Molecular Electronic Structure Calculations of Natural Products

Complex molecules permeate our world. In this project we will investigate the properties and reactivity of several such molecules computationally, together with the significant challenges they present to current molecular electronic structure methods when multiple electronic states are involved. These challenges include treating more than one electronic state sufficiently accurately as a complex molecule's geometry changes, when such calculations cannot yet be carried out accurately on the whole molecule because the resource demands are too great.Our first target is to develop our understanding of the behaviour of bioluminescent molecules. Previous computational work on bioluminescent systems is limited. The luminescence of firefly luciferase was investigated by a hybrid scheme, combining the SAC-CI method with the CIS method. The keto form of the firefly oxyluciferin shows red chemiluminescence in solution and yellow-green bioluminescence in firefly luciferase. The origin of the blue shift was proposed to be the electrostatic effect of the protein environment. However, the detailed mechanism for its luminescent behaviour has not been investigated for anything more than isolated molecules so far, neglecting interactions with the environment. To do this, reaction pathways, short lived intermediates, transition structures and crossings between potential energy surfaces must be located, including environmental effects through hybrid calculations that use accurate quantum mechanical (QM) calculations for the reaction site and molecular mechanics (MM) force fields for the surroundings.