Stereodynamics of tetramezine

The antidepressant drug tetramezine [1,2‐bis‐(3,3‐dimethyldiaziridin‐1‐yl)ethane] consists of two bridged diaziridine moieties with four stereogenic nitrogen centers, which are stereolabile and, therefore, are prone to interconversion. The adjacent substituents at the nitrogen atoms of the diaziridines moieties exist only in an antiperiplanar conformation, which results in a coupled interconversion. Therefore, three stereoisomers exist (meso form and two enantiomeric forms), which epimerize when the diaziridine moieties are regarded as stereogenic units due to the coupled interconversion. Here, we have investigated the epimerization between the meso and enantiomeric forms by dynamic gas chromatography. Temperature‐dependent measurements were performed, and reaction rate constants were determined using the unified equation of chromatography implemented in the software DCXplorer. The activation barriers of the epimerization were found to be ΔG^≠ = 100.7 kJ mol^?1 at 25°C and ΔG^≠ = 104.5 kJ mol^?1 at 37°C, respectively. The activation enthalpy and entropy were determined to be ΔH^≠ = 70.3 ± 0.4 kJ mol^?1 and ΔS^≠ = ?102 ± 2 J mol^?1 K^?1. Chirality, 2011. © 2010 Wiley‐Liss, Inc.     

Stereodynamics of Small 1,2-Dialkyldiaziridines

Diaziridines are very interesting representatives of organic compounds containing stereogenic nitrogen atoms. In particular, 1,2-dialkyldiaziridines show extraordinarily high stereointegrity. The lone electron pairs of the nitrogen atoms are in trans configuration, avoiding a four-electron repulsive interaction. Furthermore, the trans configuration of the substituents at the nitrogen atoms is energetically favored because of reduced steric interactions. Therefore only two stereoisomers (enantiomers) are observed. At elevated temperatures the enantiomers are interconverting because of the limited stereointegrity of the chirotopic nitrogen atoms. The enantiomerization rate constants and the activation parameters of interconversion are of great interest. Here, we investigated the stereodynamics of a set of small 1,2-dialkyldiaziridines bearing short substituents (Me, Et, iPr, tBu), using enantioselective dynamic gas chromatography (DGC). Separation of enantiomers of all compounds, including the highly volatile 1,2-dimethyldiaziridine, was achieved using heptakis(2,3-di-O-ethyl-6-O-tert-butyldimethylsilyl)-β-cyclodextrin in 50% PS086 (w/w) as chiral stationary phase in fused silica capillaries with a length of up to 50 m. Measurements at variable temperatures were performed and reaction rate constants were determined using the unified equation of chromatography implemented in the software DCXplorer. The activation barriers at room temperature for 1-(tert-butyl)-2-ethyldiaziridine, ΔG^╪_298K = 123.8 kJ mol^–1 (ΔH^╪ = 115.5 ± 2.9 kJ mol^–1, ΔS^╪ = –28 ± 1 J mol^–1 K^–1), and 1-ethyl-2-isopropyldiaziridine, ΔG^╪_298K = 124.2 kJ mol^–1 (ΔH^╪ = 113.1 ± 2.4 kJ mol^–1, ΔS^╪ = –37 ± 2 J mol^–1 K^–1), were determined, representing some of the highest values observed for nitrogen inversion in diaziridines. Chirality 00:000–000, 2013. © 2013 Wiley Periodicals, Inc.     

Chiral 1,2‐Dialkenyl Diaziridines: Synthesis,Enantioselective Separation,and Nitrogen Inversion Barriers

trans‐1,2‐Disubstituted diaziridines form stable enantiomers at ambient conditions because of the two stereogenic pyramidal nitrogen atoms. Functionalized trans‐1,2‐disubstituted diaziridines can be utilized as a chiral switching moiety between two enantiomeric states in more complex molecular structures. However, the synthesis of functionalized diaziridines is quite challenging, because of the limited tolerance of reaction conditions that can be applied. Here we present a strategy to make trans‐1,2‐disubstituted diaziridines accessible as versatile building blocks in C‐C‐bond formations, i.e., the Heck reaction, and therefore introducing aryl substituents. The synthesis of trans‐1,2‐dialkenyl diaziridines with terminal alkenyl substituents and their stereodynamic properties are described. Chirality 27:156–162, 2015. © 2014 Wiley Periodicals, Inc.     

