Disassembly Kinetics of Quinone‐Methide‐Based Self‐Immolative Spacers that Contain Aromatic Nitrogen Heterocycles

We prepared several pyridine‐ and pyrimidine‐based self‐immolative spacer groups to evaluate the significance of the resonance energy of the spacer aromatic ring on the kinetics of 1,4‐ and 1,6‐elimination reactions, which govern spacer disassembly. Subsequently, we relied on a photoactivation procedure to accurately analyze the disassembly kinetics. Beyond providing new results that are relevant for deriving quantitative structure–property relationships, herein, we demonstrate that pH value can be used as an efficient parameter to finely control the disassembly time of a self‐immolative spacer after an initial activation.     

Self‐Immolative Spacers: Kinetic Aspects,Structure–Property Relationships,and Applications

Self‐immolative spacers are covalent assemblies tailored to correlate the cleavage of two chemical bonds after activation of a protective part in a precursor: Upon stimulation, the protective moiety is removed, which generates a cascade of disassembling reactions leading to the temporally sequential release of smaller molecules. Originally introduced to overcome limitations for drug delivery, self‐immolative spacers have gained wide interest in medicinal chemistry, analytical chemistry, and material science. For most applications, the kinetics of the disassembly of the activated self‐immolative spacer governs functional properties. This Review addresses kinetic aspects of self‐immolation. It provides information for selecting a particular self‐immolative motif for a specific demand. Moreover, it should help researchers design kinetic experiments and fully exploit the rich perspectives of self‐immolative spacers.     

Novel Biodegradable Polymer with Redox‐Triggered Backbone Cleavage Through Sequential 1,6‐Elimination and 1,5‐Cyclization Reactions

In the past decade, the self‐immolative biodegradable polymer arose as a novel paradigm for its efficient degradation mechanism and vast potential for advanced biomedical applications. This study reports successful synthesis of a novel biodegradable polymer capable of self‐immolative backbone cleavage. The monomer is designed by covalent conjugations of both pendant redox‐trigger (p‐nitrobenzyl alcohol) and self‐immolative linker (p‐hydroxybenzyl alcohol) to the cyclization spacer (n‐2‐(hydroxyethyl)ethylene diamine), which serves as the structural backbone. The polymerization of the monomer with hexamethylene diisocyanate yields a linear redox‐sensitive polymer that can systemically degrade via sequential 1,6‐elimination and 1,5‐cyclization reactions within an effective timeframe. Ultimately, the polymer's potential for biomedical application is simulated through in vitro redox‐triggered release of paclitaxel from polymeric nanoparticles.     

Fast Cyclization of a Proline‐Derived Self‐Immolative Spacer Improves the Efficacy of Carbamate Prodrugs

Self‐immolative (SI) spacers are sophisticated chemical constructs designed for molecular delivery or material degradation. We describe herein a (S)‐2‐(aminomethyl)pyrrolidine SI spacer that is able to release different types of anticancer drugs (possessing either a phenolic or secondary and tertiary hydroxyl groups) through a fast cyclization mechanism involving carbamate cleavage. The high efficiency of drug release obtained with this spacer was found to be beneficial for the in vitro cytotoxic activity of protease‐sensitive prodrugs, compared with a commonly used spacer of the same class. These findings expand the repertoire of degradation machineries and are instrumental for the future development of highly efficient delivery platforms.     

Self‐Immolative Linkers as Caps for the Design of Gated Silica Mesoporous Supports

A new hybrid material based on sulforhodamine B dye‐loaded silica mesoporous nanoparticles capped with a self‐immolative gate has been synthesized and characterized. The gated material's controlled release behavior is monitored under different pH conditions. Under acidic and neutral conditions, a low level of dye release is detected. However, at slightly basic pH, significant dye release occurs owing to deprotonation of the phenol moiety in the capping molecule, which results in its disassembly.     

Amine‐Responsive Disassembly of AuI–CuI Double Salts for Oxidative Carbonylation

A sensitive amine‐responsive disassembly of self‐assembled Au^I‐Cu^I double salts was observed and its utilization for the synergistic catalysis was enlightened. Investigation of the disassembly of [Au(NHC)₂][CuI₂] revealed the contribution of Cu‐assisted ligand exchange of N‐heterocyclic carbene (NHC) by amine in [Au(NHC)₂]⁺ and the capacity of [CuI₂]^? on the oxidative step. By integrating the implicative information coded in the responsive behavior and inherent catalytic functions of d¹⁰ metal complexes, a catalyst for the oxidative carbonylation of amines was developed. The advantages of this method were clearly reflected on mild reaction conditions and the significantly expanded scope (51 examples); both primary and steric secondary amines can be employed as substrates. The cooperative reactivity from Au and Cu centers, as an indispensable prerequisite for the excellent catalytic performance, was validated in the synthesis of (un)symmetric ureas and carbamates.     

