Study Of Hydrophobic Effect On Chain Conformation And Self-assembling Behaviorsin Aqueous Solution

Water plays crucial roles in protein folding and hydrophobic-driven self-assembly.However,the mechanism for water effects on the self-assembling behavior is still confusing.Traditional theory considered that hydrophobic-driven self-assembly is dominated by hydrophobic interaction.However,the self-assembling behaviors are seriously dependent on the hydrophobic hydration and hydrophobic interctions.Due to the complex structures of water,the relative papers are rare and the relative theory remains in blank stage.Clarifying the hydrophobic effects on polar groups,non-polar groups and ionic groups in hydrophobic-driven self-assembly would greatly promote the development of aqueous self-assemblies.In this paper,a series of hydrophobic molecules with different ionic end groups,polypeptide homopolymers and polypeptide block copolymers were synthesized by the combination of migration insertion polymerization（MIP）,ring-opening polymerization（ROP）and atom transfer radical polymerization（ATRP）.In addition,LF-NMR,FT-IR,DSC and CD analysas were combined to study the roles of hydrophobic effects（HE）on different hydrophobic molecules,and to explain the relationship between molecular hydration states and self-assembly behaviors.Firstly,we designed the molecular model of hydrophobic P⁺X^-（P⁺:macromolecular phosphonium cation,X^-:anion）to study the influences of different terminal ionic groups on water structures.The hydration and aqueous assembling behavior were found to be dependent on the water structure perturbated by the ionic groups.The closely contacted ion pair of P⁺X^-with a chaotropic X^-,e.g.I^-,CF₃SO₃^-,and BF₄^-,ordered water molecules and acted as a kosmotrope de-hydrating the backbone of P⁺.The de-hydration induced hydrophobic interactions driving the assembly into micelle-like aggregates.In contrast,the directly contacted ion pair of P⁺X^-with a hydrophobic X^-,e.g.[BPh₄]^-or[PF₆]^-,did not perturbate the water structure,and the P⁺chains remains hydrated and assembled into hollow structures in water via hydration forces.This elucidation will progress aqueous supramolecular chemistry and researches related to Hofmeister effects.Secondly,we invested the effects of hydrophobic hydration as a hydrophobic effect（HE）on the conformations and self-assembling behaviours of terminal charged poly（L-lactide）（PLLA[PPh₂Me]I）in THF/water with various water contents.The backbone was hydrated while ionic end group was not at a lower water content（33.3%）,and the corresponding self-assembles form into fibers.As water content increased to 50.0%,ion pair end groups were hydrated and exerted a long-range effect de-hydrating the backbone,in which the dehydration endowed the chain spiralization via intramolecular C-H···O=C hydrogen bonds and driven the formation of spheres.At 60.0%of water content,the long-range effect of hydrated water for the ion pair end groups completely removed the hydration water from backbone,so the macromolecules possessed certain degree of amphiphilicity and self-assembled into vesicles.Understanding the influences of hydrophobic hydration on conformation changes and self-assembling behaviors is desirable for deciphering the behavior of biomolecules and understanding the process of aqueous self-assembly.Thirdly,the chain conformations and self-assebling behaviors of PBLG homopolymer in THF/water with various water content were investigated.We discovered that PBLG was hydrated at low water contents and adopted a helical conformation.The chain became de-hydrated by increasing water contents because of HE,which converted the long chain PBLG helix into PPII-helix.This conformational variation resulted in an alteration of self-assembled morphologies from fibers to particles.For PBLG with a short chain,the chain underwent anα-to-βtransition in the conformation due to the dehydration as water content increases,the corresponding self-assembled morphologies varied from tapes to helical ribbons,and eventually to toroids at higher water contents.We also observed that thisα-to-βtransition is accompanied by an increase in intensity of the fluorescence,which is attributed to the through-space-conjugation of tightly packed phenyl groups within theβ-sheet.The discovered effect of hydration and dehydration on PBLG chain conformation,self-assembling behaviors and optical function are essential for the innovation of polypeptide materials and understanding of water-mediated biological systems.Finally,we studied the molecular structures and self-assembling behaviors of P2VP-b-PZLys in THF/water with certain p H values and THF/water/methanol with various methanol contents,respectively.The deprotonated P2VP in alkaline conditions results in the occurrence of hydrophobic collapse,leading to anα-to-βconformational change for PZLys.Moreover,the methanol released the side chain and enlarged the packing space between the helix.Both the two approaches lead to one similar morphological transition,where the morphology converts from sphere（SPH）to disc-shape（DIS）,and then to helical-fiber（HEL）,increasing the interfacial areas of self-assemblies.Based on the cooperative regulation,HEL was formed in various conditional solutions with different p H values pairing with certain methanol contents.The discovered influences of molecular hydrophobicity and hydrophobic hydration on molecular structures and self-assembling behaviors not only provide theoretical basis for hydrophobic effects,but also open new horizons for molecular design in the field of biological simulation.