Dynamic Behavior of Clobazam on High‐Performance Liquid Chromatography Chiral Stationary Phases

Clobazam, a 1,5‐benzodiazepin‐2,4‐dione, is a chiral molecule because its ground state conformation features a nonplanar seven‐membered ring lacking reflection symmetry elements. The two conformational enantiomers of clobazam interconvert at room temperature by a simple ring‐flipping process. Variable temperature HPLC on the Pirkle type (R)‐N‐(3,5‐dinitronenzoyl)phenylglycine and (R,R)‐Whelk‐O1 chiral stationary phases (CSPs) allowed us to separate for the first time the conformational enantiomers of clobazam and to observe peak coalescence‐decoalescence phenomena due to concomitant separation and interconversion processes occurring on the same time scale. Clobazam showed temperature dependent dynamic high‐performance liquid chromatography (HPLC) profiles with interconversion plateaus on the two CSPs indicative of on‐column enantiomer interconversion. (enantiomerization) in the column temperature range between T_col = 10°C and T_col = 30°C, whereas on‐column interconversion was absent at temperature close to or lower than T_col = 5°C. Computer simulation of exchange‐deformed HPLC profiles using a program based on the stochastic model yielded the apparent rate constants for the on‐column enantiomerization and the corresponding free energy activation barriers. At T_col = 20°C the averaged enantiomerization barriers, ΔG^?, for clobazam were found in the range 21.08–21.53 kcal mol^?1 on the two CSPs. The experimental dynamic chromatograms and the corresponding interconversion barriers reported in this article are consistent with the literature data measured by DNMR at higher temperatures and in different solvents. Chirality 28:17–21, 2016. © 2015 Wiley Periodicals, Inc.     

Determination of enantiomerization barriers of hypericin and pseudohypericin by dynamic high‐performance liquid chromatography on immobilized polysaccharide‐type chiral stationary phases and off‐column racemization experiments

Direct enantiomer separation of hypericin, pseudohypericin, and protohypericin was accomplished by high‐performance liquid chromatography (HPLC) using immobilized polysaccharide‐type chiral stationary phases (CSPs). Enantioselectivities up to 1.30 were obtained in the polar‐organic elution mode whereby for hypericin and pseudohypericin Chiralpak IC [chiral selector being cellulose tris(3,5‐dichlorophenylcarbamate)] and for protohypericin Chiralpak IA (chiral selector being the 3,5‐dimethylphenylcarbamate of amylose) gave favorable results. Enantiomers were distinguished by on‐line electronic circular dichroism detection. Optimized enantioselective chromatographic conditions were the basis for determining stereodynamic parameters of the enantiomer interconversion process of hypericin and pseudohypericin. Rate constants delivered by computational simulation of dynamic HPLC elution profiles (stochastic model, consideration of peak tailing) were used to calculate averaged enantiomerization barriers (ΔG) of 97.6–99.6 kJ/mol for both compounds (investigated temperature range 25–45°C). Complementary variable temperature off‐column (i.e., in solution) racemization experiments delivered ΔG = 97.1–98.0 kJ/mol (27–45°C) for hypericin and ΔG = 98.9–101.4 kJ/mol (25–55°C) for pseudohypericin. An activation enthalpy of ΔH^# = 86.0 kJ/mol and an activation entropy of ΔS^# = ?37.7 J/(K mol) were calculated from hypericin racemization kinetics in solution, whereas for pseudohypericin these figures amounted to 74.1 kJ/mol and ?82.6 J/(K mol), respectively. Although the natural phenanthroperylene quinone pigments hypericin and pseudohypericin as well as their biological precursor protohypericin are chiral and can be separated by enantioselective HPLC low enantiomerization barriers seem to prevent the occurrence of an excess of one enantiomer under typical physiological conditions—at least as long as stereoselective intermolecular interactions with other chiral entities are absent. Chirality 2010. © 2009 Wiley‐Liss, Inc.     

