The present study is an attempt to provide an insight into the stability (in terms of interaction energy and thermodynamic parameter) and reactivity (quantified by reactivity descriptors) of the chitosan-MX system. Electronic and structural properties of chitosan during functionalization by metal were studied by density functional theory (DFT) calculations. Isolated and functionalized chitosan were optimized and their properties were evaluated. The results indicated that the properties of linking sites detect the most significant effects of functionalization process.
Degradation efficiency of and also the possibility of absorption of MX (3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone) by Chitosan nanoparticles were studied via different level of theory.
Keywords: Halogenated furanone (MX),MX analogous, Disinfection byproducts, Chitosan, Density functional theory.
During chlorine disinfection of drinking water, chlorine can react with natural organic matter (NOM) in raw water to generate halogenated disinfection byproducts (DBPs). Halogenated furanone a disinfection byproduct present in chlorinated drinking water, is one of the most potent mutagens known. MX (3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone) is formed by the reaction of chlorine with complex natural organic matter. Its genotoxic effects are well documented i,ii, and these data indicate that MX induced thyroid and bile duct tumors iii. The mechanism by which MX exerts such an intense biological effect and interaction of them with nano-materials which used in water treatment is still unclear.
MX is formed by the reaction of chlorine with complex organic matter in drinking water. Aromatic structures with an aldehyde group and substituents in the meta or para positions, e.g. like syringaldehyde or ferulic acid, can form MX upon chlorination iv, as can compounds, e.g. tryptophan, which can first form an aldehyde group v. It has been identified in drinking water MX was found at mean concentrations ranging from 2 to 80 ng/ liter vi-vii-viii-ix. Registered by WHO, MX is one of the strongest bacterial mutagens ever tested, as highlighted by the Ames Salmonella typhimurium TA100 assay x. The observed mutagenic activity is significantly correlated only to the electrophilicity response of the ring forms xi. However, analytical difficulties in measuring the low doses of MX encountered in drinking-water lead to uncertainty over whether this species would be genotoxic in vivo xii. The World Health Organization (WHO) guidelines for drinking water maintain an updated register regarding MX, considering it unnecessary at present to propose a formal guideline value for MX in drinking waterxiii,xiv.
Chitosan (poly-?-(1?4)-2-amino-2-deoxy-D-glucose) is a natural nitrogenous, amino-based, polysaccharide (Figure 2), which is produced in large quantities by N-deacetylation of chitin xv,xvi. Marine crustaceans’ shells are widely used as primary sources of chitin xvii. Chitosan (CS), which is a biocompatible and biodegradable biopolymer is a liner and cationic polymer with numerous applications mainly dependent on presence of the amine group in its structure xviii,xix. The metal-chelating property of chitosan has been mainly used in wastewater treatment. Recently, different metal chitosan complexes have been prepared to improve its activity xx. Nanoparticles displayunique physical and chemical features because of effects such asthe quantum size effect, mini size effect, surface effect andmacro-quantum tunnel effect. Chitosan tripolyphosphate nanoparticles have been synthesized and mainly used as drug carrier as reported in previous studies xxi. The major privilege of chitosan base absorption systems is the hydrophilicity combined with the polar groups, hydroxyl and also amine groups, which are capable to form long-range interaction with absorbent molecules xxii. Recently, some literature reports perceptive progress taking place with respect to a number of specialized chitosan derivatives envisaged to entangle some inherent restrictions of chitosan xxiii.
Investigations of the nature and site of interaction between chitosan and MX xxiv,xxv. Dispersion-corrected DFT methods are endowed with very good level of accuracy and have evolved as an alternative to methods that include extensive electron correlation in the calculation juxtaposed with a sufficiently large basis set xxvi,xxvii. Various dispersion corrected functional are proposed and are being applied to assess strength of weak interactions such as hydrogen bonding. Among these DFT functionals, several recent studies have outlined the suitability of CAM-B3LYP for estimating strength of hydrogen bonding interaction xxviii,xxix. CAM-B3LYP combines the hybrid qualities of Becke three parameter exchange and Lee, Yang and Parr correlation functional (B3LYP) and the dispersion correction essential for calculating interaction energy of hydrogen-bonded systems xxx,xxxi. In addition to CAM-B3LYP, M06-2X functional has also found extensive application for successful description of hydrogen-bonded systems xxxii.This functional was proven to yield much better result for exchange energy at large distancesxxxiii.
Herein, present study delves into the possibility of using chitosan nano-particles for absorbing MX from water solutions. Thermo-chemical parameters are calculated for clarifying the nature of interaction in Chitosan –MX system. Interaction energy, reactivity descriptors and electronic properties of this system are also utilized to describe the characteristics and features of interactions.
2. Computational details
The ground-state calculations were performed for the isolated systems by using a DFT approach, as implemented in the Quantum ESPRESSO package xxxiv. A plane wave basis set was used (50 Ry cutoff) with norm-conserving pseudopotentials and local-density approximation exchange correlation (XC) functional. The ground state structural calculations such as mechanical properties and optical spectrum by DFT approach were performed by SIESTA package xxxv utilizing double-zeta polarized (DZP) basis set. After the evaluation of the active bonding sites for O and Cl atom (for MX and its analogous) and O and N atoms (in chitosan molecule) and its probable recombination path, the next step was to find the activation energy along that path, so that kinetics of the recombination reaction could be predicted. An initial path was constructed and represented by a discrete set of images of the system connecting the initial and final states. To calculate the activation energy barrier, the nudged elastic band (NEB) xxxvi method implemented in the Quantum ESPRESSO package is used.
GAMESS US packagexxxvii.
Simulations were performed by using LAMMPS (large-scale atomic/molecular massively parallel simulator) open-source classical MD code xxxviii. Periodic boundary conditions were imposed in all directions of simulation cells. After the structure optimization, the system is thermalized using NVT ensemble (canonical ensemble) for 1ns where the temperature fluctuation is minimized to be around 7 K. The simulation system is then switched to NVE ensemble (microcanonical ensemble) to run for 20 ns. Non-bonded interactions were truncated at 12 Å cutoff distance and the long-range electrostatic interactions were predicted using particle-particle particle-mesh (PPPM) method xxxix. Newton’s equation of motion was solved with velocity Verlet integrator using a time step of 0.5 fs xl. During simulations, temperature and pressure were controlled via Nose-Hoover algorithm as thermostat and barostat xli.
3. Results and discussion
I. Structural properties
It is known xlii that MX(C5H3Cl3O3) can predominate as a halogenated hydroxyfuranones ring (HHF) and the open-ring form 2-chloro-3-(dichloromethyl)-4-oxobutenoic acid tautomer ((E) and (Z) Fig1.b, c) in respect to the acidity of solution. The ring form exists in solutions with pH<5.5, whereas the Z-open form will be present at standard pH of drinking-water xliii. To investigate the interaction of MX analogous with chitosan, the fully optimized structure of MX, EMX and ZMX were obtained by geometry optimization(Figure1). i Brunborg G et al. (1990) Organ-specific genotoxic effects of chemicals: The use of alkaline elution to detect DNA damage in various organs of in vivo exposed animals. Progress in Clinical and Biological Research, 340D:43–52.and Brunborg G et al. (1991) Genotoxic effects of the drinking water mutagen 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX) in mammalian cells in vitro and rats in vivo. 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