Study On The Recognition Performance Of The Fourteen-membered Melon Ring On Small Biomolecules

Cucurbit[n]urils?Q[n]s or CB[n]s?are a family of macrocyclic compounds from an acid catalyzed condensation reaction of glycoluril and formaldehyde.These pumpkin-shaped containers have a characteristic rigid hydrophobic cavity through two polar portals with carbonyl groups.In particular,their molecular recognition and coordination properties have attracted much attention in supramolecular chemistry in the last decades.Twisted cucurbit[n]uril?tQ[14]?,with 14 normal glycoluril units linked by 28 methylene bridges,has two kinds of cavities?a central cavity and two side cavities?and adopts a folded,figure-of-eight conformation.Moreover,it also displays a good solubility in both neutral water and DMSO.In this paper,we design and fabricate a supramolecular assembly based on tQ[14] and different properties of guests,for example,amino acids,organic biological dyes,pesticides and metal ions.Interaction of these guests with tQ[14] were studied in details by using different methods.In particular,the mechanism of action,sites of action and thermodynamics properties of guests in presence of tQ[14] were also investigated.Moreover,the supramolecular fluorescence probes between tQ[14] and dyes have been developed to sense metal ions and pesticide.The research contents are as follows:First,we have investigated the binding interactions between tQ[14] and the 20 standard AAs in aqueous HCl solution by isothermal titration calorimetry?ITC?and in DCl/D2 O and DMSO-d6 by 1H NMR spectroscopy.The results show that tQ[14] displays stronger binding affinity towards AAs with a positively charged side chain or containing an aromatic ring,and weaker binding affinity towards AAs with an uncharged or negatively charged side chain.Moreover,as expected,the binding mode is seen to depend on the nature of the side chains of the AAs and the solvent?DCl/D2 O or DMSO-d6?employed.Moreover,titration 1H NMR spectra in DMSO-d6 indicated two different binding interactions for t Q[14] with Arg and Lys.The driving force are contributed to the electrostatic interactions at the carbonyl portals and cavity hydrophobic interactions and ion–dipole interactions.Based on the titration 1H NMR and ITC experiments,the interactions of tQ[14] with the remaining AAs can be subdivided into three groups.For the first group,the side-chain proton signals show a clear upfield shift,suggesting that the side chains preferentially reside in the cavity of tQ[14].The particular AAs showing this behavior are His,Phe,Tyr,Trp,Cys and Met;In the second group?Pro,Val,Ala,Leu,Ile,Gly?,the side-chain proton signals show upfield shifts weakly,suggesting that the side chains reside in the cavity of tQ[14],but the bingding behavior are weak;for the third group,the side-chain proton signals show almost no change or are even downfield shifted,suggesting that the respective side chains are largely excluded from the cavity of tQ[14].The AAs showing this behavior are Ser,Thr,Gln,Glu,Asn,and Asp.So the different binding affinities of tQ[14] towards the 20 standard AAs may be rationalized in terms of a variety of different side chains.Secondly,we have examined the fluorescent properties of TO in the presence of tQ[14] in aqueous solution.TO has been found to show good affinity for tQ[14] which greatly enhances its fluorescence.Because of the special structure of tQ[14] and the more concentrated the carbonyl oxygen distribution,the interaction between tQ[14] and TO existed three binding modes:?1?the benzothiazole moiety was included in the cavity of tQ[14];?2?the dimer or higher aggregates of TO was lacated at the portal;?3?the outer surface interactions between tQ[14] and TO.Introducing different metal ions into the t Q[14]-TO interaction system induced different fluorescence responses.In particular,the tQ[14]/TO complex exhibits unique fluorescence responses to Hg²⁺,Ba²⁺ and Pb²⁺ cations in different aqueous media: only Hg²⁺ ion can lead to fluorescence enhancement of a 2:1 tQ[14]/TO complex in neutral aqueous solution;only Ba²⁺ can induce fluorescence quenching of a 2:1 tQ[14]/TO complex in aqueous HCl solution?pH 2?;and only Ba²⁺ and Pb²⁺ can induce the fluorescence quenching of a 15:1 tQ[14]/TO complex in aqueous HCl solutions?pH 2?.Moreover,the tQ[14]-TO-Ba²⁺ system exhibits reversible fluorescence enhancement and quenching by successively adding SO42-and Ba²⁺.No such fluorescence responses for the tQ[14]-TO-Hg²⁺ and tQ[14]-TO-Pb²⁺ systems were observed due to the formation of stable t Q[14]/TO/Hg²⁺ and tQ[14]/TO/Pb²⁺ ternary complexes.Thus,we have developed a fluorescent sensor based on tQ[14]/TO host-guest chemistry for the three heavy metal ions.Finally,a fluorescence enhancement of DSMI and t-DSMI were realized by the formation of inclusion complexes with tQ[14].And we presented a simple method to construct a reversible switching process between tQ[14] and styryl dyes.Dyes located at the carbonyl portal when pH=6,however,dyes prefer to into the cavity of tQ[14] when pH=2,and it is significant that the binding behavior changes could be reversibly adjusted by the protonation and deprotonation of nitrogen atom on dimethylamino.A simple assay for the determination of MV²⁺ in water solution based on tQ[14]-DSMI and tQ[14]-t-DSMI supramolecular fluorescence probes was performed.The fluorescence probes were developed to quantitative detection of MV²⁺,based upon competitive binding between tQ[14]-DSMI/t-DSMI complexes and MV²⁺.