Definition of the coordinate system
The complex in a given geometrical configuration
with the H2O2 enantiomer "a"
Computational determination of chiral molecule interaction energies
An object that cannot be superimposed with its mirror image is called chiral. Our hands are the most familiar example of such chiral objects. Image and mirror image are called enantiomers of the chiral object. Chiral molecules play important roles in nature. For example, life itself is based on single enantiomeric forms of amino acids and sugars, a fact known as "homochirality of life". Many pharmaceuticals are chiral molecules of which one enantiomer has a beneficial effect whereas the other may have severe side effects.
At the heart of it all is chiral recognition or chiral discrimination. A handshake is possible between two left or two right hands, but not between left and right hand. On the molecular level, there is an energy difference between the interaction of, for example, the right handed form of a chiral molecule and the two enantiomeric forms of another substance.
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Definition of the coordinate system
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The complex in a given geometrical configuration
with the H2O2 enantiomer "a"
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How can this subtle energy difference have such profound effect in nature? The CISS-1 Experiment gives us an opportunity to shed light on the answer. We will compute the interaction energy of two chiral molecules at many different separations and orientations. The specific system will consist of 1,2-propyleneimine (an aziridine derivative) and hydrogen peroxide. The calculations will be carried out on two different hydrogen peroxide enantiomers (see graphics). An analysis of the results will give a "chiral recognition surface" that identifies the active sites of the chiral components."
The complex in the same geometrical configuration
with the H2O2 enantiomer "b"
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The "shifting" of the H2O2 molecule with respect to the space-fixed
PI molecule.
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The chiral recognition surface will be determined by calculating the interaction energy at many different orientations and separations of the two molecules. Because of the complexity of the molecules, the energy needs to be calculated at many thousand points. Each point may require several hours of computing time on a modern workstation. In computational terms, this is an ideal parallel problem which can be distributed onto as many processors as points needed and is therefore ideally suited for the CISS Experiment.
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Click
here
to see a small movie illustrating the “shifting”
of the hydrogen peroxide molecule with respect to the space-fixed
propylene imine molecule.
One cut of the potential energy surface (the xy-plane at z = –3 Å) has been completed this morning. Click here to see a fly-by along this surface. |
The efficient MOLPRO code for electronic structure calculations will be used in the experiment. Each single point calculation is exceedingly complex. The computations will be performed on a state-of-the-art level of theory.
We thank the authors of MOLPRO, P. Knowles and H. Werner, for permission to use their program suite for this experiment.