Cross-catalytic peptide nucleic acid (PNA) replication based on templated ligation

Shorter Peptide Nucleic Acid Probes Improve Affibody-Mediated Peptide Nucleic Acid-Based Pretargeting

Affibody-mediated PNA-based pretargeting shows promise for HER2-expressing tumor radiotherapy. In our recent study, a 15-mer ZHER2:342-HP15 affibody-PNA conjugate, in combination with a shorter 9-mer [177Lu]Lu-HP16 effector probe, emerged as the most effective pretargeting strategy. It offered a superior tumor-to-kidney uptake ratio and more efficient tumor targeting compared to longer radiolabeled effector probes containing 12 or 15 complementary PNA bases. To enhance the production efficiency of our pretargeting system, we here introduce even shorter 6-, 7-, and 8-mer secondary probes, designated as HP19, HP21, and HP20, respectively. We also explore the replacement of the original 15-mer Z-HP15 primary probe with shorter 12-mer Z-HP12 and 9-mer Z-HP9 alternatives. This extended panel of shorter PNA-based probes was synthesized using automated microwave-assisted methods and biophysically screened in vitro to identify shorter probe combinations with the most effective binding properties. In a mouse xenograft model, we evaluated the biodistribution of these probes, comparing them to the Z-HP15:[177Lu]Lu-HP16 combination. Tumor-to-kidney ratios at 4 and 144 h postinjection of the secondary probe showed no significant differences among the Z-HP9:[177Lu]Lu-HP16, Z-HP9:[177Lu]Lu-HP20, and the Z-HP15:[177Lu]Lu-HP16 pairs. Importantly, tumor uptake significantly exceeded, by several hundred-fold, that of most normal tissues, with kidney uptake being the critical organ for radiation therapy. This suggests that using a shorter 9-mer primary probe, Z-HP9, in combination with 9-mer HP16 or 8-mer HP20 secondary probes effectively targets tumors while minimizing the dose-limiting kidney uptake of radionuclide. In conclusion, the Z-HP9:HP16 and Z-HP9:HP20 probe combinations offer good prospects for both cost-effective production and efficient in vivo pretargeting of HER2-expressing tumors.

Visible-light triggered templated ligation on surface using furan-modified PNAs

Oligonucleotide-templated reactions are frequently exploited for target detection in biosensors and for the construction of DNA-based materials and probes in nanotechnology. However, the translation of the specifically used template chemistry from solution to surfaces, with the final aim of achieving highly selective high-throughput systems, has been difficult to reach and therefore, poorly explored. Here, we show the first example of a visible light-triggered templated ligation on a surface, employing furan-modified peptide nucleic acids (PNAs). Tailored photo-oxidation of the pro-reactive furan moiety is ensured by the simultaneous introduction of a weak photosensitizer as well as a nucleophilic moiety in the reacting PNA strand. This allows one to ensure a localized production of singlet oxygen for furan activation, which is not affected by probe dilution or reducing conditions. Simple white light irradiation in combination with target-induced proximity between reactive functionalities upon recognition of a short 22mer DNA or RNA sequence that functions as a template, allows sensitive detection of nucleic acid targets in a 96 well plate format.

Synthetic Life with Alternative Nucleic Acids as Genetic Materials

DNA, the fundamental genetic polymer of all living organisms on Earth, can be chemically modified to embrace novel functions that do not exist in nature. The key chemical and structural parameters for genetic information storage, heredity, and evolution have been elucidated, and many xenobiotic nucleic acids (XNAs) with non-canonical structures are developed as alternative genetic materials in vitro. However, it is still particularly challenging to replace DNAs with XNAs in living cells. This review outlines some recent studies in which the storage and propagation of genetic information are achieved in vivo by expanding genetic systems with XNAs.

Prebiotic Peptides: Molecular Hubs in the Origin of Life

The fundamental roles that peptides and proteins play in today's biology makes it almost indisputable that peptides were key players in the origin of life. Insofar as it is appropriate to extrapolate back from extant biology to the prebiotic world, one must acknowledge the critical importance that interconnected molecular networks, likely with peptides as key components, would have played in life's origin. In this review, we summarize chemical processes involving peptides that could have contributed to early chemical evolution, with an emphasis on molecular interactions between peptides and other classes of organic molecules. We first summarize mechanisms by which amino acids and similar building blocks could have been produced and elaborated into proto-peptides. Next, non-covalent interactions of peptides with other peptides as well as with nucleic acids, lipids, carbohydrates, metal ions, and aromatic molecules are discussed in relation to the possible roles of such interactions in chemical evolution of structure and function. Finally, we describe research involving structural alternatives to peptides and covalent adducts between amino acids/peptides and other classes of molecules. We propose that ample future breakthroughs in origin-of-life chemistry will stem from investigations of interconnected chemical systems in which synergistic interactions between different classes of molecules emerge.

