首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献,搜索用时 31 毫秒
1.
The possibility of determining the free energy of stabilization ΔG0 of native DNA structure with the help of calorimetric data on heats ΔH of transition from the native to denaturated state is considered. Results of microcalorimetric measurements of heats of denaturation of T2 phage DNA at, different values of pH and ionic strength of solution are given. Values of free energy of stabilization of the DNA native structure ΔG0 under various conditions have been obtained. It is shown that under conditions close to physiological ΔG0 approaches 1200 cal/mole per base pair.  相似文献   

2.
When the assimilation efficiency in batch culture of a variety of heterotrophic bacteria was tested in media containing progressively smaller concentrations of carbon substrate, two groups emerged. Those in the first group assimilated carbon progressively more efficiently as the initial concentration in the culture medium was lowered below 2 mg C/ml; some displaying efficiencies exceeding 80%. Those in the second group assimilated carbon at rates that changed little, if at all, with changes in the initial concentrations of carbon substrate. Members of the second group differed from the first in their metabolic versatility as evidenced by the ability of each to catabolize hydrocarbons. The energy content (heat of combustion) of the cells of the species tested varied little with changes in cultured conditions, and values for all fell close to the average for bacteria (5411 cal/g ash-free dry wt). In continuous aerobic culture on glucose-minimal medium, the carbon assimilation and energy assimilation efficiencies were simultaneously maximal for Enterobacter aerogenes when the input carbon concentration was 0.2004 mg/ml and the dilution rate was 0.57. An equation for predicting heat of combustion of cells from knowledge of their C,H,N, and O ratios was devised and tested. Precision ranged from +6% to ?16%.  相似文献   

3.
S Makino  H Noguchi 《Biopolymers》1971,10(7):1253-1260
The measurements were made for the volume and the sound velocity changes (ΔV and ΔU) on titrating the sodium salt of poly (S-carboxymethyl L -cysteine) with dilute HCl. For the reaction, ? COO? + H+ → ? COOH, ΔV per mole of H+ bound was + 12. 7 ml and +11. 4 ml in salt-free and 0. 2 M NaCl solutions, respectively. Corresponding ΔU was about ?13 cm/sec in salt-free polymer solution where 11.5 mM carboxylate ion reacts with equimolar hydrogen ion. ΔV associated with the coil-to-β transition was found to be +2. 35 ml in H2O and +1. 90 ml in 0. 2 M NaCl per mole of amino acid residue, respectively. These values are larger than those obtained for the coil-to-helix transition of poly (L -glutamic acid). ΔU for the transition was about ?30 cm/sec in salt-free solution of polymer concentration 0.0115 mole/liter. Possible sources of ΔV and ΔU for reaction; coil → β, are (1) the formation of void volume and (2) the changes in the extent of solvation in amide linkage and in side chain.  相似文献   

4.
Melting parameters of 2U:1A complexes formed by polyuridylic acid [poly(U)] and three adenine dinucleotides, diribonucleoside monophosphonate ApA and diastereoisomers of dideoxyribonucleoside methyl phosphonate [(dApA)1 and (dApA)2], in 1M NaCl and at a number of dinucleotide concentrations were obtained from differential scanning microcalorimetric data and interpreted in terms of the theory of helix–coil equilibrium in oligonucleotide–polynucleotide systems. The apparent binding constant, 1/cm, at 39°C and melting temperatures, Tm, at 1 × 10?3 M dinucleotide concentration indicate the following order of thermodynamic stability of the complexes: 2 poly(U) · (dApA)2 (2.27 × 103M?1, 44.2°C) > 2 poly(U) · (dApA)1 (9.9 × 102M1, 39.2°C) > 2 poly(U) · (ApA) (5.9 × 102M?1, 35.8°C). Corresponding calorimetric enthalpies of melting, ΔHm: 13.5, 12.7, and 12.8 kcal/mol (UUA base triplets) were found to be considerably lower than the van't Hoff enthalpies, ΔHapp: 29.4, 16.2, and 16.2 kcal/mol, respectively, evaluated from the dependence of the melting temperatures on dinucleotide concentration. Self-association of dinucleotides and their simultaneous binding as monomers, dimers, and higher-order associated species is suggested as the most probable cause of the differences between ΔHm and ΔHapp values. The differences in thermodynamic properties of the complexes formed by (dApA)1 and (dApA)2 diastereoisomers are discussed in connection with their known conformational properties. The higher and essentially enthalpic stability of the 2 poly(U) · (dApA)2 complex correlates with a lower degree of intramolecular stacking of the (dApA)2 isomer. The hydrophobically enhanced strong self-association of the latter greatly influences the thermodynamics of its complex formation with poly(U) and results in ΔHappHm = 2.3.  相似文献   

