Abstract
Computational chemistry approaches for studying the formation of terpenes/terpenoids in wines are presented, using five particular terpenes/terpenoids (1,8-cineole, α-ylangene, botrydial, rotundone, and the wine lactone), volatile compounds (or their precursors) found in wine and/or wine grapes, as representative examples. Through these examples, we show how modern computational quantum chemistry can be employed as an effective tool for assessing the validity of proposed mechanisms for terpene/terpenoid formation.
Highlights
Terpenes are hydrocarbon natural products constructed from connected isoprene units
Removal of the pyrophosphate group from geranyl diphosphate (GPP) or farnesyl diphosphate (FPP) by terpene synthase/cyclase enzymes in vinifera grape vines generates carbocationic intermediates that rearrange within the enzyme active sites to form monoterpenes and sesquiterpenes [1,2,3,4,5]
The carbocation cyclization/rearrangement cascade that leads to α-ylangene (Scheme 2) is representative of the sort of cascades that lead to most terpenes
Summary
Terpenes are hydrocarbon natural products constructed from connected isoprene units (each containing five carbons). Removal of the pyrophosphate group from GPP or FPP by terpene synthase/cyclase enzymes in vinifera grape vines generates carbocationic intermediates that rearrange within the enzyme active sites to form monoterpenes and sesquiterpenes [1,2,3,4,5]. The computational approach generally employed involves mapping out reaction coordinates for terpene/terpenoid-forming reactions by optimizing the geometries, and calculating the relative energies, of potential minima (intermediates) and transition state structures involved in formation of terpene skeletons These structures are connected with intrinsic reaction coordinates (IRCs; steepest descent pathways from transition state structures to connected minima); in doing so, barriers for each chemical step are predicted and can be assessed as to their viability at biologically relevant temperatures. While terpene synthase enzymes appear not to be necessary, in general, for facilitating carbocation rearrangement steps, they are necessary for generating the initial carbocation for a given rearrangement, preorganizing the conformation of this carbocation and preventing premature quenching of carbocations by deprotonation or trapping by nucleophiles [42]
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