Photoinduced Electron Transfer in Zinc Naphthalocyanine–Naphthalenediimide Supramolecular Dyads

Photoinduced electron transfer was studied in self‐assembled donor–acceptor dyads, formed by axial coordination of pyridine appended with naphthalenediimide (NDI) to zinc naphthalocyanine (ZnNc). The NDI‐py:ZnNc ( 1 ) and NDI(CH₂)₂‐py:ZnNc ( 2 ) self‐assembled dyads absorb light over a wide region of the UV/Vis/near infrared (NIR) spectrum. The formation constants of the dyads 1 and 2 in toluene were found to be 2.5×10⁴ and 2.2×10⁴ M ^?1, respectively, from the steady‐state absorption and emission measurements, suggesting moderately stable complex formation. Fluorescence quenching was observed upon the coordination of the pyridine‐appended NDI to ZnNc in toluene. The energy‐level diagram derived from electrochemical and optical data suggests that exergonic charge separation through the singlet state of ZnNc (¹ZnNc*) provides the main quenching pathway. Clear evidence for charge separation from the singlet state of ZnNc to NDI was provided by femtosecond laser photolysis measurements of the characteristic absorption bands of the ZnNc radical cation in the NIR region at 960 nm and the NDI radical anion in the visible region. The rates of charge‐separation of 1 and 2 were found to be 2.2×10¹⁰ and 4.4×10⁹ s^?1, respectively, indicating fast and efficient charge separation (CS). The rates of charge recombination (CR) and the lifetimes of the charge‐separated states were found to be 8.50×10⁸ s^?1 (1.2 ns) for 1 and 1.90×10⁸ s^?1 (5.3 ns) for 2 . These values indicate that the rates of the CS and CR processes decrease as the length of the spacer increases. Their absorption over a wide portion of the solar spectrum and the high ratio of the CS/CR rates suggests that the self‐assembled NDI‐py:ZnNc and NDI(CH₂)₂‐py:ZnNc dyads are useful as photosynthetic models.     

Cooperative Binding of Divalent Diamides by N‐Alkyl Ammonium Resorcinarene Chlorides

N‐Alkyl ammonium resorcinarene chlorides, stabilized by an intricate array of hydrogen bonds leading to a cavitand‐like structure, bind amides. The molecular recognition occurs through intermolecular hydrogen bonds between the carbonyl oxygen and the amide hydrogen of the guests and the cation–anion circular hydrogen‐bonded seam of the hosts, as well as through CH ??? π interactions. The N‐alkyl ammonium resorcinarene chlorides cooperatively bind a series of di‐acetamides of varying spacer lengths ranging from three to seven carbons. Titration data fit either a 1:1 or 2:1 binding isotherm depending on the spacer lengths. Considering all the guests possess similar binding motifs, the first binding constants were similar (K₁: 10² M ^?1) for each host. The second binding constant was found to depend on the upper rim substituent of the host and the spacer length of the guests, with the optimum binding observed with the six‐carbon spacer (K₂: 10³ M ^?2). Short spacer lengths increase steric hindrance, whereas longer spacer lengths increase flexibility thus reducing cooperativity. The host with the rigid cyclohexyl upper rim showed stronger binding than the host with flexible benzyl arms. The cooperative binding of these divalent guests was studied in solution through ¹H NMR titration studies and supplemented by diffusion‐ordered spectroscopy (DOSY), X‐ray crystallography, and mass spectrometry.     

Cooperative Supramolecular Polymerization: Comparison of Different Models Applied on the Self‐Assembly of Bis(merocyanine) Dyes

Three new molecular building blocks 1 a – c for supramolecular polymerization are described that feature two dipolar merocyanine dyes tethered by p‐xylylene spacers. Concentration‐ and temperature‐dependent UV/Vis spectroscopy in chloroform combined with dynamic light scattering, capillary viscosimetry and atomic force microscopy investigations were applied to elucidate the mechanistic features of the self‐assembly of these strongly dipolar dyes. Our detailed studies reveal that the self‐assembly is very pronounced for bis(merocyanines) 1 a , b bearing linear alkyl chains, but completely absent for bis(merocyanine) 1 c bearing sterically more bulky ethylhexyl substituents. Both temperature‐ and concentration‐dependent UV/Vis data provide unambiguous evidence for a cooperative self‐assembly process for bis(merocyanines) 1 a , b , which was analyzed in detail by the Meijer–Schenning–Van‐der‐Schoot model (applicable to temperature‐dependent data) and by the Goldstein–Stryer model (applicable to concentration‐dependent data). By combining both methods all parameters of interest to understand the self‐assembly process could be derived, including in particular the nucleus size (8–10 monomeric units), the cooperativity factor (ca. 0.006), and the nucleation and elongation constants of about 10³ and 10⁶ M ^?1 in chloroform at room temperature, respectively.     