The Stereodynamics of 5,5’‐Disubstituted BIPHEPs

We investigated the stereodynamics of 5,5’‐substituted tropos BIPHEP ligands (2,2’‐bis(diphenylphosphino)‐biphenyls) by enantioselective dynamic high‐performance liquid chromatography (DHPLC) to elucidate the influence of the substitution pattern and electronics of the substituents (methyl, methoxy, and hydroxyl groups). By temperature‐dependent dynamic HPLC measurements the activation parameters ΔG^╪, ΔH^╪, and ΔS^╪ could be determined with high precision, revealing that the activation barrier of these 5,5’‐substituted BIPHEP ligands ranges in a narrow band between 87.8 and 93.0 kJ mol^–1, making them highly attractive as deracemizable dynamic chiral ligands in asymmetric catalysis. Interestingly, the activation parameters are highly influenced by a hydroxyl or methoxy group in the 5,5’‐position of the BIPHEP ligands. Chirality 25:126–132, 2013. © 2012 Wiley Periodicals, Inc.     

Studies on the Redox Characteristics of Ferrioxamine E

Thermodynamic parameters for the reduction of ferrioxamine E as calculated from redox potentials determined at four different temperatures were found to be ΔH ^≠ =7.1±3.4 kJ mol^?1 and ΔS^≠=?146 J mol^?1 K^?1. The negative entropy value is large, because the decrease in the charge at the metal center and an increase in its ionic radius force the structure of the complex to become less rigid and resemble the desferrisiderophore. The hydrophilic groups of the system are now (relatively more) available for solvent interaction. Thus, a large negative entropy change accompanies the reduction of the complex. Kinetics of reduction of ferrioxamine by V^II, Cr^II, Eu^II, and dithionite were measured at different temperatures and by dithionite at different pH values. The Cr^II and Eu^II reactions proceed by an inner‐sphere mechanism and have second‐order rate constants at 25° of 1.37×10⁴ and 1.23×10⁵ M ^?1 s^?1, respectively. For the V^II reduction, the corresponding rate constant was 1.89×10³ M ^?1 s^?1. The activation parameters for the V^II reduction were ΔH^≠ = 8.3 kJ mol^?1; ΔS^≠ = ?154 J mol^?1 K^?1. These values are indicative of an outer‐sphere mechanism for V^II reduction. The reduction by dithionite is half order in dithionite concentration indicating that SO ^. is the sole reducing species. log of reduction rate constants of different trihydroxamates by this reductant were correlated with their respective redox potentials, and the variation was found to be in approximate correspondence with the expectations of Marcus relationship.     

Stabilization of invertase by molecular engineering

Extracellular invertase (EC 3.2.1.26) of Saccharomyces cerevisiae was stabilized against thermal denaturation by intermolecular and intramolecular crosslinking of the surface nucleophilic functional groups with diisocyanate homobifunctional reagents (O?C?N(CH₂)_nN?C?O) of various lengths (n = 4, 6, 8). Crosslinking with 1,4‐diisocyanatobutane (n = 4) proved most effective in enhancing thermostability. Stability was improved dramatically by crosslinking 0.5 mg/mL of protein with 30 μmol/mL of the reagent. Molecular engineering by crosslinking reduced the first‐order thermal denaturation constant at 60°C from 1.567 min^?1 (for the native enzyme) to 0.437 min^?1 (for the stabilized enzyme). Similarly, the best crosslinking treatment increased the activation energy for denaturation from 391 kJ mol^?1 (for the native protein) to 466 kJ mol^?1 (for the stabilized enzyme). Crosslinking was confirmed by sodium dodecyl sulfate polyacrylamide gel electrophoresis. © 2009 American Institute of Chemical Engineers Biotechnol. Prog., 2010     

Kinetics and thermodynamics of catalysis and thermal inactivation of a novel α-amylase (Tp-AmyS) from Thermotoga petrophila

