Effects Of Solvent On The Nanomechanical Properties Of Amylose And Natural Cellulose

Polysaccharides are widely distributed in nature,which paly crucial roles in many vital biological activities such as energy storage,structural support and cellular recognition.Meanwhile,polysaccharides are important raw materials in food industry,textile industry,renewable energy industry and etc.Nowadays,we face serious energy(fossil fuel),environmental and food problems.The development of nanofabrication technologies based on biomass resources can help us tosolve these problems.As two most abundant polysaccharides,natural cellulose(NC)and starch have always attracted the attention of researchers.The mechanical and biological properties of NC and starch differ largely even if they share the same structure unit(glucose).Investigate the nanomechanical properties will help us to understand the mechanical properties differences of them more deeply.Studying the properties of polysaccharides in solution is neccessary if we want to better understand and utilize polysaccharides more beatter.However,there are still many problems about the properties of polysaccharides in solution remain unclear.In this thesis,single-molecule AFM is used to investigate the nanomechanical properties of NC and amylose in different solvents.Firstly,we study the effects of water content on the single-molecule mechanical properties of amylose.Secondly,we study the properties of helical structure of amylose in different solvents.Finally,the effects of water and hydrogen bonds(H-bonds)on the nanomechanical properties and functions of NC and amylose have been studied by single-molecule AFM.Based on the above systemic investigation,the main conclusions can be summarized as follows:(1)In this study,the effects of water content of the mixed solvents on the nanomechanical properties of amylose are explored by single-molecule AFM.The experimental results obtained in a series of water/alcohol mixed solvents clearly show that both the appearance and the fingerprint plateau height in the F-E curves of amylose are dependent on the water content.When water content < critical water content(CWC),no fingerprint plateau can be observed in F-E curves because there is no enough bound water to stabilize the chair conformation of sugar rings.The single-moleule mechanical strength is weak in this condition.However,the chair conformation of the sugar rings can be stabilized by bound water when water content > CWC.Therefore,the fingerprint plateau can be observed in the F-E curves.The amount of bound water around the sugar rings will be increased with the increasing of water contents beyond CWC.Thus,more energy will be consumed to rearrange the bound water in a mixed solvent with a higher water content.In other words,the higher water content of the environment,the higher plateau height in the F-E curves.The distribution of bound water around sugar rings can be influenced by alcohol molecules through regulation of H-bonds.Therefore,the hydration number in mixed solvent is less than that in neat water,even if the alcohol content is as low as 2%.This can explain the sudden increase in the plateau heights at very high water contents(98% ~ 100%).Water is a good solvent for amylose,but alcohols are not.The solvent quality to amylose will be improved with the increasing of water content of water/alcohol mixed solvents.The solubility of amylose is positively correlated with the number of H-bonds between water and amylose.When water content < CWC,mixed solvent is a poor solvent for amylose.At CWC,the solvent quality reaches a threshold value,where a plateau can be observed in some F-E curves.When water content reaches 100%(neat water),the solvent quality is maximized,where a highest plateau can be observed.The single-moleule mechanical propertites reach the best in in this condition.(2)There is no color changes when amylose is added to iodine/nonane(or N,N-dimethylformamide)mixed solvent.Meanwhile,long plateau can not be observed in the F-E curves of amylose that obtained in iodine/nonane mixed solvent(or propanol,butanol,pentanol).Interestingly,the solution is turned blue when amylose is added to aqueous iodine solution.The long plateau in the F-E curves of amylose can be observed when force experiments are carried out in water/alcohol mixed solvents.It has been reported that the long plateau in the F-E curve of amylose is caused by the unfolding of the helical structure.The blue color of amylose/aqueous iodine solution is mainly caused by the formation of the helical complex.These experimental results demonstrate that water is a key factor for the helical structure of amylose.The long plateau in the F-E curves always appears with the fingerprint plateau.There is no F-E curve that contains only the long plateau.The studies in chapter two indicate that CWC is the key factor for the stable chair conformation of sugar rings.Thus,we speculate that the stable chair conformation is the precondition for the formation of helical structure of amylose.When water content > CWC,we find that two kinds of F-E curves can be observed: one with a fingerprint plateau,the other contains both fingerprint plateau and long plateau.The helical complex is composed by amylose and aggregated alcohols.The aggregation of alcohol molecules is not stable in mixed solvents due to the disturbance of water molecules.Thus,there are two kinds of F-E curves of amylose.The aggregation of alcohol molecules will be reduced with the increasing of water content.Therefore,the long plateau is disappeared when water content is high enough.(3)The intra-chain H-bonds of NC(or amylose)can be formed in a non-polar organic solvent(nonane),while they are completely prohibited in a highly polar solvent(dimethyl sulfoxide,DMSO).The single-moleule mechanical strength of NC(or amylose)obtained in nonane is remarkably higher than that obtained in DMSO.These results suggest that the single-molecule mechanics of NC and amylose can be effectively affected by the solvent polarity through regulation of the intra-chain H-bonds.This conclusion is supported by the force measurements of CTA(cellulose triacetate)and ATA(amylose triacetate).NC and amylose share the same status of H-bonds in a given organic solvent(DMSO or nonane).In a given organic solvent amylose and NC present the same single-molecule mechanics even if the linkages between the sugar rings are different(? or ?).However,the status of H-bonds(intra-chain and polysaccharide-water)of NC and amylose differ largely in water.NC can form fewer H-bonds with water than amylose.Thus,NC is more hydrophobic than amylose.In water,NC is more rigid than amylose due to the cooperativity of intra-chain H-bonds.The nanomechanical properties and functions of NC and amylose differ largely,even though they share the same structure unit(glucose).The relative hydrophobicity and impressive mechanical strength are very important for NC that serves as the structural material,while the relative hydrophilicity is necessary for amylose that serves as the energy storage compound.These results reveal that water is a special solvent for amylose and NC: both mechanics and affinity to water of these two polysaccharides can be effectively affected by water through regulation of the H-bonds.This study casts new light on the roles of water(the key environment of organisms)in the properties and functions of polysaccharides.