Self‐Assembly and Disassembly of Vesicles as Controlled by Anion–π Interactions

Anion–π interactions have been widely studied as new noncovalent driving forces in supramolecular chemistry. However, self‐assembly induced by anion–π interactions is still largely unexplored. Herein we report the formation of supramolecular amphiphiles through anion–π interactions, and the subsequent formation of self‐assembled vesicles in water. With the π receptor 1 as the host and anionic amphiphiles, such as sodium dodecylsulfate (SDS), sodium laurate (SLA), and sodium methyl dodecylphosphonate (SDP), as guests, the sequential formation of host–guest supramolecular amphiphiles and self‐assembled vesicles was demonstrated by SEM, TEM, DLS, and XRD techniques. The intrinsic anion–π interactions between 1 and the anionic amphiphiles were confirmed by crystal diffraction, HRMS analysis, and DFT calculations. Furthermore, the controlled disassembly of the vesicles was promoted by competing anions, such as NO₃^?, Cl^?, and Br^?, or by changing the pH value of the medium.     

Self‐immolative dendrimers as novel drug delivery platforms

Self‐immolative dendrimers were recently developed and introduced as a potential platform for a single‐triggered multi‐prodrug. These unique structural dendrimers can release all of their tail units through domino‐like chain fragmentation, which is initiated by a single cleavage at the dendrimer core. The incorporation of drug molecules as the tail units and an enzyme substrate as the trigger generates a multi‐prodrug unit that is activated with a single enzymatic cleavage. We have demonstrated several examples of self‐immolative dendritic prodrug systems and have shown significant advantages with respect to the appropriate monomeric prodrug. We anticipate that single‐triggered, dendritic prodrugs will be exploited to further improve selective chemotherapeutic approaches in cancer therapy. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1569–1578, 2006     

Amphiphilic diblock copolymers bearing pendant aromatic acetal groups: Synthesis and tunable pH‐triggered assembly/disassembly transition behavior

A novel aromatic acetal‐based acid‐labile monomer 2‐phenyl‐5‐ethyl‐5‐acryloxymethyl‐1,3‐dioxacyclohexane (HEDPA) was synthesized and polymerized by reversible addition fragmentation chain transfer (RAFT) polymerization using alkynyl functional chain transfer agent (CTA‐Alk). Afterward, a series of amphiphilic diblock copolymers composed of fixed hydrophobic poly(2‐phenyl‐5‐ethyl‐5‐acryloxymethyl‐1,3‐dioxacyclohexane) (PDAEP) segments and various lengths of hydrophilic mPEG segments were prepared through click reaction between alkynyl‐terminated PDAEP and azido‐terminated mPEG. The self‐assembly behaviors of the diblock copolymers were investigated by dynamic light scattering (DLS), transmission electron microscopy (TEM), fluorescence spectroscopy, and ¹H NMR. These results indicated that the diblock copolymers could self‐assemble into nano‐sized micelles with PDAEP cores and PEG coronas in aqueous solution. DLS, fluorescence spectroscopy and UV–vis spectroscopy were used to monitor the pH‐triggered assembly/disassembly transition of the micelles. These results showed that the assembly/disassembly transition behaviors of the diblock copolymers micelles can be adjusted by changing the lengths of the mPEG segments. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 1537–1547     

Self‐Immolative Activation of β‐Galactosidase‐Responsive Probes for In Vivo MR Imaging in Mouse Models

Our lab has developed a new series of self‐immolative MR agents for the rapid detection of enzyme activity in mouse models expressing β‐galactosidase (β‐gal). We investigated two molecular architectures to create agents that detect β‐gal activity by modulating the coordination of water to Gd^III. The first is an intermolecular approach, wherein we designed several structural isomers to maximize coordination of endogenous carbonate ions. The second involves an intramolecular mechanism for q modulation. We incorporated a pendant coordinating carboxylate ligand with a 2, 4, 6, or 8 carbon linker to saturate ligand coordination to the Gd^III ion. This renders the agent ineffective. We show that one agent in particular (6‐C pendant carboxylate) is an extremely effective MR reporter for the detection of enzyme activity in a mouse model expressing β‐gal.     

