2.1. GENERALIDADES DE LOS HONGOS ENTOMOPATÓGENOS
2.1.4. Mecanismos de acción de los hongos entomopatógenos
All continuum solvation model calculations mentioned so far have used the original PCM variant, probably one of the most popular continuum solvation models in the literature. Other continuum solvation models and modifications to existing ones have been suggested over the years, for example: the conductor polarisable continuum model CPCM308, isodensity PCM (IPCM),79 self-‐consistent isodensity PCM (SCIPCM)79
and the SMD solvation model.78
From our QM/MM MD simulations we evaluated a free energy difference between A and F at the B3LYP/6-‐31+G* level. The result is consistent with the NMR
spectroscopic analysis but is in contrast to previous PCM-‐DFT results. We were curious whether other continuum solvation models were capable of predicting an energy difference between A and F closer to the QM/MM predictions.
Figure 35 Relative energies Δ(E+Gsolv) of 3F-‐GABA conformers calculated by adding gas-‐phase relative energies
(on PCM-‐B3LYP/6-‐31+G* geometries) and relative free energies of solvation from different continuum solvation models evaluated at the DFT or HF level with the 6-‐31+G* basis set.
The above mentioned solvation models, available in Gaussian 09, were used to calculate solvation free energies for each conformer, by subtracting total gas-‐phase energies from continuum solution energies, using the same level of theory for gas and solution. In order to do a consistent comparison between models, all solvation
energies were added to gas-‐phase energies at the B3LYP/6-‐31+G*//PCM-‐B3LYP/6-‐
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31+G* level (i.e. PCM-‐B3LYP geometries), giving E+Gsolv solution energies. SCIPCM
results could not be included as most calculations did not converge (those that did converge, for A, E and G, showed very similar relative energies as regular PCM results).
Δ(E+Gsolv) relative energies for the 3F-‐GABA conformers, using several different
solvation models are plotted in Figure 35. The figure also shows the relative gas-‐ phase energies of each conformer where it is evident that the intramolecular interactions strongly favour folded conformers and that any preference towards extended conformers must come from an accurate calculation of the interaction between solute and solvent.
Results using the PCM solvation model at different levels of theory result in a
dramatic drop in preference for folded conformers (compared to the gas-‐phase) but folded conformers are still favoured. Results using the PCM solvation model at
different levels of theory and the CPCM solvation model are overall quite similar. The IPCM solvation model shows a slightly different energy surface and curiously the F conformer is considerably more stable than conformers B-‐E and G-‐J and is even slightly more stable than A in agreement with the QM/MM result. The more than 5 kcal/mol energy difference between F and other extended conformers, however, seems inconsistent with the results of the unconstrained MD simulations where most extended conformers were encountered regularly (although these were performed at the PM3/MM level of theory).
Results using the SMD solvation model show dramatically different results than other solvation models. There is a slight dependence on the level of theory used to calculate the solvation energy (B3LYP vs. M05-‐2X) but overall the results seem the most
compatible with the results from the QM/MM MD simulations where conformer F is found to be slightly more stable than all other conformers (in agreement with the calculated energy difference and NMR analysis) but other conformers still being close enough in energy to be encountered during MD simulations. Also shown in Figure 35 are SMD results using only the electrostatic term (non-‐electrostatic term neglected) which shows that the performance of the SMD solvation model for our system has mainly to do with electrostatic effects but not the non-‐electrostatic (cavitation, dispersion, exchange-‐repulsion etc.) term which is of a slightly more empirical nature. The good performance of the SMD model compared to PCM (both methods use the IEFPCM protocol in Gaussian 09) thus must come from the atomic radii used in the SMD solvation model that have been specially optimised.78
Finally, we note that our E+Gsolv energies are not free energies of solution. This is due
to the fact that we do not include ZPVE or thermal effects to enthalpy and entropy. These quantities should come from vibrational frequency calculations and it has been recommended to do these calculations in the gas-‐phase.309 For 3F-‐GABA, however,
the zwitterionic form is not stable in the gas-‐phase and vibrational frequency
calculations thus cannot be performed. It is a current debate in the literature whether calculating vibrational frequencies using a continuum solvation model is a valid approximation to use for the evaluation of the free energy of solution.309,310 For our
system it is not clear whether the neglect of potentially crucial quantities like entropy is more dangerous than including an ill-‐defined thermochemical quantity. Test
calculations of the thermal correction to enthalpy and entropy from vibrational frequencies in solution, however, lead to small differences between Δ(E+Gsolv) and
We have shown that recent continuum solvation models are not in such strong disagreement with experiment (and QM/MM) for the 3F-‐GABA system. This bodes well for future studies of zwitterions as continuum solvation calculations are very straightforward to carry out and have a low computational cost compared to the orders of magnitude more involved QM/MM calculations. At least it is clear that continuum solvation models are very useful for an an initial exploration of the conformational energy surface of small molecules that interact strongly with the solvent.