5.
Saccharomyces cerevisiae CBS 426 was grown aerobically and anaerobically in a glucose-limited chemostat. The flows of biomass, glucose, ethanol, carbon dioxide, oxygen, glycerol, and the elemental composition of the biomass were measured. Models for anaerobic and aerobic growth are constructed. Values for YATP and P/O are obtained from continuous culture data for aerobic growth; this YATP value is compared with that obtained from the anaerobic growth results. The ratio between the heat produced and the oxygen consumed increases if more glucose in fermented to ethanol and carbon dioxide. An equation for ?H/?O as a function of the respiratory quotient is given.  相似文献   

6.
 The second-order rate constants for the oxidation of a series of phenol derivatives by horseradish peroxidase compound II were compared to computer-calculated chemical parameters characteristic for this reaction step. The phenol derivatives studied were phenol, 4-chlorophenol, 3-hydroxyphenol, 3-methylphenol, 4-methylphenol, 4-hydroxybenzoate, 4-methoxyphenol and 4-hydroxybenzaldehyde. Assuming a reaction of the phenolic substrates in their non-dissociated, uncharged forms, clear correlations (r = 0.977 and r = 0.905) were obtained between the natural logarithm of the second-order rate constants (ln k app and ln k 2 respectively) for their oxidation by compound II and their calculated ionisation potential, i.e. minus the energy of their highest occupied molecular orbital [E(HOMO)]. In addition to this first approach in which the quantitative structure-activity relationship (QSAR) was based on a calculated frontier orbital parameter of the substrate, in a second and third approach the relative heat of formation (ΔΔHF) calculated for the process of one-electron abstraction and H abstraction from the phenol derivatives was used as a parameter. Plots of the natural logarithms of the second-order rate constants (k app and k 2) for the reaction and the calculated ΔΔHF values for the process of one-electron abstraction also provide clear QSARs with correlation coefficients of –0.968 and –0.926 respectively. Plots of the natural logarithms of the second-order rate constants (k app and k 2) for the reaction and the calculated ΔΔHF values for the process of H abstraction provide QSARs with correlation coefficients of –0.989 and –0.922 respectively. Since both mechanisms considered, i.e. initial electron abstraction versus initial H abstraction, provided clear QSARs, the results could not be used to discriminate between these two possible mechanisms for phenol oxidation by horseradish peroxidase compound II. The computer calculation-based QSARs thus obtained for the oxidation of the various phenol derivatives by compound II from horseradish peroxidase indicate the validity of the approaches investigated, i.e. both the frontier orbital approach and the approach in which the process is described by calculated relative heats of formation. The results also indicate that outcomes from computer calculations on relatively unrelated phenol derivatives can be reliably compared to one another. Furthermore, as the actual oxidation of peroxidase substrates by compound II is known to be the rate-limiting step in the overall catalysis by horseradish peroxidase, the QSARs of the present study may have implications for the differences in the overall rate of substrate oxidation of the phenol derivatives by horseradish peroxidase. Received: 29 March 1996 / Accepted: 17 July 1996  相似文献   

