Association of star-shaped poly(D,L-lactide)s containing nucleobase multiple hydrogen bonding

Hydrogen-Bonding Interactions from Nucleobase-Decorated Supramolecular Polymer: Synthesis, Self-Assembly and Biomedical Applications

The fabrication of supramolecular materials for biomedical applications such as drug delivery, bioimaging, wound-dressing, adhesion materials, photodynamic/photothermal therapy, infection control (as antibacterial), etc. has grown tremendously, due to their unique properties, especially the formation of hydrogen bonding. Nevertheless, void space in the integration process, lack of feasibility in the construction of supramolecular materials of natural origin in living biological systems, potential toxicity, the need for complex synthesis protocols, and costly production process limits the actual application of nanomaterials for advanced biomedical applications. On the other hand, hydrogen bonding from nucleobases is one of the strategies that shed light on the blurred deployment of nanomaterials in medical applications, given the increasing reports of supramolecular polymers that promote advanced technologies. Herein, we review the extensive body of literature about supramolecular functional biomaterials based on nucleobase hydrogen bonding pertinent to different biomedical applications. It focuses on the fundamental understanding about the synthesis, nucleobase-decorated supramolecular architecture, and novel properties with special emphasis on the recent developments in the assembly of nanostructures via hydrogen-bonding interactions of nucleobase. Moreover, the challenges, plausible solutions, and prospects of the so-called hydrogen bonding interaction from nucleobase for the fabrication of functional biomaterials are outlined.

Nucleobase-Interaction-Directed Biomimetic Supramolecular Self-Assembly

ConspectusThe design and fabrication of synthetic self-assembled systems that can mimic some biological features require exquisitely sophisticated components that make use of supramolecular interactions to attain enhanced structural and functional complexity. In nature, nucleobase interactions play a key role in biological functions in living organisms, including transcription and translation processes. Inspired by nature, scientists are progressively exploring nucleobase synthons to create a diverse range of functional systems with a plethora of nanostructures by virtue of molecular-recognition-directed assembly and flexible programmability of the base-pairing interactions. To that end, nucleobase-functionalized molecules and macromolecules are attracting great attention because of their versatile structures with smart and adaptive material properties such as stimuli responsiveness, interaction with external agents, and ability to repair structural defects. In this regard, a range of nucleobase-interaction-mediated hierarchical self-assembled systems have been developed to obtain biomimetic materials with unique properties. For example, a new "grafting to" strategy utilizing complementary nucleobase interactions has been demonstrated to temporarily control the functional group display on micellar surfaces. In a different approach, complementary nucleobase interactions have been explored to enable morphological transitions in functionalized diblock copolymer assembly. It has been demonstrated that complementary nucleobase interactions can drive the morphological transformation to produce highly anisotropic nanoparticles by controlling the assembly processes at multiple length scales. Furthermore, nucleobase-functionalized bottle brush polymers have been employed to generate stimuli-responsive hierarchical assembly. Finally, such interactions have been exploited to induce biomimetic segregation in polymer self-assembly, which has been employed as a template to synthesize polymers with narrow polydispersity. It is evident from these examples that the optimal design of molecular building blocks and precise positioning of the nucleobase functionality are essential for fabrication of complex supramolecular assemblies. While a considerable amount of research remains to be explored, our studies have demonstrated the potential of nucleobase-interaction-mediated supramolecular assembly to be a promising field of research enabling the development of biomimetic materials.This Account summarizes recent examples that employ nucleobase interactions to generate functional biomaterials by judicious design of the building blocks. We begin by discussing the molecular recognition properties of different nucleobases, followed by different strategies to employ nucleobase interactions in polymeric systems in order to achieve self-assembled nanomaterials with versatile properties. Moreover, some of their prospective biological/material applications such as enhanced drug encapsulation, superior adhesion, and fast self-healing properties facilitated by complementary nucleobase interactions are emphasized. Finally, we identify issues and challenges that are faced by this class of materials and propose future directions for the exploration of functional materials with the aim of promoting the development of nucleobase-functionalized systems to design the next generation of biomaterials.