Abstract

This research is focussed on kinetic, thermodynamic and thermal inactivation of a novel thermostable recombinant α-amylase (Tp-AmyS) from Thermotoga petrophila. The amylase gene was cloned in pHIS-parallel1 expression vector and overexpressed in Escherichia coli. The steady-state kinetic parameters (V_max, K_m, k_cat and k_cat/K_m) for the hydrolysis of amylose (1.39?mg/min, 0.57?mg, 148.6?s^?1, 260.7), amylopectin (2.3?mg/min, 1.09?mg, 247.1?s^?1, 226.7), soluble starch (2.67?mg/min, 2.98?mg, 284.2?s^?1, 95.4) and raw starch (2.1?mg/min, 3.6?mg, 224.7?s^?1, 61.9) were determined. The activation energy (E_a), free energy (ΔG), enthalpy (ΔH) and entropy of activation (ΔS) at 98?°C were 42.9?kJ mol^?1, 74?kJ mol^?1, 39.9?kJ mol^?1 and ?92.3 J mol^?1 K^?1, respectively, for soluble starch hydrolysis. While ΔG of substrate binding (ΔG_E-S) and ΔG of transition state binding (ΔG_E-T) were 3.38 and ?14.1?kJ mol^?1, respectively. Whereas, EaD, Gibbs free energy (ΔG*), increase in the enthalpy (ΔH*) and activation entropy (ΔS*) for activation of the unfolding of transition state were 108, 107, 105?kJ mol^?1 and ?4.1 J mol^?1 K^?1. The thermodynamics of irreversible thermal inactivation of Tp-AmyS revealed that at high temperature the process involves the aggregation of the protein.     

Thermodynamics of the slow–fast transition in bacteriophage T2L

We have measured the thermodynamic parameters of the slow-fast tail-fiber reorientation transition on T2L bateriophage. Proportions of the virus in each form were determined from peak-height measurements in sedimention-velocity runs and from average diffusion coefficients obtained by quasielastic laser light scattering. Computer simulation of sedimentation confirmed that there were no undetected intermediates in the transition, which was analyzed as a two-state process. Van't Hoff-type plots of the apparent equilibrium constant and of the pH midpoint of the transition as function of reciprocal temperature led to the following estimates of the thermodynamic parameters for the transition at pH 6.0 and 20°C: ΔH° = ?139 ± 18Kcal mol^?1, ΔS° = ?247 ± 46 cal K^?1 mol^?1, and ΔG° = ?66 ± 22 kcal mol^?1. Per mole of protons taken up in the transition, the analogous quantities were ?15.9 ± 1.7 kcal mol^?1, ?26.3 ± 2.2 cal K^?1 mol^?1, and ?8.22 ± 1.8 kcal mol^?1. The net number of protons taken up was about 8.5 ± 1.5. The large values of the thermodynamic functions are consistent with a highly cooperative reaction and with multiple interactions between the fibres and the remainder of the phage. The negative entropy of the transition is probably due to immobilization of the fibres.     

Production,extraction, and thermodynamics protease partitioning from Aspergillus tamarii Kita UCP1279 using PEG/sodium citrate aqueous two-phase systems

Abstract

The protease from Aspergillus tamarii Kita UCP1279 extraction by aqueous two-phase PEG-Citrate (ATPS) systems, using a factorial design 2⁴, was investigated. Then, the variables studied were polyethylene glycol (PEG) molar mass (M_PEG), concentrations of PEG (C_PEG) and citrate (C_CIT), and pH. The responses analyzed were the partition coefficient (K), activity yield (Y) and purification factor (PF). The thermodynamic parameters of the ATPS partition were estimated as a function of temperature. ATPS was able to pre-purify the protease (PF = 1.6) and obtained 84% activity yield. The thermodynamic parameters ΔG°_m (?10.89?kJ mol^?1), ΔH_m (?5.0?kJ?mol^?1) and partition ΔS_m (19.74?J mol^?1 K^?1) showed that the preferential migration of almost all protein contaminants of the crude extract to the salt-rich phase, while the preferred protease was the PEG rich phase. The extracted enzyme presents optimum temperature and pH at range of 40–50?°C and 9.0–11.0, respectively. Moreover, the enzyme was identified as serine protease based on inhibition profile. ATPS showed the satisfactory performance as the first step for Aspergillus tamarii Kita UCP1279 protease pre-purification.     

Reactions of ruthenium(II) para-substituted tetraphenylporphyrin complexes with carbon monoxide oxygen: A kinetic investigation

The reaction of Ru(XTPP)(DMF)₂, where XTPP is the dianion of para substituted tetraphenylporphyrins and X is MeO, Me, H, Cl, Br, I, F, with O₂ and CO were studied in DMF. The process was found to be first-order in metalloporphyrin, first-order in molecular oxygen and carbon monoxide, and second-order overall. Second-order rate constants for the CO reaction ranged from 0.170 to 0.665 M^?1 s^?1 at 25°C, those for the O₂ reaction from 0.132 to 0.840 M^?1 s^?1 at 25°C. Similar activation parameters (ΔH_CO^± = 87 ± 1 kJ mol^?1, ΔS_CO^± = 22 ± 4 JK^?1 mol^?1; ΔH_O2^± = 81 ± 1 kJ mol^?1, and ΔS_O2^± = 11 ± 5 JK^?1 mol^?1) were found within each series. Reactivities of X substituted metalloporphyrins were found to follow different Hammett σ functions. The CO reactions correlated with σ_? following normal behavior; the O₂ reactions correlated with σ₈° indicating O₂ is π-bonded in the transition states. A dissociative mechanism is postulated for the process.     