7.
To evaluate the biological preference of [Yb(phen)2(OH2)Cl3](H2O)2 (phen is 1,10-phenanthroline) for DNA, interaction of Yb(III) complex with DNA in Tris–HCl buffer is studied by various biophysical and spectroscopic techniques which reveal that the complex binds to DNA. The results of fluorescence titration reveal that [Yb(phen)2(OH2)Cl3](H2O)2 has strongly quenched in the presence of DNA. The binding site number n, apparent binding constant K b, and the Stern–Volmer quenching constant K SV are determined. ΔH 0, ΔS 0, and ΔG 0 are obtained based on the quenching constants and thermodynamic theory (ΔH 0?>?0, ΔS 0?>?0, and ΔG 0?<?0). The experimental results show that the Yb(III) complex binds to DNA by non-intercalative mode. Groove binding is the preferred mode of interaction for [Yb(phen)2(OH2)Cl3](H2O)2 to DNA. The DNA cleavage results show that in the absence of any reducing agent, Yb(III) complex can cleave DNA. The antimicrobial screening tests are also recorded and give good results in the presence of Yb(III) complex.  相似文献   

8.
All life requires energy to drive metabolic reactions such as growth and cell maintenance; therefore, fluctuations in energy availability can alter microbial activity. There is a gap in our knowledge concerning how energy availability affects the growth of extreme chemolithoautotrophs. Toward this end, we investigated the growth of thermoacidophile Acidianus ambivalens during sulfur oxidation under aerobic to microaerophilic conditions. Calorimetry was used to measure enthalpy (ΔHinc) of microbial activity, and chemical changes in growth media were measured to calculate Gibbs energy change (ΔGinc) during incubation. In all experiments, Gibbs energy was primarily dissipated through the release of heat, which suggests enthalpy‐driven growth. In microaerophilic conditions, growth was significantly more efficient in terms of biomass yield (defined as C‐mol biomass per mole sulfur consumed) and resulted in lower ΔGinc and ΔHinc. ΔGinc in oxygen‐limited (OL) and oxygen‐ and CO2‐limited (OCL) microaerophilic growth conditions resulted in averages of ?1.44 × 103 kJ/C‐mol and ?7.56 × 102 kJ/C‐mol, respectively, and average ΔHinc values of ?1.11 × 105 kJ/C‐mol and ?4.43 × 104 kJ/C‐mol, respectively. High‐oxygen experiments resulted in lower biomass yield values, an increase in ΔGinc to ?1.71 × 104 kJ/C‐mol, and more exothermic ΔHinc values of ?4.71 × 105 kJ/C‐mol. The observed inefficiency in high‐oxygen conditions may suggest larger maintenance energy demands due to oxidative stresses and a preference for growth in microaerophilic environments.  相似文献   

9.
A Teramoto  T Norisuye 《Biopolymers》1972,11(8):1693-1700
For helix-coil transitions of polypeptide in binary mixtures consisting of helix-forming solvent and coil solvent, the transition enthalpy ΔH(T,x) has been found to depend significantly on temperature (T) and solvent composition (x). For such systems, calorimetric measurements may yield some averages of ΔH(T,x) which are no longer amenable to direct comparison with ΔH itself. Theoretical equations relating calorimetric data to ΔH(T,x) are derived and tested favorably with experimental data. It is demonstrated that the transition enthaply from heat capacity measurements is approximately equal to ΔHcfm, while those from heat of dilution and heat of solution measurements are equal to ΔHc. Here ΔHc denotes the value of ΔH at the transition point and fm represents the maximum helical content attained in a thermally induced transition. The discrepancies among calorimetric data are also discussed.  相似文献   

