue or an isoaspartyl residue, as a consequence of an in vitro deamidation reaction. Differential scanning calorimetry measurements were performed in the pH range 3.0-6.0, where the unfolding process is reversible, according to the reheating criterion used. It resulted that the monodeamidated forms have a different thermal stability with respect to the parent enzyme. In particular, the replacement of asparagine 67 with an isoaspartyl residue leads to a decrease of 6.3 degrees C of denaturation temperature and 65 kJ mol-1 of denaturation enthalpy at pH 5.0. These results are discussed and correlated to the X-ray three-dimensional structure of this derivative. The analysis leads to the conclusion that the difference in thermal stability between RNase A and (N67isoD)RNase A is due to enthalpic effects arising from the loss of two important hydrogen bonds in the loop containing residue 67, partially counterbalanced by entropic effects. Finally, the influence of cytidine-2'-monophosphate on the stability of the three ribonucleases at pH 5.0 is studied and explained in terms of its binding on the active site of ribonucleases. The analysis makes it possible to estimate the apparent binding constant and binding enthalpy for the three proteins.

Amides↗

PubMed1998

Refolding rate of stability-enhanced cytochrome c is independent of thermodynamic driving force.

N52I iso-2 cytochrome c is a variant of yeast iso-2 cytochrome c in which asparagine substitutes for isoleucine 52 in an alpha helical segment composed of residues 49-56. The N52I substitution results in a significant increase in both stability and cooperativity of equilibrium unfolding, and acts as a "global suppressor" of destabilizing mutations. The equilibrium m-value for denaturant-induced unfolding of N52I iso-2 increases by 30%, a surprisingly large amount for a single residue substitution. The folding/unfolding kinetics for N52I iso-2 have been measured by stopped-flow mixing and by manual mixing, and are compared to the kinetics of folding/unfolding of wild-type protein, iso-2 cytochrome c. The results show that the observable folding rate and the guanidine hydrochloride dependence of the folding rate are the same for iso-2 and N52I iso-2, despite the greater thermodynamic stability of N52I iso-2. Thus, there is no linear free-energy relationship between mutation-induced changes in stability and observable refolding rates. However, for N52I iso-2 the unfolding rate is slower and the guanidine hydrochloride dependence of the unfolding rate is smaller than for iso-2. The differences in the denaturant dependence of the unfolding rates suggest that the N52I substitution decreases the change in the solvent accessible hydrophobic surface between the native state and the transition state. Two aspects of the results are inconsistent with a two-state folding/unfolding mechanism and imply the presence of folding intermediates: (1) observable refolding rate constants calculated from the two-state mechanism by combining equilibrium data and unfolding rate measurements deviate from the observed refolding rate constants; (2) kinetically unresolved signal changes ("burst phase") are observed for both N52I iso-2 and iso-2 refolding. The "burst phase" amplitude is larger for N52I iso-2 than for iso-2, suggesting that the intermediates formed during the "burst phase" are stabilized by the N52I substitution.

Cytochrome c Group↗

PubMed1998

Global analysis of the thermal and chemical denaturation of the N-terminal domain of the ribosomal protein L9 in H2O and D2O. Determination of the thermodynamic parameters, deltaH(o), deltaS(o), and deltaC(o)p and evaluation of solvent isotope effects.

The stability of the N-terminal domain of the ribosomal protein L9, NTL9, from Bacillus stearothermophilus has been monitored by circular dichroism at various temperatures and chemical denaturant concentrations in H2O and D2O. The basic thermodynamic parameters for the unfolding reaction, deltaH(o), deltaS(o), and deltaC(o)p, were determined by global analysis of temperature and denaturant effects on stability. The data were well fit by a model that assumes stability varies linearly with denaturant concentration and that uses the Gibbs-Helmholtz equation to model changes in stability with temperature. The results obtained from the global analysis are consistent with information obtained from individual thermal and chemical denaturations. NTL9 has a maximum stability of 3.78 +/- 0.25 kcal mol(-1) at 14 degrees C. DeltaH(o)(25 degrees C) for protein unfolding equals 9.9 +/- 0.7 kcal mol(-1) and TdeltaS(o)++(25 degrees C) equals 6.2 +/- 0.6 kcal mol(-1). DeltaC(o)p equals 0.53 +/- 0.06 kcal mol(-1) deg(-1). There is a small increase in stability when D2O is substituted for H2O. Based on the results from global analysis, NTL9 is 1.06 +/- 0.60 kcal mol(-1) more stable in D2O at 25 degrees C and Tm is increased by 5.8 +/- 3.6 degrees C in D2O. Based on the results from individual denaturation experiments, NTL9 is 0.68 +/- 0.68 kcal mol(-1) more stable in D2O at 25 degrees C and Tm is increased by 3.5 +/- 2.1 degrees C in D2O. Within experimental error there are no changes in deltaH(o) (25 degrees C) when D2O is substituted for H2O.

