• Dhurairajan senthilnathan

      Articles written in Journal of Chemical Sciences

    • Biocatalysis of azidolysis of epoxides: Computational evidences on the role of halohydrin dehalogenase (HheC)

      Dhurairajan senthilnathan Venkatachalam Tamilmani Ponnambalam Venuvanalingam

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      Biocatalytic azidolysis of 9 unsymmetrical epoxides by halohydrin dehalogenase enzyme (HheC) in gas phase and uncatalysed azidolysis of the same epoxides in gas phase and in aqueous solution have been modelled at DFT level. Aliphatic epoxides (1-6) and aromatic epoxides (9) undergo 𝛽 cleavage while styrene oxide (7) and 𝑝-nitro styrene (8) oxide prefer 𝛼 cleavage in the gas phase. Inclusion of aqueous solvation effect via Polarizable Continuum Model (PCM) increases the activation barrier and makes the reaction endothermic due to extensive solvation of azide anion and oxido anionic products, but does not alter the regioselectivity. Halohydrin dehalogenase from Agrobacterium radiobactor AD1 catalyses (E1-E9) ring opening of all these epoxides by azide ion with 𝛽 selectivity and the reversal of selectivity in epoxide 7 and 8 is notable. These reactions follow, in both enzymatic and non-enzymatic environment, S$_N$2 mechanism. Calculations while agreeing totally with experimental results offer better insights on the factors determining the regioselectivity and particularly the role of enzyme. Active site model and crystal structure data reveal that the Tyr145 and Ser132 form weak hydrogen bonds with epoxide oxygen lone pair and form reactant enzyme complex (REC). The enzyme complex activates the epoxide ring towards azidolysis. The NBO deletion and second order perturbation analyses clearly bring out the role of catalytic duo Tyr145 and Ser132 and particularly shed light on the dominant contribution of Tyr145 in selectively activating C$_{\beta}$-O bond. The present results indicate that Arg149 or other residues in the pocket do not seem to have any significant effect on the reaction.

    • Evidence for the powerful catalytic ability of imidozirconocene complex from its epoxide ring cleavage reactions - A DFT mechanistic view

      Dhurairajan Senthilnathan Rajadurai Vijay Solomon Ponnambalam Venuvanalingam

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      Imidozirconocene complex is known for its bifunctional reactivity and catalytic ability and this complex mediates ring cleavage of epoxides. Cyclooctene oxide (1) Norbornene oxide (2) and 2,5-dimethyl cyclohexene oxide (3) undergo ring cleavage in the presence of imidozirconocene complex. Epoxide 1 has accessible 𝛽-hydrogens (type I) while epoxide 2 and 3 do not have them (type II). Normally type I epoxides undergo elimination while type II epoxides prefer insertion. All the insertion reactions lead to five-membered metallacycle formation and elimination results in thermodynamically stable allyl-alkoxy product. The insertion is a two-step process following either diradical or zwitterionic pathway, while elimination is a one-step concerted reaction. DFT (density functional theory) modelling of these reactions at B3LYP/LANL2DZ level show that epoxide 1 undergoes elimination in agreement with experiment. However, calculations indicate that epoxide (2) proceeds through diradical intermediate in contrast to experimental observations. Surprisingly, epoxide (3) that has both the 𝛽 positions blocked by methyl groups undergoes elimination rather than insertion. AIM and EDA analyses offer further insights on the reaction mechanism and bifunctional reactivity of imidozirconozene complex.

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