Molecular Engineering of DNA-inspired Janus base nanomaterials

Introduction

With the development of DNA nanotechnology, Janus base nanomaterials are a novel family of non-covalent and DNA-inspired nanostructures. They self-assembled from small-molecule building blocks derived from G^C or A^T DNA base pairs. Scientists have made a lot of progress toward the design, synthesis, engineering and applications of Janus base nanomaterials. Here, we will give a brief review on the development and synthesis of Janus base nanomaterials.

Development of Janus Molecules

Millions of years ago, Janus Kinases

JAK, “just another kinase”, were discovered in a PCR-based screen of kinases in 1989 [1]. They possess two near-identical phosphate-transferring domains. One domain exhibits the kinase activity, while the other negatively regulates the kinase activity of the first. This architecture gives JAK another name “Janus kinase” taken from the two-faced Roman god Janus.

1980s, Janus-Wedge molecules

Janus-Wedge molecules were designed to bind by inserting between two nucleobases to form a base-wedge-base triad motif [2]. They can recognize either matched or unmatched base pair by maximizing all Watson-Crick H-bonds. Janus-Wedge PNA (Peptide Nucleic Acid) could be used to form DNA/RNA triplex for sequence-specific gene silence or DNA/RNA-protein interaction.

1990s, 2D Janus planes

In 1992, Dr. Whitesides and co-workers reported solid-state structures of crinkled tape and rosette obtained by combination of N,N’-bis(p-substituted phenyl)melamine and 5,5-diethylbarbituric acid [3]. By varying X groups of melamine, the crystalline architecture changes due to the competition between steric interactions of the substituents and a tendency for a high packing coefficient in the crystal.

In 1999, Dr. Mascal and co-workers synthesized a DNA base hybrid in 5 steps from the malononitrile dimer [4]. One face of the molecule acts as cytosine and the other guanine. Situated at 120° angles to each other, the H-bond pattern leads to the self-assembly of six molecules into a rosette in the solid state through the formation of 18 strong H-bonds.

After 2000, 3D Janus base nanotubes

In 2001, Dr. Fenniri and co-workers reported the helical rosette nanotubes [5]. The module is composed by three building blocks: 1. The hydrophobic guanine-cytosine base unit makes G^C module form a six-membered rosette; 2. The ethylene tether links the base unit to the chiral center which allows intramolecular ionic H-bond; 3. The amino acid moiety dictates the supramolecular chirality. This tri-block design drives G^C module self-assembly into helical nanotubular architectures in water. The authors also synthesized 3 other modules to demonstrate the essential role of Janus base and side chain’s chirality in the assembly of the tubular architectures.

In 2010s, Drs. Yu and Chens synthesized adenine-thymine base module [6], in which adenine face was modified with 1 more H-bond donor. The authors successfully applied the nanotubes on delivery of nucleic acids, drugs into cells or bodily tissues.

Synthesis of Janus Molecules

Fenniri and co-workers’ strategy started from Vilsmeier reaction of commercially available barbituric acid 1 to obtain compound 2. Allylamine, methylamine, benzylalcohol were introduced into 3 electrophilic sites sequentially and regioselectively which afforded compound 3. After the methylamine was Boc-protected, aldehyde was converted to nitrile via an oxime intermediate. Compound 4 was treated with chlorocarbonyl isocyanate after which 7 M NH₃ in methanol was added to form the second ring. The resulting amine 5 was protected, then oxidative cleavage of the double bond was achieved by the 2-step procedure using OsO₄/NMMO and NaIO₄. Reductive amination of compound 6 with L-lysine derived side chain 7 followed by global deprotection using TFA/thioanisole provided G^C module in an overall 14 steps.

Drs. Yu and Chens synthesis began with the condensation of cyanoacetic acid 8 and ethylcarbamate 9 which yielded compound 10. Treatment of 10 with carbon disulfide and methyl iodide afforded 11. Compound 11 and allylamine were refluxed in ethanol which closed the first ring. The second ring was formed by refluxing compound 12 with guanidine. After primary amines and imide were all Boc-protected, following oxidative cleavage, reductive amination, global deprotection provided A^T module in an overall 10 steps.

Applications and Future Perspective

One promising application of Janus base nanomaterial is for drug delivery. Previously, various nanostructures, including liposomes, polymers, carbon nanotubes and magnetic nanoparticles have been used as carriers in drug delivery systems. However, clinical use of such structures has often been hampered by undesirable or toxic side effects. Compared with conventional delivery vehicles, the non-covalent Janus base nanomaterials have much better biocompatibility and biodegradability. Subsequently, they present low cytotoxicity and low immunogenicity due to their biological nature. Thus, they can be designed to incorporate and deliver therapeutic agents such as anticancer drugs, RNAs, small peptides and protein molecules [7–12]. Moreover, Janus bases can be engineered into versatile nanostructures and nanodevices for tissue engineering and regenerative medicine [13–14]. As a summary, these DNA-inspired Janus base nanostructures will be a family of safe, versatile and effective nanomaterials for biomedical applications.

Figure 1.

History of Janus base molecules

Figure 2.

Synthetic route of G^C module

Figure 3.

Synthetic route of A^T module