A microcalorimetric study of the reaction catalysed by pyruvate kinase

The enthalpy change for phosphorylation of ADP^3? by PEP^3? catalysed by pyruvate kinase has been determined at 25°C using flow microcalorimetry. Measurements were made at pH 8 in three buffer systems TRIS, TEA and HEPES and also at pH 8.5 in TRIS buffer. The values of ΔH obtained, ?8.75 kJ mol^?1 in TRIS, ?7.3₉ kJ mol^? in TEA and ?6.1₉ kJ mol^?1 in HEPES surprisingly display a dependence on the buffer system used. The enthalpy change was combined with free energy data to calculate the entropy change for the catalysed reaction.     

The conformation and aggregation of bovine β-casein A. II. Thermodynamics of thermal association and the effects of changes in polar and apolar interactions on micellization

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.     

Co-permeability of 3H-labelled water and 14C-labelled organic acids across isolated Prunus laurocerasus cuticles: effect of temperature on cuticular paths of diffusion

Co‐permeability of ³H‐labelled water and ¹⁴C‐labelled benzoic acid or 2,4‐dichlorophenoxyacetic acid across isolated cuticular membranes of Prunus laurocerasus L. was measured at temperatures ranging from 15 to 50 °C. The water and benzoic acid permeances were highly correlated over the whole temperature range investigated, whereas water and 2,4‐dichlorophenoxyacetic acid permeances were only correlated between 15 and 30 °C. The activation energies of cuticular permeability calculated from Arrhenius plots were 40 kJ mol^?1 for water and benzoic acid and 115 kJ mol^?1 for 2,4‐dichlorophenoxyacetic acid. The slopes of the Arrhenius plots of 2,4‐dichlorophenoxyacetic acid were linear between 15 and 50 °C, whereas pronounced phase transitions around 30 °C were observed for water and benzoic acid permeability. However, with isolated polymer matrix membranes, where cuticular waxes forming the transport‐limiting barrier of cuticles have been extracted, phase transitions were not observed for water and benzoic acid. It is concluded that temperatures above 30 °C caused structural changes in the transport‐limiting barrier of the cuticles leading to additional paths of diffusion for water and benzoic acid but not for 2,4‐dichlorophenoxyacetic acid.     

Kinetic and thermodynamics study of the pyrolytic process of the freshwater macroalga,Chara vulgaris

This is the first study where the pyrolysis of the freshwater macroalga Chara vulgaris was explored to reveal its bioenergy potential. The suitability of C. vulgaris to bioenergy conversion via pyrolysis was accessed in terms of kinetic triplet and thermodynamic parameters. For this purpose, pyrolysis experiments under a thermogravimetric scale were conducted at multiple heating rates (5, 10, and 20 °C min^?1) in an inert atmosphere. The mass-loss profiles of C. vulgaris pyrolysis showed that there are two dominant decomposition stages that are related to distinct chemical components inside its structure and that directly affect the calculated kinetic triplet and thermodynamics parameters. The average activation energy obtained using isoconversional methods of Flynn-Wall-Ozawa, Kissinger-Akahira-Sunose, Starink, and Friedman was in the range of 52.87–72.91 kJ mol^?1 for the first decomposition stage and 156.14–174.65 kJ mol^?1 for the second decomposition stage. The pyrolytic conversion of C. vulgaris initially follows a second-order reaction model (first decomposition stage), while in second decomposition stage is controlled by a second-order Avrami-Erofeev reaction model. The kinetic results derived from the non-isothermal decomposition of C. vulgaris proved its suitable characteristics for pyrolysis. Additionally, multi-stage kinetic interpretation was successfully attained based on two kinetic triplets, where reconstructed pyrolysis behavior correlated well with experimental pyrolysis behavior. The changes in enthalpy, Gibbs free energy, and entropy for first decomposition stage were 67.58±0.25 kJ mol^?1, 180.77±5.30 kJ mol^?1, and ?176.65±0.41 J mol^?1 K^?1, and for the second decomposition stage the values were 166.70±0.09 kJ mol^?1, 285.51±1.29 kJ mol^?1, and ?124.29±0.09 J mol^?1 K^?1, respectively. Based on thermodynamic aspects, C. vulgaris is particularly interesting for use as a raw material to produce biofuels and bioenergy.

