Synthesis and oligomerization of Fmoc/Boc-protected PNA monomers of 2,6-diaminopurine, 2-aminopurine and thymine

A safety-catch protecting group strategy compatible with Boc-chemistry for the synthesis of peptide nucleic acids (PNAs)

Peptide Nucleic Acids (PNAs) are an intriguing class of synthetic biomolecules with great potential in medicine. Although PNAs could be considered analogs of oligonucleotides, their synthesis is more like that of peptides. In both cases, a Solid-Phase Synthesis (SPS) approach is used. Herein, the advantage using Boc as a temporal protecting group has been demonstrated to be more favored than Fmoc. In this context, a new PNA SPS strategy has been developed based on a safety-catch protecting group scheme for the exocyclic nitrogen of the side-chain bases and the linker. Sulfinyl (sulfoxide)-containing moieties are fully stable to the trifluoroacetic acid (TFA) used to remove the Boc group, but they can be reduced to the corresponding sulfide derivatives, which are labile in the presence of TFA. The efficiency of this novel synthetic strategy has been demonstrated in the synthesis of the PNA pentamer H-PNA(TATCT)-βAla-OH.

Discovery of phosphonate derivatives containing different substituted 1,2,3-triazole motif as promising tobacco mosaic virus (TMV) helicase inhibitors for controlling plant viral diseases

BACKGROUND

The discovery and identification of targets is of far-reaching significance for developing novel pesticide candidates and increasing the probability of success. To explore and identify highly effective tobacco mosaic virus (TMV) helicase-targeted lead structures, a series of novel phosphonate derivatives containing a 1,2,3-triazole motif were rationally engineered and their antiviral activity was assessed.

RESULTS

Bioassay results showed that the optimized B17 exhibited more potent curative activity (EC50  = 271.5 μg mL-1 ) against TMV in vivo, which was superior to that of commercial Ribavirin (EC50  = 689.3 μg mL-1 ). B17 presented a stronger binding capacity through binding analysis with helicase, affording a corresponding value of 12.7 μM. Enzyme activity assay showed B17 exhibited excellent inhibitory activity on TMV helicase (39.2% at 300 μM). Furthermore, molecular docking simulations demonstrated that B17 displayed strong hydrogen-bond interactions (2.1, 2.1, 2.2, and 3.2 Å) with Ala-33, Gly-10, Gly-8, and Glu-217 of TMV helicase. Encouragingly, transmission electron microscopy analysis revealed that B17 could remarkably disrupt surface morphology and inhibit TMV proliferation. Additionally, these compounds also displayed potential anti-CMV (cucumber mosaic virus) and antipathogens (Xanthomonas oryzae pv. oryzae and Xanthomonas axonopodis pv. citri) by expanding their applications in agriculture.

CONCLUSION

Current research demonstrated that B17 could be considered as a potential antiviral agent alternative though targeting TMV helicase. © 2023 Society of Chemical Industry.

A simple fluorescent assay for the detection of peptide nucleic acid-directed double strand duplex invasion

Peptide nucleic acid (PNA) is a mimic of nucleic acids that is able to bind complementary oligonucleotides with high affinity and excellent selectivity. As such, the use of PNA has been proposed in numerous applications in biochemistry, medicine, and biotechnology. Sequences of pseudo-complementary PNAs containing diaminopurine (D)-2-thiouracil (^S U) base pairs bind to complementary regions within double-stranded DNA targets. This type of binding is termed "double duplex invasion" and involves unwinding of the duplex accompanied by simultaneous hybridization of both DNA strands by the two pseudo-complementary PNAs. In this study, a simple method of assaying DNA strand invasion by pseudo-complementary PNAs was developed. This method is based on the incorporation of a single fluorescent cytidine analog, phenylpyrrolocytidine (PhpC), into the double-stranded DNA target such that upon invasion by PNA, the PhpC is displaced to a single-stranded region resulting in the turn-on of fluorescence emission. This fluorescent assay is applicable to studies both at equilibrium and approach-to-equilibrium (time-dependent) conditions.

Incorporation of Pseudo-complementary Bases 2,6-Diaminopurine and 2-Thiouracil into Serinol Nucleic Acid (SNA) to Promote SNA/RNA Hybridization

Serinol nucleic acid (SNA) is a promising candidate for nucleic acid-based molecular probes and drugs due to its high affinity for RNA. Our previous work revealed that incorporation of 2,6-diaminpurine (D), which can form three hydrogen bonds with uracil, into SNA increases the melting temperature of SNA-RNA duplexes. However, D incorporation into short self-complementary regions of SNA promoted self-dimerization and hindered hybridization with RNA. Here we synthesized a SNA monomer of 2-thiouracil (sU), which was expected to inhibit base pairing with D by steric hindrance between sulfur and the amino group. To prepare the SNA containing D and sU in high yield, we customized the protecting groups on D and sU monomers that can be readily deprotected under acidic conditions. Incorporation of D and sU into SNA facilitated stable duplex formation with target RNA by suppressing the self-hybridization of SNA and increasing the stability of the heteroduplex of SNA and its complementary RNA. Our results have important implications for the development of SNA-based probes and nucleic acid drugs.