Quadruple Hydrogen Bond-Containing A-AB-A Triblock Copolymers: Probing the Influence of Hydrogen Bonding in the Central Block

This work reveals the influence of pendant hydrogen bonding strength and distribution on self-assembly and the resulting thermomechanical properties of A-AB-A triblock copolymers. Reversible addition-fragmentation chain transfer polymerization afforded a library of A-AB-A acrylic triblock copolymers, wherein the A unit contained cytosine acrylate (CyA) or post-functionalized ureido cytosine acrylate (UCyA) and the B unit consisted of n-butyl acrylate (nBA). Differential scanning calorimetry revealed two glass transition temperatures, suggesting microphase-separation in the A-AB-A triblock copolymers. Thermomechanical and morphological analysis revealed the effects of hydrogen bonding distribution and strength on the self-assembly and microphase-separated morphology. Dynamic mechanical analysis showed multiple tan delta (δ) transitions that correlated to chain relaxation and hydrogen bonding dissociation, further confirming the microphase-separated structure. In addition, UCyA triblock copolymers possessed an extended modulus plateau versus temperature compared to the CyA analogs due to the stronger association of quadruple hydrogen bonding. CyA triblock copolymers exhibited a cylindrical microphase-separated morphology according to small-angle X-ray scattering. In contrast, UCyA triblock copolymers lacked long-range ordering due to hydrogen bonding induced phase mixing. The incorporation of UCyA into the soft central block resulted in improved tensile strength, extensibility, and toughness compared to the AB random copolymer and A-B-A triblock copolymer comparisons. This study provides insight into the structure-property relationships of A-AB-A supramolecular triblock copolymers that result from tunable association strengths.

Functional Systems Derived from Nucleobase Self-assembly

Dynamic and reversible non-covalent interactions endow synthetic systems and materials with smart adaptive functions that allow them to response to diverse stimuli, interact with external agents, or repair structural defects. Inspired by the outstanding performance and selectivity of DNA in living systems, scientists are increasingly employing Watson-Crick nucleobase pairing to control the structure and properties of self-assembled materials. Two sets of complementary purine-pyrimidine pairs (guanine:cytosine and adenine:thymine(uracil)) are available that provide selective and directional H-bonding interactions, present multiple metal-coordination sites, and exhibit rich redox chemistry. In this review, we highlight several recent examples that profit from these features and employ nucleobase interactions in functional systems and materials, covering the fields of energy/electron transfer, charge transport, adaptive nanoparticles, porous materials, macromolecule self-assembly, or polymeric materials with adhesive or self-healing ability.

Entrapment of an adenine derivative by a photo-irradiated uracil-functionalized micelle confers controlled self-assembly behavior

HYPOTHESIS

Invoking cooperative assembly of the uracil-functionalized supramolecular polymer BU-PPG [uracil end-capped poly(propylene glycol)] upon association with the nucleobase adenine derivative A-MA [methyl 3-(6-amino-9H-purin-9-yl)propanoate] as a model drug provides a new concept to control and tune the properties of supramolecular complexes and holds significant potential for the development of safer, more effective drug delivery systems.

EXPERIMENTS

BU-PPG and A-MA were successfully developed and exhibited specific recognition and high affinity, which enabled reversible complementary adenine-uracil (A-U) hydrogen bonding-induced formation of spherical micelles in aqueous solution. The self-assembly and controllable A-MA release behavior of BU-PPG/A-MA micelles were studied using morphological analysis and optical and light scattering techniques to investigate the effect of photoirradiation and temperature on the complementary hydrogen bond interactions between BU-PPG and A-MA.