Circular Dichroism↗

PubMed2002

Interaction of Hsp90 with 20S proteasome: thermodynamic and kinetic characterization.

The proteasome and heat shock proteins have been found in the centrosome. The evidence of their copurification reported by several studies suggests that they form stable complex. In addition, Hsp90 is involved in the loading of proteasome-generated antigenic peptides to the class I major histocompatibility complex. In this article, we report a detailed thermodynamic and kinetic characterization of the Hsp90-20S proteasome interaction, using a surface plasmon resonance technique. The modulation exerted by protons in solution has been investigated, and the results have been discussed, taking into account structural motifs characterizing the binding interface between the two macromolecules.

Animals↗

PubMed2001

Contribution of Thr29 to the thermodynamic stability of goat alpha-lactalbumin as determined by experimental and theoretical approaches.

The Thr29 residue in the hydrophobic core of goat alpha-lactalbumin (alpha-LA) was substituted with Val (Thr29Val) and Ile (Thr29Ile) to investigate the contribution of Thr29 to the thermodynamic stability of the protein. We carried out protein stability measurements, X-ray crystallographic analyses, and free energy calculations based on molecular dynamics simulation. The equilibrium unfolding transitions induced by guanidine hydrochloride demonstrated that the Thr29Val and Thr29Ile mutants were, respectively, 1.9 and 3.2 kcal/mol more stable than the wild-type protein (WT). The overall structures of the mutants were almost identical to that of WT, in spite of the disruption of the hydrogen bonding between the side-chain O-H group of Thr29 and the main-chain C=O group of Glu25. To analyze the stabilization mechanism of the mutants, we performed free energy calculations. The calculated free energy differences were in good agreement with the experimental values. The stabilization of the mutants was mainly caused by solvation loss in the denatured state. Furthermore, the O-H group of Thr29 favorably interacts with the C=O group of Glu25 to form hydrogen bonds and, simultaneously, unfavorably interacts electrostatically with the main-chain C=O group of Thr29. The difference in the free energy profile of the unfolding path between WT and the Thr29Ile mutant is discussed in light of our experimental and theoretical results.

Amino Acid Substitution↗

PubMed2004

Folding thermodynamics of three beta-sheet peptides: a model study.

We study the folding thermodynamics of a beta-hairpin and two three-stranded beta-sheet peptides using a simplified sequence-based all-atom model, in which folding is driven mainly by backbone hydrogen bonding and effective hydrophobic attraction. The native populations obtained for these three sequences are in good agreement with experimental data. We also show that the apparent native population depends on which observable is studied; the hydrophobicity energy and the number of native hydrogen bonds give different results. The magnitude of this dependence matches well with the results obtained in two different experiments on the beta-hairpin.

Hydrogen Bonding↗

PubMed2005

Both helical propensity and side-chain hydrophobicity at a partially exposed site in alpha-helix contribute to the thermodynamic stability of ubiquitin.

Improving helical propensity of residues was proposed as one of the approaches to increase protein stability. Here the contribution of the helix propensity and hydrophobicity of residues at partially buried positions of alpha-helix to the stability of a model protein-ubiquitin- is explored. Thermodynamic stabilities of 13 ubiquitin variants with substitutions at a partially buried helical residue were measured by differential scanning calorimetry. It was found that the dynamic range of stabilities for different amino acid residues at this partially buried position is 3 times larger than that expected based on helical propensity alone. Correlation analysis shows that both helical propensity and hydrophobicity are important in defining the relative stabilities of the studied ubiquitin variants. These results provide experimental evidence that partially buried positions are potentially useful sites for engineering proteins with enhanced thermostability.

Escherichia coli Proteins↗