Antibacterial Peptide Nucleic Acids-Facts and Perspectives

Antibiotic resistance is an escalating, worldwide problem. Due to excessive use of antibiotics, multidrug-resistant bacteria have become a serious threat and a major global healthcare problem of the 21st century. This fact creates an urgent need for new and effective antimicrobials. The common strategies for antibiotic discovery are based on either modifying existing antibiotics or screening compound libraries, but these strategies have not been successful in recent decades. An alternative approach could be to use gene-specific oligonucleotides, such as peptide nucleic acid (PNA) oligomers, that can specifically target any single pathogen. This approach broadens the range of potential targets to any gene with a known sequence in any bacterium, and could significantly reduce the time required to discover new antimicrobials or their redesign, if resistance arises. We review the potential of PNA as an antibacterial molecule. First, we describe the physicochemical properties of PNA and modifications of the PNA backbone and nucleobases. Second, we review the carriers used to transport PNA to bacterial cells. Furthermore, we discuss the PNA targets in antibacterial studies focusing on antisense PNA targeting bacterial mRNA and rRNA.

On the Necessity of Nucleobase Protection for 2-Thiouracil for Fmoc-Based Pseudo-Complementary Peptide Nucleic Acid Oligomer Synthesis

A selection of benzyl-based protecting groups for thiouracil (^SU) for the synthesis of pseudo-complementary peptide nucleic acid (PNA) has been evaluated. The 4-methoxybenzyl-protecting group that has found use for ^SU during Boc-based oligomerization is also suitable for Fmoc-based oligomerization. Furthermore, it is demonstrated that ^SU protection is unnecessary for the successful synthesis of thiouracil-containing PNA. The new 2-thiothymine (^ST) PNA monomer has also been prepared and incorporated into an oligomer and its binding to complementary PNA evaluated.

Clickable PNA Probes for Imaging Human Telomeres and Poly(A) RNAs

The ability to bind strongly to complementary nucleic acid sequences, invade complex nucleic acid structures, and resist degradation by cellular enzymes has made peptide nucleic acid (PNA) oligomers as very useful hybridization probes in molecular diagnosis. For such applications, the PNA oligomers have to be labeled with appropriate reporters as they lack intrinsic labels that can be used in biophysical assays. Although solid-phase synthesis is commonly used to attach reporters onto PNA, development of milder and modular labeling methods will provide access to PNA oligomers labeled with a wider range of biophysical tags. Here, we describe the establishment of a postsynthetic modification strategy based on bioorthogonal chemical reactions in functionalizing PNA oligomers in solution with a variety of tags. A toolbox composed of alkyne- and azide-modified monomers were site-specifically incorporated into PNA oligomers and postsynthetically click-functionalized with various tags, ranging from sugar, amino acid, biotin, to fluorophores, by using copper(I)-catalyzed azide-alkyne cycloaddition, strain-promoted azide-alkyne cycloaddition, and Staudinger ligation reactions. As a proof of utility of this method, fluorescent PNA hybridization probes were developed and used in imaging human telomeres in chromosomes and poly(A) RNAs in cells. Taken together, this simple approach of generating a wide range of functional PNA oligomers will expand the use of PNA in molecular diagnosis.

Synthesis of Fmoc-Protected ( S, S)- trans-Cyclopentane Diamine Monomers Enables the Preparation and Study of Conformationally Restricted Peptide Nucleic Acids

An efficient synthesis of Fmoc-protected ( S, S)- trans-cyclopentane PNA ( tcypPNA) monomers starting from mono-Boc-protected ( S, S)-1,2-cyclopentanediamine is reported. A general synthetic strategy was developed so that tcypPNA monomers with each nucleobase can be made in sufficient quantity and purity for use in solid-phase peptide synthesis (SPPS). The newly synthesized monomers were then successfully incorporated into 10-residue PNA oligomers using standard Fmoc chemistry for SPPS. The different tcypPNAs allow different positions in the sequence to be conformationally constrained with ( S, S)- trans-cyclopentane to determine the effects on binding to complementary DNA.

