DeltaDeltaH Degrees Research Articles

Enantiomer separation of triazole fungicides by high‐performance liquid chromatography

Enantiomer separation is one of the most important prerequisites for the investigation of environmental enantioselective behaviors for chiral pesticides. In the present study, the enantiomer separation of 7 triazole fungicides, i.e., hexaconazole (1), triadimefon (2), tebuconazole (3), diniconazole (4), flutriafol (5), propiconazole (6), and difenoconazole (7), were evaluated using normal phase high-performance liquid chromatography. Chrialcel OD column and Chrialcel OJ column were used. The influence of column temperature was studied for the optimization of the resolution as well as the type and percentage of organic modifier in the mobile phase. The retention factors for the enantiomers of all investigated compounds decreased as the temperature increased. The natural logarithms of the selectivity factors (lnalpha) of hexaconazole (1), tebuconazole (3), flutriafol (5), propiconazole (6) and difenoconazole (7) depended linearly on the inverse of temperature (1/T) while the corresponding values for triadimefon (2) and diniconazole (4) kept unchanged in the studied temperature range 10-35 degrees C. Van't Hoff plots afforded thermodynamic parameters, such as the apparent change in enthalpy DeltaH degrees , the apparent change in entropy DeltaS degrees and the apparent change in DeltaDeltaH degrees and DeltaDeltaS degrees . The thermodynamic parameters (DeltaH degrees , DeltaS degrees , DeltaDeltaH degrees and DeltaDeltaS degrees ) were calculated in order to provide an understanding of the thermosynamic driving forces for enantioseparation. The established method shows perspective to be used for preparing micro-scale amount of pure enantiomers of the chiral triazoles studied.

Why does cyclopropene have the acidity of an acetylene but the bond energy of methane?

The gas-phase acidity of 3,3-dimethylcyclopropene (1) has been measured by bracketing and equilibrium techniques. Consistent with simple hybridization arguments, our value (deltaH degrees (acid) = 382.7 +/- 1.3 kcal mol(-)(1)) is indistinguishable from that for methylacetylene (i.e., deltadeltaH degrees (acid)(1 - CH(3)Ctbd1;CH) = 1.6 +/- 2.5 kcal mol(-)(1)). The electron affinity of 3,3-dimethylcyclopropenyl radical (1r) was also determined (EA = 37.6 +/- 3.5 kcal mol(-)(1)), and these quantities were combined in a thermodynamic cycle to afford the homolytic C-H bond dissociation energy. To our surprise, the latter quantity (107 +/- 4 kcal mol(-)(1)) is the same as that for methane, which cannot be explained in terms of the s-character in the C-H bonds. An orbital explanation (delocalization) is proposed to account for the extra stability of 1r. All of the results are supplemented with G3 and B3LYP computations, and both approaches are in good accord with the experimental values. We also note that for simple hydrocarbons which give localized carbanions upon deprotonation there is an apparent linear correlation between any two of the following three quantities: deltaH degrees (acid), BDE, and EA. This observation could be of considerable value in many diverse areas of chemistry.

Structural and thermodynamic consequences of introducing alpha-aminoisobutyric acid in the S peptide of ribonuclease S.

The S protein-S peptide interaction is a model system to study binding thermodynamics in proteins. We substituted alanine at position 4 in S peptide by alpha-aminoisobutyric acid (Aib) to investigate the effect of this substitution on the conformation of free S peptide and on its binding to S protein. The thermodynamic consequences of this replacement were studied using isothermal titration calorimetry. The structures of the free and complexed peptides were studied using circular dichroic spectroscopy and X-ray crystallography, respectively. The alanine4Aib replacement stabilizes the free S peptide helix and does not perturb the tertiary structure of RNase S. Surprisingly, and in contrast to the wild-type S peptide, the DeltaG degrees of binding of peptide to S pro, over the temperature range 5-30 degrees C, is virtually independent of temperature. At 25 degrees C, the DeltaDeltaG degrees, DeltaDeltaH degrees, DeltaDeltaS and DeltaDeltaCp of binding are 0.7 kcal/mol, 2.8 kcal/mol, 6 kcal/mol x K and -60 kcal/mol x K, respectively. The positive value of DeltaDeltaS is probably due to a decrease in the entropy of uncomplexed alanine4Aib relative to the wild-type peptide. The positive value of DeltaDeltaH: degrees is unexpected and is probably due to favorable interactions formed in uncomplexed alanine4Aib. This study addresses the thermodynamic and structural consequences of a replacement of alanine by Aib both in the unfolded and complexed states in proteins.

