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The Ground State Conformation

Due to the coarse nature of the grid used in the calculations described in the previous subsection, the location of the absolute minimum on the energy surface is uncertain. In order to identify this minimum, a series of unconstrained relaxations were performed. In each case, the starting point was chosen to lie within one of the minima identified on the relaxed conformational energy map. The starting conformations, the final relaxed conformations and the relative energies of the minima are given in Table gif.


Starting Geometry tex2html_wrap_inline3119 Relaxed Geometry tex2html_wrap_inline3119 Relative Energy (kcal tex2html_wrap_inline3057 )
tex2html_wrap_inline3125 tex2html_wrap_inline3127 0
tex2html_wrap_inline3129 tex2html_wrap_inline3131 16
tex2html_wrap_inline3133 tex2html_wrap_inline3135 6
Table: The relaxed geometries and relative energies for minima on the conformational energy map of acetylcholine.


From these data we can see that the most stable conformation of acetylcholine is the trans, gauche gif conformation of the dihedral angles D2 and D3 (see Figure gif). This is in agreement with studies by Pullman and Port [65] and Edvardsen and Dahl [66]. In the fully relaxed geometry the angles D1 and D4 are found to be tex2html_wrap_inline3153 and tex2html_wrap_inline3155 respectively.



Figure: The calculated ground state conformation of acetylcholine showing isosurfaces of charge density at 0.5 tex2html_wrap_inline2772 , 1.2 tex2html_wrap_inline2772 and 1.8 tex2html_wrap_inline2772 .

If we examine the Mulliken populations of the atoms in the ground state conformation, as shown in Figure gif, we find that the cationic alkyl ammonium head has a net charge of +0.63e. This is in good agreement with calculations by Pullman and Port [65]. Similarly, in agreement with Pullman and Port, this positive charge is found to be distributed among the hydrogen atoms of the three methyl groups bound to the nitrogen. Thus, the positive charge is spread over the exterior of the cationic head. This contrasts with the results of Beveridge and Radna [63] who find that the majority of the positive charge resides on the N and C atoms. The distribution of this charge is relevant to the interaction of the molecule with a nucleophile such as water or a cationic receptor cite. The Mulliken populations do not differ significantly between the three minima we have investigated. This is because all of the minima correspond to a trans arrangement of the D2 bond which results in an extended geometry for the molecule. Thus, there is little interaction between the alkyl ammonium head and the tex2html_wrap_inline2653 methylene group in any of the conformations examined. A more compact form of the molecule would result in greater interactions between the different sections of the molecule and hence would probably lead to a significant rearrangement of the atomic charges.



Figure: The Mulliken charges of the acetylcholine molecule in its ground state. Oxygen atoms are shown as red, nitrogen as light blue, carbon as grey and hydrogen as white. Charges are in electronic units and are labeled in the colour corresponding to the species with the exception of hydrogen atoms which are labeled in black.

next up previous contents
Next: Summary Up: Results and Discussion Previous: Conformation Energy Maps

Matthew Segall
Wed Sep 24 12:24:18 BST 1997