10.
Y C Fu  H V Wart  H A Scheraga 《Biopolymers》1976,15(9):1795-1813
The enthalpy change associated with the isothermal pH-induced uncharged coil-to-helix transition ΔHh° in poly(L -ornithine) in 0.1 N KCl has been determnined calorimetrically to be ?1530 ± 210 and ?1270 ± 530 cal/mol at 10° and 25°C, respectively. Titration data provided information about the state of charge of the polymer in the calorimetric experiments, and optical rotatory dispersion data about its conformation. In order to compute ΔHh°, the observed calorimetric heat was corrected for the heat of breaking the sample cell, the heat of dilution of HCl, the heat of neutralization of the OH? ion, and the heat of ionization of the δ-amino group in the random coil. The latter was obtained from similar calorimetric measurements on poly(D ,L -ornithine). Since it was discovered that poly(L -ornithine) undergoes chain cleavage at high pH, the calorimetric measurements were carried out under conditions where no degradation occurred. From the thermally induced uncharged helix–coil transition curve for poly(L -ornithine) at pH 11.68 in 0.1 N KCl in the 0°–40°C region, the transition temperature Ttr and the quantity (?θh/?T)Ttr have been obtained. From these values, together with the measured values of ΔHh°, the changes in the standard free energy ΔGh° and entropy ΔGh°, associated with the uncharged coil-to-helix transition at 10°C have been calculated to be ?33 cal/mol and ?5.3 cal/mol deg, respectively. The value of the Zimm–Bragg helix–coil stability constant σ has been calculated to be 1.4 × 10?2 and the value of s calculated to be 1.06 at 10°C, and between 0.60 and 0.92 at 25°C.  相似文献   

11.
M Heinecke  D Bode  U Schernau 《Biopolymers》1974,13(1):227-235
The heats of the conformational conversion ΔHc of polyuridylic acid (polyU) in the presence of various cations were determined using a differential scanning microcalorimeter and an isothermal mixing calorimeter. The first method yields a value of ΔHc = 5.2 kcal/mol of base pairs, the second one a value of ΔHc = 6.4 kcal/mol of base pairs. Knowing ΔHc and the maximal slope of the degree of conversion, which was determined from the temperature dependence of the absorbance of polyU solutions at 260 nm, we were able to evaluate the parameter of cooperativity σ. From the relation σ = e the stacking free energy ΔFstack could be calculated. Dividing the apparent enthalpy of conversion by the calorimetrically measured enthalpy yields the cooperative length N0 of one helical segment at the midpoint of conversion. The results show that the type of the cation has no influence on the magnitude of ΔHc, whereas the cooperativity of polyU is influenced by the various cations.  相似文献   

12.
A sedimentation analysis has been used to determine the proportion of protein present as monomer and aggregate in 0.5 and 1.0 g/dl solutions of β-casein A in pH 7 phosphate buffer over the temperature range 10–40°C. The amount and molecular weight of the aggregate increase with temperature; under the conditions used, the aggregation number (n) of β-casein is given approximately by n = 0.6t + 2 with t in degrees centigrade. The concentration of β-casein in monomeric and aggregated states at different temperatures is used to calculate the standard enthalpy of aggregation ΔH° (Van't Hoff) by assuming that β-casein undergoes a cooperative, two-state, micellization process; aggregation is an endothermic process and ΔH° = 66.0 ± 2.6 kJ mol?1. Combination of this ΔH° with the amount of protein calculated to dissociate when 1 g/dl solutions are diluted isothermally to 0.5 g/dl gives the heat of dilution at various temperatures. These calculated heats of dilution are compared with the experimental values obtained by carrying out the same dilutions in a microcalorimeter. The heat of dilution decreases linearly with β-casein concentration, but the extrapolated zero-concentration values of 65.8 ± 1.6 kJ mol?1 is the same as the Van't Hoff enthalpy. This agreement in the enthalpy values indicates that the micellization of β-casein occurs cooperatively. The effect of modifying the hydrophobic/hydrophilic balance of the system on the micellization of β-casein A has been investigated. The hydrophobic interaction between the protein molecules is decreased by removing the three C-terminal residues (Ileu Ileu Val) with carboxypeptidase-A. This modification drastically reduces the ability of the β-casein molecule to form micelles. Substitution of 2H2O for H2O at constant temperature perturbs the monomer–micelle equilibrium in favor of micelles because of enhanced hydrophobic interactions in the former solvent. The results are consistent with β-casein micellization involving a delicate balance of the hydrophobic forces favoring aggregation and electrostatic forces opposing it.  相似文献   