Spacer‐length‐dependent association in polymers with multiple‐hydrogen‐bonded end groups

Herein, we investigate the influence of spacer length on the homoassociation and heteroassociation of end‐functionalized hydrogen‐bonding polymers based on poly(n‐butyl acrylate). Two monofunctional ureido‐pyrimidinone (UPy) end‐functionalized polymers were prepared by atom transfer radical polymerization using self‐complementary UPy‐functional initiators that differ in the spacer length between the multiple‐hydrogen‐bonding group and the chain initiation site. The self‐complementary binding strength (K_dim) of these end‐functionalized polymers was shown to depend critically on the spacer length as evident from ¹H NMR and diffusion‐ordered spectroscopy. In addition, the heteroassociation strength of the end‐functionalized UPy polymers with end‐functionalized polymers containing the complementary 2,7‐diamido‐1,8‐naphthyridine (NaPy) hydrogen‐bond motif is also affected when the aliphatic spacer length is too short. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Sugar‐Decorated Sugar Vesicles: Lectin–Carbohydrate Recognition at the Surface of Cyclodextrin Vesicles

An artificial glycocalix self‐assembles when unilamellar bilayer vesicles of amphiphilic β‐cyclodextrins are decorated with maltose and lactose by host–guest interactions. To this end, maltose and lactose were conjugated with adamantane through a tetra(ethyleneglycol) spacer. Both carbohydrate–adamantane conjugates strongly bind to β‐cyclodextrin (K_a≈4×10⁴ M ^?1). The maltose‐decorated vesicles readily agglutinate (aggregate) in the presence of the lectin concanavalin A, whereas the lactose‐decorated vesicles agglutinate in the presence of peanut agglutinin. The orthogonal multivalent interaction in the ternary system of host vesicles, guest carbohydrates, and lectins was investigated by using isothermal titration calorimetry, dynamic light scattering, UV/Vis spectroscopy, and cryogenic transmission electron microscopy. It was shown that agglutination is reversible, and the noncovalent interaction can be suppressed and eliminated by the addition of competitive inhibitors, such as D ‐glucose or β‐cyclodextrin. Also, it was shown that agglutination depends on the surface coverage of carbohydrates on the vesicles.     

Concurrent Block Copolymer Polymersome Stabilization and Bilayer Permeabilization by Stimuli‐Regulated “Traceless” Crosslinking

The fabrication of block copolymer (BCP) vesicles (polymersomes) exhibiting synchronized covalent crosslinking and bilayer permeabilization remains a considerable challenge as crosslinking typically leads to compromised membrane permeability. Herein it is demonstrated how to solve this dilemma by employing a stimuli‐triggered crosslinking strategy with amphiphilic BCPs containing photolabile carbamate‐caged primary amines. Upon self‐assembling into polymersomes, light‐triggered self‐immolative decaging reactions release primary amine moieties and extensive amidation reactions then occur due to suppressed amine pK_a within hydrophobic milieu. This leads to serendipitous vesicle crosslinking and the process is associated with bilayer hydrophobicity‐to‐hydrophilicity transition and membrane permeabilization.     

The Methylene Alkoxy Carbamate Self‐Immolative Unit: Utilization for the Targeted Delivery of Alcohol‐Containing Payloads with Antibody–Drug Conjugates

A strategy for the conjugation of alcohol‐containing payloads to antibodies has been developed and involves the methylene alkoxy carbamate (MAC) self‐immolative unit. A series of MAC β‐glucuronide model constructs were prepared to evaluate stability and enzymatic release, and the results demonstrated high stability at physiological pH in a substitution‐dependent manner. All the MAC model compounds efficiently released alcohol drug surrogates under the action of β‐glucuronidase. To assess the MAC technology for ADCs, the potent microtubule‐disrupting agent auristatin E (AE) was incorporated through the norephedrine alcohol. Conjugation of the MAC β‐glucuronide AE drug linker to the anti‐CD30 antibody cAC10, and an IgG control antibody, gave potent and immunologically specific activities in vitro and in vivo. These studies validate the MAC self‐immolative unit for alcohol‐containing payloads within ADCs, a class that has not been widely exploited.     