Spontaneous Emergence of Self-Replicating Molecules Containing Nucleobases and Amino Acids

The conditions that led to the formation of the first organisms and the ways that life originates from a lifeless chemical soup are poorly understood. The recent hypothesis of “RNA-peptide coevolution” suggests that the current close relationship between amino acids and nucleobases may well have extended to the origin of life. We now show how the interplay between these compound classes can give rise to new self-replicating molecules using a dynamic combinatorial approach. We report two strategies for the fabrication of chimeric amino acid/nucleobase self-replicating macrocycles capable of exponential growth. The first one relies on mixing nucleobase- and peptide-based building blocks, where the ligation of these two gives rise to highly specific chimeric ring structures. The second one starts from peptide nucleic acid (PNA) building blocks in which nucleobases are already linked to amino acids from the start. While previously reported nucleic acid-based self-replicating systems rely on presynthesis of (short) oligonucleotide sequences, self-replication in the present systems start from units containing only a single nucleobase. Self-replication is accompanied by self-assembly, spontaneously giving rise to an ordered one-dimensional arrangement of nucleobase nanostructures.

Nitrogen heterocycles form peptide nucleic acid precursors in complex prebiotic mixtures

The ability to store information is believed to have been crucial for the origin and evolution of life; however, little is known about the genetic polymers relevant to abiogenesis. Nitrogen heterocycles (N-heterocycles) are plausible components of such polymers as they may have been readily available on early Earth and are the means by which the extant genetic macromolecules RNA and DNA store information. Here, we report the reactivity of numerous N-heterocycles in highly complex mixtures, which were generated using a Miller-Urey spark discharge apparatus with either a reducing or neutral atmosphere, to investigate how N-heterocycles are modified under plausible prebiotic conditions. High throughput mass spectrometry was used to identify N-heterocycle adducts. Additionally, tandem mass spectrometry and nuclear magnetic resonance spectroscopy were used to elucidate reaction pathways for select reactions. Remarkably, we found that the majority of N-heterocycles, including the canonical nucleobases, gain short carbonyl side chains in our complex mixtures via a Strecker-like synthesis or Michael addition. These types of N-heterocycle adducts are subunits of the proposed RNA precursor, peptide nucleic acids (PNAs). The ease with which these carbonylated heterocycles form under both reducing and neutral atmospheres is suggestive that PNAs could be prebiotically feasible on early Earth.

Chemical Systems Involving Two Competitive Self-Catalytic Reactions

Self-catalytic reactions are chemical phenomena, in which a product catalyzes the reactions of substrates further to yield products. A significant amplification of product concentration occurs during the reactions in a dilute solution, which exhibit notable properties such as sigmoidal kinetics, seeding effects, and thermal hysteresis. Chemical systems involving two competitive self-catalytic reactions can be considered, in which the competitive formation of two products occurs, which is affected by environmental changes, subtle perturbations, and fluctuations, and notable chemical phenomena appear such as formation of different structures in response to slow/fast temperature changes, chiral symmetry breaking, shortcut in reaction time, homogeneous-heterogeneous transitions, and mechanical responses. Studies on such chemical systems provide understanding on biological systems and can also be extended to the development of novel functional materials.

Exploring the emergence of complexity using synthetic replicators

A significant number of synthetic systems capable of replicating themselves or entities that are complementary to themselves have appeared in the last 30 years. Building on an understanding of the operation of synthetic replicators in isolation, this field has progressed to examples where catalytic relationships between replicators within the same network and the extant reaction conditions play a role in driving phenomena at the level of the whole system. Systems chemistry has played a pivotal role in the attempts to understand the origin of biological complexity by exploiting the power of synthetic chemistry, in conjunction with the molecular recognition toolkit pioneered by the field of supramolecular chemistry, thereby permitting the bottom-up engineering of increasingly complex reaction networks from simple building blocks. This review describes the advances facilitated by the systems chemistry approach in relating the expression of complex and emergent behaviour in networks of replicators with the connectivity and catalytic relationships inherent within them. These systems, examined within well-stirred batch reactors, represent conceptual and practical frameworks that can then be translated to conditions that permit replicating systems to overcome the fundamental limits imposed on selection processes in networks operating under closed conditions. This shift away from traditional spatially homogeneous reactors towards dynamic and non-equilibrium conditions, such as those provided by reaction-diffusion reaction formats, constitutes a key change that mimics environments within cellular systems, which possess obvious compartmentalisation and inhomogeneity.

Versatility of peptide nucleic acids (PNAs): role in chemical biology, drug discovery, and origins of life

This review briefly discussed nomenclature, synthesis, chemistry, and biophysical properties of a plethora of PNA derivatives reported since the discovery of aegPNA. Different synthetic methods and structural analogs of PNA synthesized till date were also discussed. An insight was gained into various chemical, physical, and biological properties of PNA which make it preferable over all other classes of modified nucleic acid analogs. Thereafter, various approaches with special attention to the practical constraints, characteristics, and inherent drawbacks leading to the delay in the development of PNA as gene therapeutic drug were outlined. An explicit account of the successful application of PNA in different areas of research such as antisense and antigene strategies, diagnostics, molecular probes, and so forth was described along with the current status of PNA as gene therapeutic drug. Further, the plausibility of the existence of PNA and its role in primordial chemistry, that is, origin of life was explored in an endeavor to comprehend the mystery and open up its deepest secrets ever engaging and challenging the human intellect. We finally concluded it with a discussion on the future prospects of PNA technology in the field of therapeutics, diagnostics, and origin of life.

