Heath Watts

Publications

• 4.12

  Impact points

  Evaluation of potential reaction mechanisms leading to the formation of coniferyl alcohol α-linkages in lignin: a density functional theory study.

  Heath D Watts, Mohamed Naseer Ali Mohamed, James D Kubicki

  Physical chemistry chemical physics : PCCP. 12/2011; 13(47):20974-85.

  Five potential reaction mechanisms, each leading to the formation of an α-O-4-linked coniferyl alcohol dimer, and one scheme leading to the formation of a recently proposed free-radical coniferyl alcohol trimer were assessed using density functional theory (DFT) calculations. These potential reactio... [more] Five potential reaction mechanisms, each leading to the formation of an α-O-4-linked coniferyl alcohol dimer, and one scheme leading to the formation of a recently proposed free-radical coniferyl alcohol trimer were assessed using density functional theory (DFT) calculations. These potential reaction mechanisms were evaluated using both the calculated Gibbs free energies, to predict the spontaneity of the constituent reactions, and the electron-density mapped Fukui function, to determine the most reactive sites of each intermediate species. The results indicate that each reaction in one of the six mechanisms is thermodynamically favorable to those in the other mechanisms; what is more, the Fukui function for each free radical intermediate corroborates with the thermochemical results for this mechanism. This mechanism proceeds via the formation of two distinct free-radical intermediates, which then react to produce the four α-O-4 stereoisomers.
• 3.47

  Impact points

  Comparison of multistandard and TMS-standard calculated NMR shifts for coniferyl alcohol and application of the multistandard method to lignin dimers.

  Heath D Watts, Mohamed Naseer Ali Mohamed, James D Kubicki

  The journal of physical chemistry. B. 02/2011; 115(9):1958-70.

  Coniferyl alcohol is a monomeric building block of lignin, the second most abundant biopolymer. During lignification, the monomer forms a variety of linkages through free radical additions. A large NMR database has been constructed that reports the (1)H and (13)C chemical shifts for thousands of lig... [more] Coniferyl alcohol is a monomeric building block of lignin, the second most abundant biopolymer. During lignification, the monomer forms a variety of linkages through free radical additions. A large NMR database has been constructed that reports the (1)H and (13)C chemical shifts for thousands of lignin oligomers. Herein, Boltzmann averaged (1)H and (13)C GIAO NMR calculations were performed on coniferyl alcohol and four of its dimers, β-O-4, β-β, β-5, and 5-5, to compare the calculated chemical shifts with experiment. Six B3LYP/6-311++G(d,p) energy-minimized conformational isomers of coniferyl alcohol were subjected to single-point GIAO NMR calculations. Initially, four NMR shift calculation methods were compared: three were performed using the TMS-standard method at the HF/6-311+G(2d,p), B3LYP/6-311+G(2d,p), and mPW1PW91/6-31G(d) theory levels, and the fourth was performed with a multistandard approach using a mPW1PW91/6-31G(d) theory level. For the multistandard method, benzene was used as the standard for aromatic C and H atoms and methanol was used for aliphatic C and H atoms. The hydroxyl-H of methanol was used as the standard for hydroxyl-H atoms. The Boltzmann averaged results for six conformers showed that the multistandard method is more accurate for coniferyl alcohol and its dimers than the often used TMS-standard method, based on the mean unsigned, root-mean-squared, and maximum errors, as well as linear correlations between observed and calculated values. The (13)C results were more accurate than the (1)H results, due to poorer agreement between calculated hydroxyl-H results and observed data. Further Boltzmann-averaged, multistandard NMR calculations compared the (13)C and (1)H chemical shifts with experiment for the four stereoisomers of the β-O-4 dimer, as well as the 5-5, β-5, and β-β dimers of coniferyl alcohol. The (13)C results correlated well with experiment (r(2)>0.99) for all dimers and showed small statistical errors, compared with experiment. The correlation with experiment for (1)H NMR was generally inferior to the (13)C NMR results for the dimers.
• 2.03

  Impact points

  MP2, density functional theory, and molecular mechanical calculations of C-H...pi and hydrogen bond interactions in a cellulose-binding module-cellulose model system.

  Mohamed Naseer Ali Mohamed, Heath D Watts, Jing Guo, Jeffrey M Catchmark, James D Kubicki

  Carbohydrate research. 08/2010; 345(12):1741-51.

  Exploring non-covalent interactions, such as C-H...pi stacking and classical hydrogen bonding (H-bonding), between carbohydrates and carbohydrate-binding modules (CBMs) is an important task in glycobiology. The present study focuses on intermolecular interactions, such as C-Hcdots, three dots, cente... [more] Exploring non-covalent interactions, such as C-H...pi stacking and classical hydrogen bonding (H-bonding), between carbohydrates and carbohydrate-binding modules (CBMs) is an important task in glycobiology. The present study focuses on intermolecular interactions, such as C-Hcdots, three dots, centeredpi (sugar-aromatic stacking) and H-bonds, between methyl beta-d-glucopyranoside and l-tyrosine-a proxy model system for a cellulose-CBM complex. This work has made use of various types of quantum mechanics (QM) and molecular mechanics (MM) methods to determine which is the most accurate and computationally efficient. The calculated interaction potential energies ranged between -24 and -38kJ/mol. The larger interaction energy is due to H-bonding between the phenyl hydroxyl of tyrosine and the O4 of the sugar. Density functional theory (DFT) methods, such as BHandHLYP and B3LYP, exaggerate the H-bond. Although one of the MM methods (viz. MM+) considered in this study does maintain the C-Hcdots, three dots, centeredpi stacking configuration, it underestimates the interaction energy due to the loss of the H-bond. When the O-H bond vector is in the vicinity of O4 (O-Hcdots, three dots, centeredO4 approximately 2A, e.g., in the case of MP2/6-31G(d)), the torsional energy drops to a minimum. For this configuration, natural bond orbital (NBO) analysis also supports the presence of this H-bond which arises due to orbital interaction between one lone pair of the sugar O4 and the sigma *(O-H) orbital of the phenyl group of tyrosine. The stabilization energy due to orbital delocalization of the H-bonded system is approximately 13kJ/mol. This H-bond interaction plays an important role in controlling the CH/pi interaction geometry. Therefore, the C-Hcdots, three dots, centeredpi dispersive interaction is the secondary force, which supports the stabilization of the complex. The meta-hybrid DFT method, M05-2X, with the 6-311++G(d,p) basis set agrees well with the MP2 results and is less computationally expensive. However, the M05-2X method is strongly basis set dependent in describing this CH/pi interaction. Computed IR spectra with the MP2/6-31G(d) method show blue shifts for C1-H, C3-H, and C5-H stretching frequencies due to the C-Hcdots, three dots, centeredpi interaction. However, the M05-2X/6-311++G(d,p) method shows a small red shift for the C1-H stretching region and blue shifts for the C2-H and C3-H stretches. For the aromatic tyrosine C(delta1)-C(epsilon1) and C(delta2)-C(epsilon2) bonds in the complex, the calculated IR spectra show red shifts of 12cm(-1) (MP2/6-31G(d)) and 5cm(-1) (M05-2X/6-311++G(d,p)). This study also reports the upfield shifts of computed (1)H NMR chemical shifts due to the C-Hcdots, three dots, centeredpi interaction.