Poster Title: National Needs for Nanoscale Science and Engineering in Forest Resources
Authors: Mohamed Naseer Ali Mohamed, Heath Watts, James D. Kubicki, J. Catchmark, N. Brown, M. Tien, D. Archibald
Poster Abstract: The intermolecular interactions, such as C-H/pi and hydrogen bonds, between O-methy-beta-D glucose and tyrosine, a dimer model system of cellulose-celluose binding module (CBM), have been analyzed using various levels of ab initio theory and molecular mechanical methods such as MM+, OPLS-AA, Amber94, MM2, and MM3 in conjunction with ITC experiment. From these studies, the interaction energy was found to vary from -24 to -38 kJ/mol in the order of MP2/6-311+G**>MP2/6-31G*>MM+. Hydrogen bonding between phenol hydroxyl of tyrosine and O4 of sugar causes an increase in the interaction energy in the case of MP2 results. Density functional theory (DFT) methods, such as BHandHLYP and B3LYP, over-emphasize this H-bonding at the expense of the C-H/ π interaction, and the resulting dimer structure has the rings of the two monomers perpendicular rather than parallel to one another. MM+ inadequately reproduces this H-bonding interaction although it does predict C-H/π interaction. We investigated the existence of this hydrogen bonding by varying the phenol O-H bond configuration with respect to O4 through torsional scan and found that when O-H bond vector comes in the vicinity of O4 (O-H---O4 = 2.08 Å), the torsional energy becomes the lowest one. For this configuration, NBO analysis also supports the presence of this hydrogen bond which arises due orbital interaction of one lone pair of the sugar O4 and the σ*(O-H) orbital of the phenyl group of tyrosine. The stabilization energy of the H-bonded system due to orbital delocalization is approximately 13 kJ/mol. 1H NMR chemical shifts are also reported to characterize the existence of C-H/pi interaction. Appreciable upfield characteristic Aromatic Solvent Induced Shifts (ASIS) have been found to be existing. Analysis of isothermal titration calorimetric data reveals that the Glc-Tyr binding phenomenon is the entropically favorable as evident from the positive value of T∆S and ∆G agrees very well with earlier reports in the literature. Interestingly, it is found from these investigations that computed binding free energy reflects the data obtained through experiments to a greater extent.