A Lucifer-Based Environment-Sensitive Fluorescent PNA Probe for Imaging Poly(A) RNAs

Fluorescence‐based oligonucleotide (ON) hybridization probes greatly aid the detection and profiling of RNA sequences in cells. However, certain limitations such as target accessibility and hybridization efficiency in cellular environments hamper their broad application because RNAs can form complex and stable structures. In this context, we have developed a robust hybridization probe suitable for imaging RNA in cells by combining the properties of 1) a new microenvironment‐sensitive fluorescent nucleobase analogue, obtained by attaching the Lucifer chromophore (1,8‐naphthalimide) at the 5‐position of uracil, and 2) a peptide nucleic acid (PNA) capable of forming stable hybrids with RNA. The fluorescence of the PNA base analogue labeled with the Lucifer chromophore, when incorporated into PNA oligomers and hybridized to complementary and mismatched ONs, is highly responsive to its neighboring base environment. Notably, the PNA base reports the presence of an adenine repeat in an RNA ON with reasonable enhancement in fluorescence. This feature of the emissive analogue enabled the construction of a poly(T) PNA probe for the efficient visualization of polyadenylated [poly(A)] RNAs in cells—poly(A) being an important motif that plays vital roles in the lifecycle of many types of RNA. Our results demonstrate that such responsive fluorescent nucleobase analogues, when judiciously placed in PNA oligomers, could generate useful hybridization probes to detect nucleic acid sequences in cells and also to image them.

Fluorogenic PNA probes

Fluorogenic oligonucleotide probes that can produce a change in fluorescence signal upon binding to specific biomolecular targets, including nucleic acids as well as non-nucleic acid targets, such as proteins and small molecules, have applications in various important areas. These include diagnostics, drug development and as tools for studying biomolecular interactions in situ and in real time. The probes usually consist of a labeled oligonucleotide strand as a recognition element together with a mechanism for signal transduction that can translate the binding event into a measurable signal. While a number of strategies have been developed for the signal transduction, relatively little attention has been paid to the recognition element. Peptide nucleic acids (PNA) are DNA mimics with several favorable properties making them a potential alternative to natural nucleic acids for the development of fluorogenic probes, including their very strong and specific recognition and excellent chemical and biological stabilities in addition to their ability to bind to structured nucleic acid targets. In addition, the uncharged backbone of PNA allows for other unique designs that cannot be performed with oligonucleotides or analogues with negatively-charged backbones. This review aims to introduce the principle, showcase state-of-the-art technologies and update recent developments in the areas of fluorogenic PNA probes during the past 20 years.

PNA monomers fully compatible with standard Fmoc-based solid-phase synthesis of pseudocomplementary PNA

Here we report the synthesis of new PNA monomers for pseudocomplementary PNA (pcPNA) that are fully compatible with standard Fmoc chemistry. The thiocarbonyl group of the 2-thiouracil (sU) monomer was protected with the 4-methoxy-2-methybenzyl group (MMPM), while the exocyclic amino groups of diaminopurine (D) were protected with Boc groups. The newly synthesized monomers were incorporated into a 10-mer PNA oligomer using standard Fmoc chemistry for solid-phase synthesis. Oligomerization proceeded smoothly and the HPLC and MALDI-TOF MS analyses indicated that there was no remaining MMPM on the sU nucleobase. The new PNA monomers reported here would facilitate a wide range of applications, such as antigene PNAs and DNA nanotechnologies.

A Bifunctional Monomer for On-Resin Synthesis of Polyfunctional PNAs and Tailored Induced-Fit Switching Probes

A synthetic strategy for the production of polyfunctional PNAs bearing substituent groups both on the nucleobase and on the backbone C5 carbon of the same monomer is described; this is based on the use of a tris-orthogonally protected monomer and subsequent solid-phase selective functionalization. This strategy can be used for synthesizing PNAs that are not readily accessible by use of preformed modified monomers. As an example, a PNA-based probe that undergoes a switch in its fluorescence emission upon hybridization with a target oligonucleotide, induced by tailor-made movement of two pyrene substituent groups, was synthesized.

Responsive Fluorescent PNA Analogue as a Tool for Detecting G-quadruplex Motifs of Oncogenes and Activity of Toxic Ribosome-Inactivating Proteins

Fluorescent oligomers that are resistant to enzymatic degradation and report their binding to target oligonucleotides (ONs) by changes in fluorescence properties are highly useful in developing nucleic-acid-based diagnostic tools and therapeutic strategies. Here, we describe the synthesis and photophysical characterization of fluorescent peptide nucleic acid (PNA) building blocks made of microenvironment-sensitive 5-(benzofuran-2-yl)- and 5-(benzothiophen-2-yl)-uracil cores. The emissive monomers, when incorporated into PNA oligomers and hybridized to complementary ONs, are minimally perturbing and are highly sensitive to their neighboring base environment. In particular, benzothiophene-modified PNA reports the hybridization process with significant enhancement in fluorescence intensity, even when placed in the vicinity of guanine residues, which often quench fluorescence. This feature was used in the turn-on detection of G-quadruplex-forming promoter DNA sequences of human proto-oncogenes (c-myc and c-kit). Furthermore, the ability of benzothiophene-modified PNA oligomer to report the presence of an abasic site in RNA enabled us to develop a simple fluorescence hybridization assay to detect and estimate the depurination activity of ribosome-inactivating protein toxins. Our results demonstrate that this approach with responsive PNA probes will provide new opportunities to develop robust tools to study nucleic acids.