FINDINGS

The resulting micelles possess unusual physical properties, including controlled photoreactivity kinetics, controllable self-assembled morphology and low cytotoxicity in vitro, as well as reversible temperature-responsive behavior. Importantly, irradiated micelles exhibited excellent long-term structural stability under normal physiological conditions and serum disturbance. Increasing the temperature triggered rapid release of A-MA by disrupting A-U complexes. These findings represent an entirely new, promising strategy for the development of multi-controlled release drug delivery nanocarriers based on complementary hydrogen bonding interactions.

Tunable Supramolecular Assemblies from Amphiphilic Nucleoside Phosphoramidate Nanofibers by Enzyme Activation

Enzymes possess unique qualities that make them ideal regulators of supramolecular assembly. They are uniquely sensitive to biomolecules and biological compartments, catalytic in effecting chemical reactions, and present a biocompatible and degradable platform for assembly regulation. We demonstrate the novel utility of Histidine Triad Nucleotide Binding Protein 1 (HINT1) in regulating supramolecular hydrogel formation. We synthesized nucleoside-phosphoramidate-functionalized self-assembling peptides that we observed to form nanofibers. We found HINT1's catalytic hydrolysis of the nucleoside phosphoramidate moieties within the nanofiber structures to induce nanofiber organization and higher ordered assembly. The role of HINT1 in effecting this structural change was confirmed with experiments utilizing a high-affinity HINT1 inhibitor and catalytically dead HINT1 mutant. In addition, the kinetics and morphology of hydrogel formation were found to be dependent on the structure of the released nucleoside monophosphate. This work highlights the self-assembly of phosphoramidate nanofibers and their higher organization triggered by HINT1 enzymatic activity.

CO2-Driven stereochemical inversion of sugars to create thymidine-based polycarbonates by ring-opening polymerisation

A strategy using CO₂ to invert the stereochemistry and cyclise natural sugars is presented and applied towards thymidine-based polymers by ring-opening polymerisation.

Lamellar mesoscopic organization of supramolecular polymers: a necessary pre-ordering secondary structure

Thy-functionalized PPGs organize in lamellae due to the alignment of amide links in a β-sheet-like secondary structure analogous to proteins.

Adenine as an organocatalyst for the ring-opening polymerization of lactide: scope, mechanism and access to adenine-functionalized polylactide

Straightforward access to a bioconjugate is reported via adenine initiated ring-opening polymerization without a co-catalyst. The mechanism is elucidated by DFT.

Nucleobase-functionalized ABC triblock copolymers: self-assembly of supramolecular architectures

RAFT polymerization afforded acrylic ABC triblock copolymers with self-complementary nucleobase-functionalized external blocks and a low-Tg soft central block. ABC triblock copolymers self-assembled into well-defined lamellar microphase-separated morphologies for potential applications as thermoplastic elastomers. Complementary hydrogen bonding within the hard phase facilitated self-assembly and enhanced mechanical performance.

