Interactions Of Cyclodextrins With Typical Persistent Organic Pollutants And Effects Of Their Conjugates On The Biodegration Of Organic Pollutants

Persistent organic pollutants(POPs)are the organic compounds with the long-term residual,bioaccumulation,semi-volatile,and high toxicity.As typical POPs,polycyclic aromatic hydrocarbons(PAHs)and organochlorine pesticides have resulted in the massive soil contaminations in China.They are difficult to be biodegraded,making the recovery of the contaminated soil a long-term and arduous task.Microbial remediation has been one of the promising methods for soil recovery,due to the low cost,the long existence,and the long-lasting efficiency.However,microbial remediation has not been the primary soli recovery strategy,since POPs usually have high hydrophobicity to interact with the organic matters in soil,which results in their low bioavailbilities.Cyclodextrins(CDs)have shown the strong ability to recognize the the small hydrophobic molecules,by which they may efficiently increase their solubilities via the hydrophobic inner cavity.CDs have been extensively employed as the solubilizers for POPs in soil recovery.However,the effects of different CDs on POPs differ,which limits their applications.Currently,the study on microbial remediation of POPs is still at the early stage.How POPs enter microbial cells for degradation and whether this process is crucial for soil recovery have been elusive.Here,we combine the simulations and experiments to sysmetically study the effects of CDs on the solubility and biodegradation of POPs in different phases,focusing on the molecular recongnitions between CD and POPs,the dynamic and energetic properties of CD/POPs complexes.The resutls illustrate the mechanism of CDs in enhancing the biodegration of POPs.The results and conclusions are as follows:Firstly,the molecular dockings are employed to evaluate the molecular interations between CDs(α-CD,β-CD,γ-CD,hydroxypropyl-CD:HP-β-CD and sulfobutylguanidine-CD:SBE-β-CD)and the typical POPs(naphthalene:Nap,anthracene:Ant,phenanthrene:Phe,and pyrene:Pyr).Due to the small molecular size,Nap,Ant and Phe favor α-CD.The interactions of Pyr with β-CD are stronger than α-CD and γ-CD.Two CD derivatives,HP-β-CD and SBE-β-CD,can form the stronger interactions with four POPs than three natural CDs,indicating that the CD derivatives are more efficient for accomodating POPs.This is in line with the experimental conclusions,indicating the ability of CDs in enhancing the solubility and biodegradation of POPs.Secondly,by using the molecular dynamics simulations and adaptive-biasing force calculations,we investigate the mechanism of β-CD in promoting the solubility and biodegradation of Phe.The results suggest β-CD acts as a "carrier" to promote the biodegradation of Phe.The simulations show that β-CD and Phe may rapidly form the complexes in the aqueous phase,which then interact stably with the membrane.However,β-CD and the β-CD-included complexes show the large energy barrier for passing through the membrane.This indicats that on the membrane,Phe should escape from β-CD and penetrate the membrane alone.These results highlight the importance of membrane interactions in the CD-enhanced biodegradation of POPs.Thirdly,three natural CDs and DDT molecules are selected and studeid by the simulations and experiments to explore the effect of the size of the CD hydrophobic cavities on DDT biodegradation.The experiments show that α-CD slightly promotes the DDT biodegradatio,while β-and y-CDs cannot.MD simulations illustrate the recognitions between CDs and DDT.α-CD with smaller hydrophobic cavity partically include DDT,forming the complexes with both hydrophilic and hydrophobic surfaces.β-and γ-CD encapsulate DDT molecules with the large hydrophobic cavities,leading to the complexes consisting mainly the hydrophilic surfaces.The α-CD-DDT complex exhibits the stronger membrane interactions,compared to the β-CD-DDT and y-CD-DDT complexes.These results indicate that the size of CD’s cavity may alter the molecaulr interactions between CD and DDT and affect the biodegration of DDT.Finally,two CD derivatives(SBE-β-CD、HP-β-CD)and Phe are selected to investigate the roles of the side chains of the CD derivatives in the solubility and biodegration of Phe.The expriments have suggested that SBE-β-CD is stronger in promoting the solubility and biodegration of Phe than HP-β-CD.The simulations suggest that the differences derive from the different dynamic properties of the side chains in SBE-β-CD and HP-β-CD.The negatively charged SBE motifs in SBE-β-CD exhibit the electrostatic repulsion force,which may keep the CD’s opening available for Phe inclusions.The HP motifs in HP-β-CD are shorter with the hydrogen bonding sites and hydrophobic groups,which may entangle to block the CD’s opening and interfere with the Phe inclusions.These results illustrate the importance of side chain dynamics of CDs in the solubility and biodegradation of POPs.In summary,this work studies the mechanism of CDs in promoting the solubility and biodegradation of POPs,illustrating the molecular recognitions between CDs and POPs and the dynamic and energetic properties of CD/POP complexes in solution phase and on membrane.The results illustrate the key factors of CDs in affecting the solubility and biodegradation of POPs,emphasizing the importance of the membrane interactions,the size of hydrophobic cavityies and the dynamics of side chains.We expect that this work may improve our understanding of the enhancement of POPs biodegradation by CDs and provide the theoretical basis for chemical modificiation and rational applications of CDs in enviromental recovery.