13.
Summary The effect on metabolism of hyperbaric dissolved oxygen tension in batch cultures of Candida tropicalis (Cast.) Berkhout and Rhodococcus erythropolis with three different carbon sources was studied in a 20-l bioreactor under controlled conditions. The respiratory quotient was not significantly influenced by dissolved oxygen concentrations up to 40 mg/l oxygen. Elementary cell composition and proportional contents of DNA, RNA, and protein were not markedly influenced by the various oxygen concentrations but depended mainly on the growth rate. It was found that the production of trehalose lipid by R. erythropolis was dependent on the growth rate which could be enhanced by raising the oxygen concentration. The specific activity of catalase was affected more by the nature of the carbon source than by increased oxygen concentration. On the basis of the experimental data the application of oxygen-enriched air for biotechnological processes is discussed.Symbols and abbreviations kLa Specific volumetric oxygen transfer rate - Oxygen consumption rate, grams oxygen per hour and per liter - Carbon dioxide production rate, grams carbon dioxide per hour and per liter - RQ Respiratory quotient, - t Cultivation time - YX/S Yield coefficient, grams cell dry weight/grams substrate - Yield coefficient, grams cell dry weight/grams oxygen consumed - YkJ Yield coefficient, grams cell dry weight/heat of combustion of the consumed substrate - Yave– Yield coefficient, grams cell dry weight/mol available electrons of the consumed substrate  相似文献   

14.
We characterized bacteria from contaminated aquifers for their ability to utilize aromatic hydrocarbons under hypoxic (oxygen-limiting) conditions (initial dissolved oxygen concentration about 2 mg/l) with nitrate as an alternate electron acceptor. This is relevant to current intense efforts to establish favorable conditions forin situ bioremediation. Using samples of granular activated carbon slurries from an operating groundwater treatment system, we isolated bacteria that are able to use benzene, toluene, ethylbenzene, orp-xylene as their sole source of carbon under aerobic or hypoxic-denitrifying conditions. Direct isolation on solid medium incubated aerobically or hypoxically with the substrate supplied as vapor yielded 103 to 105 bacteria ml–1 of slurry supernatant, with numbers varying little with respect to isolation substrate or conditions. More than sixty bacterial isolates that varied in colony morphology were purified and characterized according to substrate utilization profiles and growth condition (i.e., aerobic vs. hypoxic) specificity. Strains with distinct characteristics were obtained using benzene compared with those isolated on toluene or ethylbenzene. In general, isolates obtained from direct selection on benzene minimal medium grew well under aerobic conditions but poorly under hypoxic conditions, whereas many ethylbenzene isolates grew well under both incubation conditions. We conclude that the conditions of isolation, rather than the substrate used, will influence the apparent characteristic substrate utilization range of the isolates obtained. Also, using an enrichment culture technique, we isolated a strain ofPseudomonas fluorescens, designated CFS215, which exhibited nitrate dependent degradation of aromatic hydrocarbons under hypoxic conditions.Abbreviations BTEX benzene, toluene, ethylbenzene, andp-xylene - HPLC high performance liquid chromatography - GAC granular activated carbon  相似文献   

15.
J Alvarez  R Biltonen 《Biopolymers》1973,12(8):1815-1828
The heats of solution of thymine in water and ethanol have been determined calorimetrically as a function of temperature. These data, along with solubility data, have been used to calculate the thermodynamic quantities (ΔGt, ΔHt, ΔSt and ΔCp,t) associated with the transfer of thymine from ethanol to water. Since ΔSt = ?2 cal/mole deg and ΔCp,t = 0, it has been concluded that hydrophobic bonding does not play an important role in the thermocynamic stability of nucleic acids. However, large heat capacities of solution of thymine are observed in both solvents (ΔC°p2 = 45 ± 4 cal/mole deg). This is explained in terms of temperature variation in the degree of solvent–solute hydrogen bonding. It is our proposal that the components of macromolecules (i.e., nucleic acid bases and amino acids) do not make all possible hydrogen bonds with the solvent in the vicinity of room temperature. Thus the thermodynamic contribution of hydrogen bonding to the stability of macromolecules in aqueous solution must be reassessed.  相似文献   