Biomimetic polyorganosiloxanes: model compounds for new materials

The chemistry of N-organosilylalkyl-substituted heterocyclic bases (thymine, adenine and cytosine) is described, covering the structures of model compounds, the synthesis of substituted oligo-siloxanes and a preliminary report of the synthesis of a poly(organosiloxane) with pendant N-alkyl(heterocycle) functionalities. N-Alkenylthymines CH2=CH(CH2)(n)T (T = thymine, n = 1 (1), 2 (2), 3 (3)) have been prepared and 2 hydrosilylated to form PhMe2Si(CH2)4T (5). Alternatively, 5 was prepared by reaction of PhMe2Si(CH2)4Br (6) with (O,O-SiMe3)2T, a method which has also been used to prepare PhMe2Si(CH2)4A (7) and PhMe2Si(CH2)4C (8) (A = adenine, C = cytosine). Model di- and tri-siloxanes [Br(CH2)4(Me)2Si]2O (10), Me3SiOSi(Me)2(CH2)4Br (11), PhMe2SiOSi(Me)2(CH2)4Br (12) and (Me3SiO)2(Me)Si(CH2)4Br (13) have been prepared by hydrosilylation of H2C[double bond, length as m-dash]C(H)(CH2)4Br with an appropriate hydrosiloxane and used to prepare Me3SiO(Me)2Si(CH2)4T (14), Me3SiO(Me)2Si(CH2)4A (15) (both from 11), and (Me3SiO)2(Me)Si(CH2)4T (16), (Me3SiO)2(Me)Si(CH2)4A (17) (both from 13). 10 reacts with thymine to give a mixture of the pyrimidocyclophane cyclo-T-N,N-[(CH2)4(Me)2Si]2O (19) and [T(CH2)4Si(Me)2]2O (20), while cytosine reacts similarly to form cyclo-C-N,N-[(CH2)4(Me)2Si]2O (21; as an imine) and [C(CH2)4Si(Me)2]2O (22); adenine only generates [A(CH2)4Si(Me)2]2O (18) in an analogous synthesis. Using a related protocol, polymeric {[MeSi(O)(CH2)4Br]2[Me2SiO]98}n (23) has been converted to {[MeSi(O)(CH2)4T]2[Me2SiO]98}n (24) and {[MeSi(O)(CH2)4A]2[Me2SiO]98}n (25). The structures of 4, 5, 8, 19 and 21, along with a 2 : 1 adduct of 5 with Ni(dithiobiuret)2 (9) are reported.

A systematic investigation on biological activities of a novel double zwitterionic Schiff base Cu(II) complex

Double zwitterionic amino acid Schiff base, o-vanillylidene-L-histidine (OVHIS) and its copper complex (CuOVHIS) have been synthesized and characterized. CuOVHIS has distorted octahedral geometry, and OVHIS coordinates the copper ion in a tetradentate manner (N2O2). The pKa of OVHIS in aqueous solution was studied by potentiometric and spectrophotometric methods. DNA binding behavior of the compounds was investigated using spectrophotometric, cyclic voltammetric, and viscosity methods. The efficacy of DNA cleaving nature was tested on pUC19 DNA. The in vitro biological activity was tested against various micro organisms. The effect of CuOVHIS on the surface feature of Escherichia coli was analyzed by SEM. DPPH assay studies revealed that CuOVHIS has higher antioxidant activity. OVHIS inhibits proliferation of HCT117 cells with half maximal inhibition (IC50) of 71.15±0.67. Chelation of OVHIS with Cu(II) ion enhances the inhibition of proliferation action (IC50=53.14±0.67).

Synthesis, characterization and biological evaluation of paeonol thiosemicarbazone analogues as mushroom tyrosinase inhibitors

A series of hydroxy- and methoxy-substituted paeonol thiosemicarbazone analogues were synthesized as potential tyrosinase inhibitors and their inhibitory effects on mushroom tyrosinase and inhibitory mechanism were evaluated. Paeonol thiosemicarbazone analogues have been found exhibiting more remarkable inhibition than their indexcompounds on mushroom tyrosinase. Among them, compound 2,4-dihydroxy acetophenone-4-phenyl-3-thiosemicarbazone (d1) had the most potent inhibition activity with the IC50 value of 0.006 ± 0.001 mM, displayed as a reversible competitive inhibitor. The inhibitory ability of o- or p-substituted acetophenone thiosemicarbazones was: di-substituted acetophenone thiosemicarbazones>mono-substituted acetophenone thiosemicarbazones>non-substituted acetophenone thiosemicarbazones. Copper ions chelation assay explained that compound d1 exhibited competitive inhibition by forming a chelate with the copper ions at the catalytic domain of tyrosinase as well as indicate a 1.5:1 binding ratio of compound d1 with copper ions. In the fluorescence spectrum study, compound d1 behaved stronger fluorescence quenching on tyrosinase towards d1-Cu(2+) complex, inhibiting tyrosinase mainly by means of chelating the two copper ions in the active site. The newly synthesized compounds may serve as structural templates for designing and developing novel tyrosinase inhibitors.

