- Deuterium kinetic isotope effects on the dissociation of a protein-fatty acid complex in the gas phase
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Deuterium kinetic isotope effects (KIEs) are reported for the first time for the dissociation of a protein-ligand complex in the gas phase. Temperature-dependent rate constants were measured for the loss of neutral ligand from the deprotonated ions of the 1:1 complex of bovine β-lactoglobulin (Lg) and palmitic acid (PA), (Lg + PA)n- → Lgn- + PA, at the 6- and 7- charge states. At 25 °C, partial or complete deuteration of the acyl chain of PA results in a measurable inverse KIE for both charge states. The magnitude of the KIEs is temperature dependent, and Arrhenius analysis of the rate constants reveals that deuteration of PA results in a decrease in activation energy. In contrast, there is no measurable deuterium KIE for the dissociation of the (Lg + PA) complex in aqueous solution at pH 8. Deuterium KIEs were calculated using conventional transition-state theory with an assumption of a late dissociative transition state (TS), in which the ligand is free of the binding pocket. The vibrational frequencies of deuterated and non-deuterated PA in the gas phase and in various solvents (n-hexane, 1-chlorohexane, acetone, and water) were established computationally. The KIEs calculated from the corresponding differences in zero-point energies account qualitatively for the observation of an inverse KIE but do not account for the magnitude of the KIEs nor their temperature dependence. It is proposed that the dissociation of the (Lg + PA) complex in aqueous solution also proceeds through a late TS in which the acyl chain is extensively hydrated such that there is no significant differential change in the vibrational frequencies along the reaction coordinate and, consequently, no significant KIE.
- Liu, Lan,Michelsen, Klaus,Kitova, Elena N.,Schnier, Paul D.,Brown, Alex,Klassen, John S.
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scheme or table
p. 5931 - 5937
(2012/05/07)
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- Synthesis and use of deuterated palmitic acids to decipher the cryptoregiochemistry of a Δ13 desaturation
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(Chemical Equation Presented) The synthesis of two hexadeuterated palmitic acids differing in the position of the diagnostic labels, and their use to decipher the cryptoregiochemistry of a Δ13 desaturation are described. A dithiane and a triple bond functionalities were used to introduce the diagnostic (C 13 or C14) and tagging (C8 and C9) labels, respectively, in the palmitic acid skeleton. Using these probes, the cryptoregiochemistry of the Δ13 desaturation involved in the biosynthesis of Thaumetopoea pityocampa sex pheromone was studied by means of kinetic isotope effect determinations. Transformation of both (Z)-11-hexadecenoic and 11-hexadecynoic acids into (Z, Z)-11,13-hexadecadienoic and (Z)-13-hexadecen-11-ynoic acids, respectively, is initiated by abstraction of the hydrogen atom at the C13 position, followed by the fast elimination of the C14 hydrogen to give the double bond.
- Abad, Jose-Luis,Serra, Montserrat,Camps, Francisco,Fabrias, Gemma
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p. 760 - 764
(2007/10/03)
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