16.
An Exact theoretical expression for the apparent diffusion coefficient Dapp(K) of a thin rigid rod with arbitrary anisotropy of its translational diffusion diffusion coefficient is derived from the first cumulant of its dynamic structure factor. Dapp(K) is predicted to reach a limiting plateau value at extermely large values of KL, where K is the scattering vector and L the rod length. Howerver, that limiting plateau value is approached only very slowly along a quasi-plateau with a very gradual slope. Dynamic light-scattering studies have been performed on tobacco mosaic virus from K2 = (0.4–20) × 1010 cm?2 using 632-8-nm laser radiation. The present data yield D0 = (4.19 ± 0.10) × 10?8 cm2/s (corrected to 20,w conditions) and, with literature data to establish L = 2980 Å and the rotational diffusion coefficient DR = 318s?1, yield also Δ ≡ D ? D = (1.79 ± 0.38) × 10?8 cm2/s. The experimental data closely follow the curve of Dapp(K) vs K2 calcuated for these parameters. The present value of D0 substantially exceeds all previous dynamic light-scattering values, but is in good aggreement with previous sedimentation data, which were confirmed for the presemt sample. The anisotropy ratio Δ/D0 = 0.43 ± 0.09 is in accord with theoretical predictions based on the modified Kirkwood algorithm, despite the fact the D0 lies significantly below its corresponding theoretical value. The present data largely predlude the possibility that both D0 and Δ/D0 could simultaneously match their theoretical predictions. We present a detailed comparison of the experimental data with the calculations of Tirado and Garcia de la Torre based on the modified Kirkwood algorithm and with the Broersma formulas.  相似文献   

17.
Abstract.Oxygen uptake and carbon dioxide release at 28°C were determined in worker castes of twenty-six species of forest termites from the Danum Valley Conservation Area, south-east Sabah, by Warburg manometry. Metabolic rate varied inversely with body weight in a suite of soil-, wood/soil- and wood-feeding species, giving a slope (in a log–log plot) of – 0.63. However, a number of large species, actively foraging forms such as Macrotermes malaccensis, M. gilvus, Havilanditermes atripennis and Hospitalitermes hospitalis, but also the wood-feeding Schedorhinotermes sarawakensis, showed an oxygen consumption greater than expected for their body weight. Rates of methane emission were above 0.100 μmol g–1 h–1 in seventeen species, with very high fluxes in two wood/soil-feeders, Termes borneensis (0.546 ± 0.163 μmol g–1 h–1) and Prohamitermes mirabilis (0.303 ± 0.123 μmol g–1 h–1). Of the fifteen remaining species, seven were soil-feeders, five were wood-feeders, two were wood/litter-feeders and a single species fed on lichen and moss. Low or negligible CH4 emissions (< 0.100 μmol g–1 h–1) were observed in three other species, all wood-feeders. An apparent respiratory quotient (RQapp) was calculated using xCO2 and xO2 (corrected for methane emission, but not hydrogen). Mean RQapp was at or above 1.00 in eleven species and between 0.95 and 1.00 in a further six species, the two sets of species together representing all trophic groups, including lichen-feeders. This is argued to be consistent with carbohydrate being the principal substrate supporting respiration.  相似文献   

18.
The biotechnical platform strain Ralstonia eutropha H16 was genetically engineered to express a cox subcluster of the carboxydotrophic Oligotropha carboxidovoransOM5, including (i) the structural genes coxM, ‐S and ‐L, coding for an aerobic carbon monoxide dehydrogenase (CODH) and (ii) the genes coxD, ‐E, ‐F and ‐G, essential for the maturation of CODH. The coxOc genes expressed under control of the CO2‐inducible promoter PL enabled R. eutropha to oxidize CO to CO2 for the use as carbon source, as demonstrated by 13CO experiments, but the recombinant strains remained dependent on H2 as external energy supply. Therefore, a synthetic metabolism, which could be described as ‘carboxyhydrogenotrophic’, was established in R. eutropha. With this extension of the bacterium's substrate range, growth in CO‐, H2‐ and CO2‐containing artificial synthesis gas atmosphere was enhanced, and poly(3‐hydroxybutyrate) synthesis was increased by more than 20%.  相似文献   

