Differential Transition State

Szefczyk B, Mulholland AJ, Ranaghan KE, Sokalski WA (2004) Differential transition-state stabilization in enzyme catalysis Quantum chemical analysis of interactions in the chorismate mutase reaction and prediction of the optimal catalytic field. J Am Chem Soc 126 16148—16159... 

Thus, by application of Eq. (16) it is generally concluded that transition state enthalpy differences are the major source of differential transition state free energies. 

Differential transition state/product stabilization approach... 

Already in 1948 Linus Pauling has put forward hypothesis 34,35] that catalytic activity of enzymes is due to transition state stabilization by enzyme active site. Extending this idea to the relative stabilization of transition state with respect to initial enzyme - substrate complex, one may obtain Differential Transition State Stabilization (DTSS) model [7], with activation barrier change A representing quantitatively the catalytic activity of any catalyst C... 

Electrostatic Differential Transition State Stabilization energies resulting from aminoacid... 

FIGURE 10 6 Confor mations and electron delo calization in 1 3 butadiene The s CIS and the s trans con formations permit the 2p or bitalsto be aligned parallel to one another for maxi mum TT electron delocaliza tion The s trans conformation is more stable than the s CIS Stabilization resulting from tt electron de localization is least in the perpendicular conformation which IS a transition state for rotation about the C 2—C 3 single bond The green and yellow colors are meant to differentiate the orbitals and do not indicate their phases... 

Three kinetically equivalent rate terms involving intramolecular participation are shown in Table 6-3 with representations of appropriate transition states (mechanisms). Differentiation among these possibilities can be difficult. 

The first column corresponds to six-membered transition states where two types can be differentiated situation A, where the proton transfer leads to a neutral tautomer, and situation B (often Tinuvin P or TIN 75), where the proton transfer leads to a zwitterionic tautomer (Scheme 25). 

The chiral catalyst 142 achieves selectivities through a double effect of intramolecular hydrogen binding interaction and attractive tt-tt donor-acceptor interactions in the transition state by a hydroxy aromatic group [88]. The exceptional results of some Diels-Alder reactions of cyclopentadiene with substituted acroleins catalyzed by (R)-142 are reported in Table 4.21. High enantio- and exo selectivity were always obtained. The coordination of a proton to the 2-hydroxyphenyl group with an oxygen of the adjacent B-0 bond in the nonhelical transition state should play an important role both in the exo-endo approach and in the si-re face differentiation of dienophile. 

When the carbonyl groups are present, the transition state for syn attack is sta-bihzed by interactions between the in-phase combination of the NN lone pairs and the antisymmetric n orbital of the CO-X-CO bridge (100). Although the secondary effect (SOI) operates only during syn approach and contributes added stabilization to this transition state, the primary orbital interaction (see 103) between the HOMO of the cyclohexadiene moiety of 100 and the n orbital of the dienophile (NN, Fig. 16) is differentiated with respect to the direction of attack, i.e., syn or anti, of triazolinedione (NN, Fig. 16). 

Transition state theory has been useful in providing a rationale for the so-called kinetic isotope effect. The kinetic isotope effect is used by enzy-mologists to probe various aspects of mechanism. Importantly, measured kinetic isotope effects have also been used to monitor if non-classical behaviour is a feature of enzyme-catalysed hydrogen transfer reactions. The kinetic isotope effect arises because of the differential reactivity of, for example, a C-H (protium), a C-D (deuterium) and a C-T (tritium) bond. 

The interpretation of phenomenological electron-transfer kinetics in terms of fundamental models based on transition state theory [1,3-6,10] has been hindered by our primitive understanding of the interfacial structure and potential distribution across ITIES. The structure of ITIES was initially studied by electrochemical and thermodynamic analyses, and more recently by computer simulations and interfacial spectroscopy. Classical electrochemical analysis based on differential capacitance and surface tension measurements has been extensively discussed in the literature [11-18]. The picture that emerged from... 

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