Pyrrolidinyl PNA with α/β-Dipeptide Backbone: From Development to Applications

The specific pairing between two complementary nucleobases (A·T, C·G) according to the Watson-Crick rules is by no means unique to natural nucleic acids. During the past few decades a number of nucleic acid analogues or mimics have been developed, and peptide nucleic acid (PNA) is one of the most intriguing examples. In addition to forming hybrids with natural DNA/RNA as well as itself with high affinity and specificity, the uncharged peptide-like backbone of PNA confers several unique properties not observed in other classes of nucleic acid analogues. PNA is therefore suited to applications currently performed by conventional oligonucleotides/analogues and others potentially beyond this. In addition, PNA is also interesting in its own right as a new class of oligonucleotide mimics. Unlimited opportunities exist to modify the PNA structure, stimulating the search for new systems with improved properties or additional functionality not present in the original PNA, driving future research and applications of these in nanotechnology and beyond. Although many structural variations of PNA exist, significant improvements to date have been limited to a few constrained derivatives of the privileged N-2-aminoethylglycine PNA scaffold. In this Account, we summarize our contributions in this field: the development of a new family of conformationally constrained pyrrolidinyl PNA having a nonchimeric α/β-dipeptide backbone derived from nucleobase-modified proline and cyclic β-amino acids. The conformational constraints dictated by the pyrrolidine ring and the β-amino acid are essential requirements determining the binding efficiency, as the structure and stereochemistry of the PNA backbone significantly affect its ability to interact with DNA, RNA, and in self-pairing. The modular nature of the dipeptide backbone simplifies the synthesis and allows for rapid structural optimization. Pyrrolidinyl PNA having a (2'R,4'R)-proline/(1S,2S)-2-aminocyclopentanecarboxylic backbone (acpcPNA) binds to DNA with outstanding affinity and sequence specificity. It also binds to RNA in a highly sequence-specific fashion, albeit with lower affinity than to DNA. Additional characteristics include exclusive antiparallel/parallel selectivity and a low tendency for self-hybridization. Modification of the nucleobase or backbone allowing site-specific incorporation of labels and other functions to acpcPNA via click and other conjugation chemistries is possible, generating functional PNAs that are suitable for various applications. DNA sensing and biological applications of acpcPNA have been demonstrated, but these are still in their infancy and the full potential of pyrrolidinyl PNA is yet to be realized. With properties competitive with, and in some aspects superior to, the best PNA technology available to date, pyrrolidinyl PNA offers great promise as a platform system for future elaboration for the fabrication of new functional materials, nanodevices, and next-generation analytical tools.

A self-replicating peptide nucleic acid

While the non-enzymatic ligation and template-directed synthesis of peptide nucleic acids (PNA) have been reported since 1995, a case of self-replication of PNA has not been achieved yet. Here, we present evidence for autocatalytic feedback in a template directed synthesis of a self-complementary hexa-PNA from two trimeric building blocks. The course of the reaction was monitored in the presence of increasing initial concentrations of the product by RP-HPLC. Kinetic modeling with the SimFit program revealed parabolic growth characteristics. The observed template effect, as well as the rate of ligation, was significantly influenced by nucleophilic catalysts, pH value, and uncharged co-solvents. Systematic optimization of the reaction conditions allowed us to increase the autocatalytic efficiency of the system by two orders of magnitude. Our findings contribute to the hypothesis that PNA may have served as a primordial genetic molecule and was involved in a potential precursor of a RNA world.

DNA cross-triggered cascading self-amplification artificial biochemical circuit

A novel DNA cross-triggered cascading self-amplification artificial biochemical circuit can be triggered by either of two fully independent oligonucleotide factors (∼2 amol) and amplify both of them by 10⁵–10⁷ fold.

The construction of compact and robust artificial biochemical circuits based on nucleic acids can help researchers to understand the essential mechanisms of complex biological systems, and design sophisticated strategies for various requirements. In this study, a novel DNA cross-triggered cascading self-amplification artificial biochemical circuit was developed. Once triggered by trace amounts (as low as 2 amol) of either of two fully independent oligonucleotide factors under homogeneous isothermal conditions, the circuit simultaneously amplified both factors by 10⁵–10⁷ fold, which was proved using mass spectrometry. The compact and robust circuit was successfully used to construct a multi-input Boolean logic operation and a sensitive DNA biosensor based on the dual-amplification of both the target and reporter. The circuit showed great potential for signal gain in complicated molecular programming, and flexible control of nucleic acid nanomachines in biochemical network systems and nanotechnology.