Synthesis and Reactivity of trans-Tricyclo[4.2.0.01,3]oct-4-ene

The first synthesis of trans-tricyclo[4.2.0.0(1,3)]oct-4-ene (1), an ethenyl bridged spirohexane, was accomplished in four steps starting from Carpino et al. gem-dichloro ketone 6. An X-ray crystal structure of 1 with one substituent was obtained to provide geometry data on this novel ring system and to confirm the stereochemical assignment of the penultimate synthetic intermediate. Tricyclo[4.2.0. 0(1,3)]oct-4-ene is surprisingly stable. It reacts with glacial acetic acid but only slowly at 145 degrees C; the products were isolated and identified. A unimolecular rearrangement takes place at elevated temperatures (165 degrees C and higher), presumably, via a biradical intermediate to afford tricyclo[4.2.0.0(1,5)]oct-3-ene (23). The structure of this 1,5-bridged bicyclo[2.1.0]pentane derivative was established by NMR and an X-ray crystal structure of its Diels-Alder adduct with isobenzofuran. Tricyclo[4.2.0.0(1, 3)]oct-4-ene equilibrates with 23, so equilibrium constants and reaction rates were measured over a 20 degrees C temperature range from 180 degrees C to 200 degrees C. The difference in the heats of formation (DeltaDeltaH degrees (f) (23 - 1)) is -2.1 kcal/mol, which is in good agreement with ab initio (HF and MP2) calculations using the 6-31G(d) basis set (-1.9 (HF) and -1.4 (MP2) kcal/mol). Computations on trans-tricyclo[4.2.0.0(1,3)]octane and spirohexane also were carried out, and the structures and energies were compared.

Weak electrophile selective characteristics of the rat preneoplastic marker enzyme glutathione S-transferase P-form, GST-P (7-7): a theory of linear free energy relationships for evaluation of the active site hydrophobicity of isoenzymes.

Subsite (the H-site) hydrophobicity of the rat glutathione S-transferase P-form (GST-P, 7-7, pi class species) and six other GST species of the alpha and mu classes was examined theoretically and experimentally by application of linear free energy relationships (LFERs) with a series of S-alkylated glutathiones, GS(CH2)n-1CH3 (n = 1-12). Plots of log Ki (inhibition constant) versus n were used to generate LFERs for microscopic hydrophobic interactions. The free enthalpic change per methylene group (-deltadeltaG degrees, absolute value) evaluated for GST-P (1.8 kJ/mol) was lower than those for the other six forms (2.4-3.5 kJ/mol). In addition, the enthalpic change (deltadeltaH degrees) determined from van't Hoff plots was much smaller for GST-P (0.5 kJ/mol) than the GST1-1 value (5.9 kJ/mol). As these thermodynamic parameters, deltadeltaG degrees and deltadeltaH degrees, may be considered as indirect and direct measures of GST hydrophobicity respectively, the H-site hydrophobicity of GST-P is thus very low as compared with those of other forms, clearly indicating that the pi class GST-P selectively targets for weak electrophiles, i.e. water-soluble carcinogens such as acrolein and hydroxyalkenals. The finding also defines a host-defensive role of the preneoplastic cells against the carcinogenic insult, although paradoxical.

Redox thermodynamics of the native and alkaline forms of eukaryotic and bacterial class I cytochromes c.

The reduction potentials of beef heart cytochrome c and cytochromes c2 from Rhodopseudomonas palustris, Rhodobacter sphaeroides, and Rhodobacter capsulatus were measured through direct electrochemistry at a surface-modified gold electrode as a function of temperature in nonisothermal experiments carried out at neutral and alkaline pH values. The thermodynamic parameters for protein reduction (DeltaS degrees rc and DeltaH degrees rc) were determined for the native and alkaline conformers. Enthalpy and entropy terms underlying species-dependent differences in E degrees and pH- and temperature-induced E degrees changes for a given cytochrome were analyzed. The difference of about +0.1 V in E degrees between cytochromes c2 and the eukaryotic species can be separated into an enthalpic term (-DeltaDeltaH degrees rc/F) of +0.130 V and an entropic term (TDeltaDeltaS degrees rc/F) of -0.040 V. Hence, the higher potential of the bacterial species appears to be determined entirely by a greater enthalpic stabilization of the reduced state. Analogously, the much lower potential of the alkaline conformer(s) as compared to the native species is by far enthalpic in origin for both protein families, and is largely determined by the substitution of Met for Lys in axial heme ligation. Instead, the biphasic E degrees /temperature profile for the native cytochromes is due to a difference in reduction entropy between the conformers at low and high temperatures. Temperature-dependent 1H NMR experiments suggest that the temperature-induced transition also involves a change in orientation of the axial methionine ligand with respect to the heme plane.