I was able to model the first step of proposed hydrolysis mechanism (Koshland et al.) for a cyanogenic beta glucoside and couple of its analogs including a alpha anomer. I identified the barrier heights (BH) following the reaction coordinate suggested by said paper.
All the calculation were done at B3LYP/6-311+G** level of theory.
I have been faced with about a 10.0 kcal/mol difference in BH for the alpha anomer. You will find that there is no significant difference in any structure. I would appreciate if someone can point me out a comparison of BHs for alpha/beta glucoside hydrolysis or a possible explanation for this difference.
Below is the attached TS structures and their BHs calculated.
L1 - 29.58 kcal/mol
L3 - 26.58 kcal/mol
L2 (alpha) - 15.24 kcal/mol
Regards,
D L S Dinuka
Mr. Dinuka,
It can be carried out only experimentally by means of mass spectrometry. In this context pay attention to the following reference:
1. Quantitative collision induced mass spectrometry of substituted piperazines – A correlative analysis between theory and experimen, Journal of Molecular Structure, Volume 1149, 2017, 243-256, Bojidarka Ivanova, Michael Spiteller.
Together with the references therein devoted to: Iribarne-Thomson model to MS reaction kinetics. The corresponding thermodynamic parameters are directly derived from the MS kinetic ones.
Thank you sir Bojidarka B. Ivanova I'll go through this. Would you be kind enough to elaborate on your ideas for the reported energy difference in the original question?
Assuming i have scanned the reaction coordinate properly and the resultant structures are as should be would such difference in barrier heights be possible (given the only change is alpha-beta change in leaving group)?
This could likely be explained by the anomeric effect.
Anomeric effect is stabilization alpha anomer over beta anomer due to donation of lone pair into sigma-star orbital of anomeric substituent.
Hydrolysis of your alpha anomer must proceed through a transition state or intermediate where that adjacent heteroatom stabilizes an antibonding orbital which is, presumably, not possible for beta anomer.
- Badges
- Science topic
- Similar topics
- Physical Sciences
- Quantum Physics
- Quantum Mechanics