A base-modified PNA-graphene oxide platform as a turn-on fluorescence sensor for the detection of human telomeric repeats

Given the biological and therapeutic significance of telomeres and other G-quadruplex forming sequences in human genome, it is highly desirable to develop simple methods to study these structures, which can also be implemented in screening formats for the discovery of G-quadruplex binders. The majority of telomere detection methods developed so far are laborious and use elaborate assay and instrumental setups, and hence, are not amenable to discovery platforms. Here, we describe the development of a simple homogeneous fluorescence turn-on method, which uses a unique combination of an environment-sensitive fluorescent nucleobase analogue, the superior base pairing property of PNA, and DNA-binding and fluorescence quenching properties of graphene oxide, to detect human telomeric DNA repeats of varying lengths. Our results demonstrate that this method, which does not involve a rigorous assay setup, would provide new opportunities to study G-quadruplex structures.

Influence of PNA containing 8-aza-7-deazaadenine on structure stability and binding affinity of PNA·DNA duplex: insights from thermodynamics, counter ion, hydration and molecular dynamics analysis

This paper describes the synthesis of a novel 8-aza-7-deazapurin-2,6-diamine (DPP)-containing peptide nucleic acid (PNA) monomer and Boc protecting group-based oligomerization of PNA, replacing adenine (A) with DPP monomers in the PNA strand. The PNA oligomers were synthesized against the biologically relevant SV40 promoter region (2494-AATTTTTTTTATTTA-2508) of pEGFP-N3 plasmid. The DPP-PNA·DNA duplex showed enhanced stability as compared to normal duplex (A-PNA·DNA). The electronic distribution of DPP monomer suggested that DPP had better electron donor properties over 2,6-diamino purine. UV melting and thermodynamic analysis revealed that the PNA oligomer containing a diaminopyrazolo(3,4-d)pyrimidine moiety (DPP) stabilized the PNA·DNA hybrids compared to A-PNA·DNA. DPP-PNA·DNA duplex showed higher water activity (Δnw = 38.5) in comparison to A-PNA·DNA duplex (Δnw = 14.5). The 50 ns molecular dynamics simulations of PNA·DNA duplex containing DPP or unmodified nucleobase-A showed average H-bond distances in the DPP-dT base pair of 2.90 Å (OH-N bond) and 2.91 Å (NH-N bond), which were comparably shorter than in the A-dT base pair, in which the average distances were 3.18 Å (OH-N bond) and 2.97 Å (NH-N bond), and there was one additional H-bond in the DPP-dT base pair of around 2.98 Å (O2H-N2 bond), supporting the higher stability of DPP-PNA·DNA. The analysis of molecular dynamics simulation data showed that the system binding free energy increased at a rate of approximately -4.5 kcal mol(-1) per DPP base of the PNA·DNA duplex. In summary, increased thermal stability, stronger hydrogen bonding and more stable conformation in the DPP-PNA·DNA duplex make it a better candidate as antisense/antigene therapeutic agents.

A solid-phase CuAAC strategy for the synthesis of PNA containing nucleobase surrogates

The synthesis of an azide containing PNA monomer is described. The monomer was incorporated into two PNA sequences for the purpose of synthesizing an intercalating fluorophore-labeled PNA and a metal binding hairpin using a solid phase copper catalyzed azide-alkyne Huisgen cycloaddition (CuAAC). Click chemistry was performed using 2-ethynylfluorene or 1-ethynylpyrene to add a fluorophore to the PNA, which were tested for their ability to recognize an abasic site on a DNA target. A PNA hairpin possessing azide monomers at each termini was synthesized and reacted with 2-ethynylpyridine to form a hairpin that is stabilized by Ni²⁺.

Peptide Nucleic Acid Monomers: A Convenient and Efficient Synthetic Approach to Fmoc/Boc Monomers

A convenient and cost-effective method for the synthesis of Fmoc/Boc-protected peptide nucleic acid monomers is described. The Fmoc/Boc strategy was developed in order to eliminate the solubility issues during peptide nucleic acid solid-phase synthesis, in particular that of the cytosine monomer, that occurred when using the commercialized Bhoc chemistry approach.