Syntheses of aliphatic polycarbonates from 2'-deoxyribonucleosides

Poly(2'-deoxyadenosine) and poly(thymidine) constructed of carbonate linkages were synthesized by polycondensation between silyl ether and carbonylimidazolide at the 3'- and 5'-positions of the 2'-deoxyribonucleoside monomers. The N-benzoyl-2'-deoxyadenosine monomer afforded the corresponding polycarbonate together with the cyclic oligomers. However, the deprotection of the N-benzoyl group resulted in the scission of the polymer main chain. Thus, the N-unprotected 2'-deoxyadenosine monomers were examined for polycondensation. However, there was involved the undesired reaction between the adenine amino group and the carbonylimidazolide to form the carbamate linkage. In order to exclude this unfavorable reaction, dynamic protection was employed. Strong hydrogen bonding was used in place of the usual covalent bonding for reducing the nucleophilicity of the adenine amino group. Herein, 3',5'-O-diacylthymidines that form the complementary hydrogen bonding with the adenine amino group were added to the polymerization system of the N-unprotected 2'-deoxyadenosine monomer. Consequently, although the oligomers (M(n) = 1000-1500) were produced, the contents of the carbamate group were greatly reduced. The dynamic protection reagents were easily and quantitatively recovered as the MeOH soluble parts from the polymerization mixtures. In the polycondensation of the thymidine monomer, there tended to be involved another unfavorable reaction of carbonate exchange, which consequently formed the irregular carbonate linkages at not only the 3'-5' but also the 3'-3' and 5'-5' positions. Employing the well-designed monomer suppressed the carbonate exchange reaction to produce poly(thymidine) with the almost regular 3'-5'carbonate linkages.

Hydrogen-bond-guided self-assembly of nucleotides on a receptor-array surface

The hydrogen-bond-guided self-assembly of 5'-ribonucleotides bearing adenine(A), cytosine (C), uracil (U), or guanine (G) bases from aqueous solution on a lipid-like surface decorated with synthetic bis(Zn(II)-cyclen) (cyclen=1,4,7,10-tetraazacyclodododecane) metal-complex receptor sites is described. The process was studied by using surface plasmon resonance spectroscopy. The data show that the mechanism of nucleotide binding to the 2D template is influenced by the chemistry of the bases and the pH value of the solution. In a neutral solution of pH 7.5, the process is cooperative and selective with respect to Watson-Crick pairs (A-U and C-G), which form stable double planes in accordance with the Chargaff rule. In a more acidic solution at pH 6.0, the interactions between complementary partners become non-cooperative and the surface also stabilizes mismatched and wobble pairs due to the pH-induced changes in the receptor coordination state. The results suggest that hydrogen bonding plays a key role in the self-assembly of complementary nucleotides at the lipid-like interface, and the cooperative character of the process stems from the ideal matching of the orientation and chemistry of all the interacting components with respect to each other in neutral solution.

Aliphatic polyester polymer stars: synthesis, properties and applications in biomedicine and nanotechnology

A critical review: the ring-opening polymerization of cyclic esters provides access to an array of biodegradable, bioassimilable and renewable polymeric materials. Building these aliphatic polyester polymers into larger macromolecular frameworks provides further control over polymer characteristics and opens up unique applications. Polymer stars, where multiple arms radiate from a single core molecule, have found particular utility in the areas of drug delivery and nanotechnology. A challenge in this field is in understanding the impact of altering synthetic variables on polymer properties. We review the synthesis and characterization of aliphatic polyester polymer stars, focusing on polymers originating from lactide, ε-caprolactone, glycolide, β-butyrolactone and trimethylene carbonate monomers and their copolymers including coverage of polyester miktoarm star copolymers. These macromolecular materials are further categorized by core molecules, catalysts employed, self-assembly and degradation properties and the resulting fields of application (262 references).