An accurate theoretical study of energy barriers of alkaline hydrolysis of carboxylic esters

Understanding the mechanism of enhanced anaerobic biodegradation of biodegradable plastics after alkaline pretreatment

Biodegradable plastics (BPs) tend to replace conventional plastics, which increases the amount of BP waste entering the environment. The anaerobic environment exists extensively in nature, and anaerobic digestion has become a widely used technique for organic waste treatment. Many kinds of BPs have low biodegradability (B_D) and biodegradation rates under anaerobic condition due to the limitation of hydrolysis, so they still have harmful environmental consequences in anaerobic environment. There is an urgent need to find an intervention method to improve the biodegradation of BPs. Therefore, this study aimed to investigate the effectiveness of alkaline pretreatment in accelerating the thermophilic anaerobic degradation of ten widely used BPs, such as poly (lactic acid) (PLA), poly (butylene adipate-co-terephthalate) (PBAT), thermoplastic starch (TPS), poly (butylene succinate-co-butylene adipate) (PBSA), cellulose diacetate (CDA), etc. The results showed that NaOH pretreatment significantly improved the solubility of PBSA, PLA, poly (propylene carbonate) (PPC), and TPS. Except for PBAT, pretreatment with an appropriate NaOH concentration could improve the B_D and degradation rate. The pretreatment also reduced the lag phase in the anaerobic degradation of BPs such as PLA, PPC, and TPS. Specifically, for CDA and PBSA, the B_D increased from 4.6 % and 30.5 % to 85.2 % and 88.7 %, with increments of 1752.2 % and 190.8 %, respectively. Microbial analysis indicated that NaOH pretreatment promoted the dissolution and hydrolysis of PBSA and PLA and the deacetylation of CDA, which contributed to rapid and complete degradation. This work not only provides a promising method for improving the degradation of BP waste but also lays the foundation for its large-scale application and safe disposal.

Fractional Charge Density Functional Theory and Its Application to the Electro-inductive Effect

We employed the chemical potential equalization principle to demonstrate that fractional electrons are involved in the electro-inductive effect as well as the vibrational Stark effect. By the chemical potential model, we were able to deduce that the frontier molecular orbitals of immobilized molecules can provide valuable insight into these effects. To further understand and quantify these findings, we introduced fractional charge density functional theory (FC-DFT), a canonical ensemble approach for open systems. This method allows for the calculation of electronic energies, nuclear gradients, and the Hessian matrix of fractional electronic systems. To correct the spurious delocalization error commonly found in approximate density functionals for small systems, we imposed the Perdew-Parr-Levy-Balduz (PPLB) condition through linear interpolation of two adjacent integer points (LI-FC-DFT). Although this approach is relatively simple in terms of molecular modeling, the results obtained through LI-FC-DFT calculations predict the same trend seen in experimental reactivity and the frequency change of immobilized molecules.

Simulations of the Biodegradation of Citrate-Based Polymers for Artificial Scaffolds Using Accelerated Reactive Molecular Dynamics

In this study, we investigate the reactivity and mechanical properties of poly(1,6-hexanediol-co-citric acid) via ReaxFF molecular dynamics simulations. We implement an accelerated scheme within the ReaxFF framework to study the hydrolysis reaction of the polymer which is provided with a sufficient amount of energy known as the restrain energy after a suitable pretransition-state configuration is obtained to overcome the activation energy barrier and the desired product is obtained. The validity of the ReaxFF force field is established by comparing the ReaxFF energy barriers of ester and ether hydrolysis with benchmark DFT values in the literature. We perform chemical and mechanical degradation of polymer chain bundles at 300 K. We find that ester hydrolyzes faster than ether because of the lower activation energy barrier of the reaction. The selectivity of the bond-boost scheme has been demonstrated by lowering the boost parameters of the accelerated simulation, which almost stops the ether hydrolysis. Mechanical degradation of prehydrolyzed and intermittent hydrolyzed polymer bundles is performed along the longitudinal direction at two different strain rates. We find that the tensile modulus of the polymers increases with increase in strain rates, which shows that polymers show a strain-dependent behavior. The tensile modulus of the polyester-ether is higher than polyester but reaches yield stress faster than polyester. This makes polyester more ductile than polyester-ether.

Validating computer simulations of enantioselective catalysis; reproducing the large steric and entropic contributions in Candida Antarctica lipase B

The prospect for computer-aided refinement of stereoselective enzymes is further validated by simulating the ester hydrolysis by the wild-type and mutants of CalB, focusing on the challenge of dealing with strong steric effects and entropic contributions. This was done using the empirical valence bond (EVB) method in a quantitative screening of the enantioselectivity, considering both k(cat) and k(cat)/K(M) of the R and S stereoisomers. Although the simulations require very extensive sampling for convergence they give encouraging results and major validation, indicating that our approach offers a powerful tool for computer-aided design of enantioselective enzymes. This is particularly true in cases with large changes in steric effects where alternative approaches may have difficulties in capturing the interplay between steric clashes with the reacting substrate and protein flexibility.