Impact of bPNA Backbone Structural Constraints and Composition on Triplex Hybridization with DNA

Intracellular RNA and DNA tracking by uridine-rich internal loop tagging with fluorogenic bPNA

The most widely used method for intracellular RNA fluorescence labeling is MS2 labeling, which generally relies on the use of multiple protein labels targeted to multiple RNA (MS2) hairpin structures installed on the RNA of interest (ROI). While effective and conveniently applied in cell biology labs, the protein labels add significant mass to the bound RNA, which potentially impacts steric accessibility and native RNA biology. We have previously demonstrated that internal, genetically encoded, uridine-rich internal loops (URILs) comprised of four contiguous UU pairs (8 nt) in RNA may be targeted with minimal structural perturbation by triplex hybridization with 1 kD bifacial peptide nucleic acids (bPNAs). A URIL-targeting strategy for RNA and DNA tracking would avoid the use of cumbersome protein fusion labels and minimize structural alterations to the RNA of interest. Here we show that URIL-targeting fluorogenic bPNA probes in cell media can penetrate cell membranes and effectively label RNAs and RNPs in fixed and live cells. This method, which we call fluorogenic U-rich internal loop (FLURIL) tagging, was internally validated through the use of RNAs bearing both URIL and MS2 labeling sites. Notably, a direct comparison of CRISPR-dCas labeled genomic loci in live U2OS cells revealed that FLURIL-tagged gRNA yielded loci with signal to background up to 7X greater than loci targeted by guide RNA modified with an array of eight MS2 hairpins. Together, these data show that FLURIL tagging provides a versatile scope of intracellular RNA and DNA tracking while maintaining a light molecular footprint and compatibility with existing methods.

Structural and Thermodynamic Control of Supramolecular Polymers and DNA Assemblies with Cyanuric Acid: Influence of Substituents and Intermolecular Interactions

Understanding the interactions and thermodynamic parameters that govern the structure and stability of supramolecular polymers is challenging because of their flexible nature and high sensitivity to weak intermolecular interactions. The application of both experimental and computational analyses reveals the role that substituents on cyanuric acid (Cy), and other nitrogen-containing heterocycles, play in the formation of novel helical supramolecular structures. In this report, we focus on how noncovalent interactions, including steric and stacking interactions, modulate the structural and physical properties of these assemblies. In-depth analyses and several examples of critical steric and electrostatic effects provide insight into the relationship between intermolecular interactions of Cy with nucleic acids and the structure and thermodynamic stability of the supramolecular polymers they form.

Structural Analysis of the Poly(thymidine)-Melamine Assembly

Hydrogen bonding between the DNA nucleobases and small organic molecules, such as melamine, is a new strategy for the design of novel DNA materials. Poly(thymidine) DNA and melamine self-assemble into a duplex structure containing two antiparallel DNA strands hydrogen bonded to central melamine units. In this Article, molecular dynamics simulations rationalize the observed antiparallel duplex structure. Alternative duplex and triplex structures with parallel and antiparallel strand orientations are shown to be unstable because of the increase in unfavorable interactions between the DNA backbones.

Synthesis of bifacial Peptide Nucleic Acids with diketopiperazine backbones

We report herein synthesis of bifacial peptide nucleic acids (bPNAs) with novel diketopiperazine (DKP) backbones that display unnatural melamine (M) bases as well as native bases. To examine the structure-function scope of diketopiperazine bPNAs, we synthesized a set of bPNAs using diaminopropionic acid, diaminobutyric acid, ornithine and lysine derivatives to display the base-tripling motifs, which result in 1, 2, 3, and 4 carbons linking alpha carbon to sidechain amine, respectively. Thermal denaturation of DNA hybrids with these bPNAs revealed that the optimal sidechain linkage was 4 carbons, corresponding to the lysine derivative. Accordingly, monomers displaying two bases per sidechain were prepared via double reductive alkylation of the ε-amine of Fmoc-Lysine with acetaldehyde derivatives of adenine, cytidine, uridine and melamine. With these building blocks in hand, diketopiperazine bPNAs were prepared to display a combination of native and synthetic (melamine) bases. Preliminary melting studies indicate binding signatures of cytidine and melamine-displaying bPNAs to T-rich DNAs, though full characterization of this behavior is ongoing. We anticipate that the straightforward synthetic methodology developed herein will enable further studies on noncanonical nucleic acid hybridization with diketopiperazine backbones.