Synthesis and characterization of photosensitive liquid crystalline poly(benzylidene‐ether)s with alkanones and methylene spacers in the main chain

Photosensitive main chain liquid crystalline poly (benzylidene‐ether)s were synthesized by Claisen‐Schmidt polycondensation reaction of 4,4′‐Diformyl‐α,ω‐diphenoxyalkanes with acetone, cyclopentanone and cyclohexanone. The diformyl precursors were synthesized from 4‐hydroxybenzaldehyde and dibromoalkanes of varying spacer lengths. The structure of the monomers and polymers was confirmed by ELEM ANAL , Fourier transform infrared, ¹H NMR and ¹³C NMR spectral analyses. Molecular weight of the polymers was determined by gel permeation chromatography and was found to be moderate. The thermogravimetric analysis data revealed that the polymers were stable up to 280 °C and start degrading thereafter. Cyclopentanone containing polymers are more stable than acetone and cyclohexanone containing polymers. The self‐extinguishing properties of the synthesized polymers were studied by the determination of the limiting oxygen index values with Van Krevelen's equation. The influence of the length of methylene spacer on phase transition was investigated with differential scanning calorimetry and proved that the isotropic temperature decreases as the spacer length increases. Polarized optical microscopic study showed that cyclohexanone containing polymer IIIb exhibit nematic schlieren texture. Crystallinity of the polymers was studied with X‐ray diffractograms. The photolysis of liquid crystalline poly(benzylidene‐ether)s revealed that the entgegen, zusammen (EZ) photoisomerization proceeds in the system. Fluorescence spectra showed violet and green emission maxima. The band gap energy is calculated from absorption spectra are in the range of 3.17–3.40 eV. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Design and Sensing Properties of a Self‐Assembled Supramolecular Oligomer

Supramolecular polymers are a class of macromolecules stabilized by weak non‐covalent interactions. These self‐assembled aggregates typically undergo stimuli‐induced reversible assembly and disassembly. They thus hold great promise as so‐called functional materials. In this work, we present the design, synthesis, and responsive behavior of a short supramolecular oligomeric system based on two hetero‐complementary subunits. These “monomers” consist of a tetrathiafulvalene‐functionalized calix[4]pyrrole (TTF‐C[4]P) and a glycol diester‐linked bis‐2,5,7‐trinitrodicyanomethylenefluorene‐4‐carboxylate (TNDCF), respectively. We show that when mixed in organic solvents, such as CHCl₃, CH₂ClCH₂Cl, and methylcyclohexane, supramolecular aggregation takes place to produce short oligomers stabilized by hydrogen bonding and donor–acceptor charge‐transfer (CT) interactions. The self‐associated materials were characterized by ¹H NMR and UV/Vis/NIR absorption spectroscopy, as well as by concentration‐ and temperature‐dependent absorption spectroscopy and dynamic light scattering (DLS) analyses of both the monomeric and oligomerized species. The self‐associated system produced from TTF‐C[4]P and TNDCF exhibits a concentration‐dependent aggregation behavior typical of supramolecular polymers. Further support for the proposed self‐assembly came from theoretical calculations. The fluorescence emitting properties of TNDCF are quenched under conditions that promote the formation of supramolecular aggregates containing TTF‐C[4]P and TNDCF. This quenching effect has been utilized as a probe for the detection of substrates in the form of anions (i.e., chloride) and nitroaromatic explosives (i.e., 1,3,5‐trinitrobenzene). Specifically, the addition of these substrates to mixtures of TTF‐C[4]P and TNDCF produced a fluorescence “turn‐on” response.     

Fluorescent Cross‐Linked Supramolecular Polymer Constructed by Orthogonal Self‐Assembly of Metal–Ligand Coordination and Host–Guest Interaction

A new host molecule consists of four terpyridine groups as the binding sites with zinc(II) ion and a copillar[5]arene incorporated in the center as a spacer to interact with guest molecule was designed and synthesized. Due to the 120 ° angle of the rigid aromatic segment, a cross‐linked dimeric hexagonal supramolecular polymer was therefore generated as the result of the orthogonal self‐assembly of metal–ligand coordination and host–guest interaction. UV/Vis spectroscopy, ¹H NMR spectroscopy, viscosity and dynamic light‐scattering techniques were employed to characterize and understand the cross‐linking process with the introduction of zinc(II) ion and guest molecule. More importantly, well‐defined morphology of the self‐assembled supramolecular structure can be tuned by altering the adding sequence of the two components, that is, the zinc(II) ion and the guest molecule. In addition, introduction of a competitive ligand suggested the dynamic nature of the supramolecular structure.