Chemical Stimulus‐responsive Folding and Unfolding of a Dendrimeric Assembly

Abstract

Ad endrimeric trimer undergoes folding and unfolding in response to ac hemical stimulus. The trimer of interest contains ac entral dendrimer with ab utadiynelinked zinc porphyrin dimer ((ZnP) 2 )c ore, in addition to two terminal dendrimers with zinc porphyrin (ZnP) cores. The obtaineda bsorption spectra indicate that the unfolded form is the exclusive conformer in chloroform, while the addition of 1,4-diazabicyclo[2.2.2]octane (DABCO) in chloroform leads to transformation from the unfolded to the folded structure containing two DABCO units per trimer;t he folded structure originates from the cross-linking of (ZnP) 2 and ZnP with DABCO. Moreover,t he addition of excess DABCO promotes the generation of the unfolded structure containing four DABCO units.The folding of proteinsi sk nown to create organized three-dimensional structurest hat are essential for imparting aw ide range of biological functions. This process can also be simulated by the cross-linkage of single-chain polymers to produce discrete nanoscale macromolecules, such as organic nanoparticles (ONPs).[1] However,t he application of such methods to the construction of organized arrays of functional units is challenging due to structural diversity at the molecular level. To date, dendrimers have attracted attention as building blocks for ONPs, as the well-defineds tructures of dendrimers allow the precise arrangemento ff unctional units.[2] Indeed,w ep reviously reported au nique method for the construction of ONPs using dendrimers with rigid backbones. [3] In this method, dendrimersw ere linearly connected using alkyl chains and the self-folding of the resulting assembly produced ONPs with rigid backbones. Although such rigid backbonesa ppear to be au seful framework for the precise arrangement of functional units in ONPs,t he random aggregation of these backbones limits their utility.In contrast, folding of the dendrimer assembly through the cross-linking of rigid backbones is am ore sophisticated approach to create ONPs since the rigid cross-linkers can produce an extended backbone with ap redictable structure (Figure 1a). Thus, we herein report the preparation of at rimer of dendrimers containing ab utadiyne-linked zinc porphyrin dimer( (ZnP) 2 ) as the core of the central dendrimer,i na ddition to two terminal dendrimers containing zinc porphyrin (ZnP) cores (Figure 1b). Such porphyrins are considered particularly valuable for the formation of noncovalentc ross-linkages.[4] We expect that upon the addition of suitable bidentate ligands as crosslinkers, the trimer will undergo folding and unfolding as shown in Figure 1a.I na ddition, ZnP and (ZnP) 2 can be independently monitored by spectroscopicm easurements, thereby allowing the folding and unfolding processes to be disclosed. [5] As such, we will employ spectroscopic techniques to examine trimerf olding and unfolding in response to ac hemical stimulus.Trimer 1 andd endrimers 2 and 3 were synthesized as outlined in Scheme1.F ollowing the initial reaction of two hydroxyl groups in 2a wi...

Ligand-Binding Ability of a Porphyrin Core in a Dendrimer with Rigid Branched Terminal Components

“…The syntheses of dendrimers 2 and 3 are summarized in Schemes and , respectively. Dendrons 4 and 5 were prepared according to our reported procedures . Dendrimer 6 was prepared in 59% yield by copper-free Sonogashira coupling reactions between 4 and 5 .…”

“…Unfortunately, intense broad signals of aromatic protons in the branching chains overlapped the singlet of the triazole proton (H g ) that should be observed around 7.5 ppm . The singlet at 5.10 ppm and broad triplet at 4.33 ppm were reasonably assigned to the methylene protons H e and H f , respectively. , MALDI-TOF MS of 1 clearly showed the molecular ion peaks at m / z = 12042 (calcd for C 844 H 830 N 10 O 54 Zn, 12032) (Figure ). The cleavage of the ether bond in the branching chains gave a series of fragment ions.…”

“…Recently, we prepared dendrimers with branched oligomers that adopt folded or unfolded higher order structures depending on the temperature and solvent used (Scheme ). Moreover, the higher order structure of a dendrimer with four porphyrin units and branched oligomers was successfully controlled by adding a small bridging ligand as an external stimulus . When a functional site is introduced at the center of the dendrimers with branched oligomers, the stimulus-responsive transformation of the higher order structure is often accompanied by a significant change in function.…”

Inhibition of Ligand Binding Ability of Three Porphyrins by an Organic Effector

“…It has been established that axial coordination of ligandsw ith zinc porphyrins produces absorption bands in longer wavelength region compared to the corresponding bands of ligand-free zinc porphyrins. [11] The observeda bsorption change indicates the axial coordination of PYo nt he zinc atom of P1. On the other hand, blue shift and ad ecrease in intensity were observed at l max = 655 nm which is assigned to P2, indicating the interaction betweent he shielding panel and P2.…”

“…[6] The coordi-nation between PY and P2 is insignificant under these conditions because of the negligible intensity of ac haracteristic band in the wavelength region longert han 655 nm by the axial coordination. [11] The binding constanto f1·2 was evaluated as K 1·2 = 6.73 AE 0.67 10 6 m À1 from the globalc urve-fitting by the theoretical curveso btained from the 1:1b inding model (1 + 2Q1·2). [12] These results indicate the nearly quantitative formation of 1·2 under these conditions.…”