19.
Many macromolecular interactions, including protein‐nucleic acid interactions, are accompanied by a substantial negative heat capacity change, the molecular origins of which have generated substantial interest. We have shown previously that temperature‐dependent unstacking of the bases within oligo(dA) upon binding to the Escherichia coli SSB tetramer dominates the binding enthalpy, ΔHobs, and accounts for as much as a half of the observed heat capacity change, ΔCp. However, there is still a substantial ΔCp associated with SSB binding to ssDNA, such as oligo(dT), that does not undergo substantial base stacking. In an attempt to determine the origins of this heat capacity change, we have examined by isothermal titration calorimetry (ITC) the equilibrium binding of dT(pT)34 to SSB over a broad pH range (pH 5.0–10.0) at 0.02 M, 0.2 M NaCl and 1 M NaCl (25°C), and as a function of temperature at pH 8.1. A net protonation of the SSB protein occurs upon dT(pT)34 binding over this entire pH range, with contributions from at least three sets of protonation sites (pKa1 = 5.9–6.6, pKa2 = 8.2–8.4, and pKa3 = 10.2–10.3) and these protonation equilibria contribute substantially to the observed ΔH and ΔCp for the SSB‐dT(pT)34 interaction. The contribution of this coupled protonation (∼ −260 to −320 cal mol−1 K−1) accounts for as much as half of the total ΔCp. The values of the “intrinsic” ΔCp,0 range from −210 ± 33 cal mol−1 °K−1 to −237 ± 36 cal mol−1K−1, independent of [NaCl]. These results indicate that the coupling of a temperature‐dependent protonation equilibria to a macromolecular interaction can result in a large negative ΔCp, and this finding needs to be considered in interpretations of the molecular origins of heat capacity changes associated with ligand‐macromolecular interactions, as well as protein folding. Proteins 2000;Suppl 4:8–22. © 2000 Wiley‐Liss, Inc.  相似文献   

20.
The objective have been to establish if those ions which are known to change the stability of the structure of proteins, have any influence on the properties of ionizable polypeptides. Potentiometric titrations and complementary optical rotation data are presented for aqueous solutions of poly-L -lysine (PLL) in the presence of KSCN, KCl, and KF, and for poly(L -glutamic acid) (PLGA) in the presence of KSCN, KCl, and LiCl. The following measured quantities which are affected by salt concentration were obtained: intrinsic pK (pK0), slope of pKapp versus degree of ionization (α) curves, the degree of ionization at which the helix to coil transition occurs, and the free energy of this transition for the uncharged molecule (δG°hel). The effects of nonspecific salts (KCl and LiCl for PLL and KSCN and KCl for PLGA) are small, and about, as expected from general electrostatic considerations. In line with the observations made with isoelectric and cat ionic collagen, specific, effects were noted with KSCN–PLL and with LiCl–PLGA. In the presence of KSCN, the poly-L -lysine helix becomes stabilized at much lower degree of ionization than in the presence of KCl, and the slope of the pKapp versus α plots is greatly reduced. However, ΔG°hel (for the uncharged molecule) is not affected, and pK0 is only slightly higher. We interpret these data in terms of binding of SCN? primarily to the side-chain amino groups (both to R? NH3+ and to R? NH2) solutions. (L -glutamic acid) in LiCl solution has its transition at the same α value as in KCl solution. However, both the slopes of the pKapp versus α plots and the absolute values of ΔG°hel are lower than in KCl solution. We interpret these results in terms of binding of Li+ to side chains as well as to the peptide bond.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号