     

Purification,kinetic and thermodynamic characterization of soluble acid invertase from sugarcane (Saccharum officinarum L.)

We report for the first time kinetic and thermodynamic properties of soluble acid invertase (SAI) of sugarcane (Saccharum officinarum L.) salt sensitive local cultivar CP 77-400 (CP-77). The SAI was purified to apparent homogeneity on FPLC system. The crude enzyme was about 13 fold purified and recovery of SAI was 35%. The invertase was monomeric in nature and its native molecular mass on gel filtration and subunit mass on SDS-PAGE was 28 kDa. SAI was highly acidic having an optimum pH lower than 2. The acidic limb was missing. Proton transfer (donation and receiving) during catalysis was controlled by the basic limb having a pKa of 2.4. Carboxyl groups were involved in proton transfer during catalysis. The kinetic constants for sucrose hydrolysis by SAI were determined to be: k_m = 55 mg ml^?1, k_cat = 21 s^?1, k_cat/k_m = 0.38, while the thermodynamic parameters were: ΔH* = 52.6 kJ mol^?1, ΔG* = 71.2 kJ mol^?1, ΔS* = ?57 J mol^?1 K^?1, ΔG*_E–S = 10.8 kJ mol^?1 and ΔG*_E–T = 2.6 kJ mol^?1. The kinetics and thermodynamics of irreversible thermal denaturation at various temperatures 53–63 °C were also determined. The half -life of SAI at 53 and 63 °C was 112 and 10 min, respectively. At 55 °C, surprisingly the half -life increased to twice that at 53 °C. ΔG*, ΔH* and ΔS* of irreversible thermal stability of SAI at 55 °C were 107.7 kJ mol^?1, 276.04 kJ mol^?1 and 513 J mol^?1K^?1, respectively.     

Triple helix–coil transition of covalently bridged collagenlike peptides

The thermal triple helix–coil transition of covalently bridged collagenlike peptides with repeating sequences of (Ala-Gly-Pro)_n, n = 5–15, was studied optically. The peptides were soluble in water/acetic acid (99:1) and were found to form triple-helical structures in this solvent system beginning with n = 8. The thermodynamic analysis of the transition equilibrium curves for n = 9–13 yielded the parameters ΔH°_s = ?7.0 kJ per tripeptide unit, ΔS°_s = ?23.1 J deg^?1 mol^?1 per tripeptide unit for the coil-to-helix transition, and the apparent nucleation parameter σ ? 5 × 10^?2. It was suggested through double-jump temperature experiments that the rate-limiting step during refolding is not only influenced by the difficulties of nucleation, but also by cis–trans isomerization of the Gly-Pro peptide bond.     

Kinetics and thermodynamics of the P870+Q−A → P870+Q−B reaction in isolated reaction centers from the photosynthetic bacterium Rhodopseudomonas sphaeroides

The temperature dependences of the P870⁺Q^?_A → P870Q_A and P870⁺Q^?_B → P870Q_B recombination reactions were measured in reaction centers from Rhodopseudomonas sphaeroides. The data indicate that the P870⁺Q^?_B state decays by thermal repopulation of the P870⁺Q^?_A state, followed by recombination. ΔG° for the P870⁺Q^?_A → P870⁺Q^?_B reaction is ?6.89 kJ · mol^?1, while ΔH° = ?14.45 kJ · mol^?1 and ?TΔS° = + 7.53 kJ · mol^?1. The activation ethalpy, $\text{H}{}^3$, for the P870⁺Q^?_A Δ P870⁺Q^?_B reaction is +56.9 kJ · mol^?1, while the activation entropy is near zero. The results permit an estimate of the shape of the potential energy curve for the P870⁺Q^?_A → P870⁺Q^?_B electron transfer reaction.