Elemental exchange: Bioisosteric replacement of phosphorus by boron in drug design

Continuous efforts are being directed toward the employment of boron in drug design due to its advantages and unique characteristics including a plethora of target engagement modes, lower metabolism, and synthetic accessibility, among others. Phosphates are components of multiple drug molecules as well as clinical candidates, since they play a vital role in various biochemical functions, being components of nucleotides, energy currency- ATP as well as several enzyme cofactors. This review discusses the unique chemistry of boron functionalities as phosphate bioisosteres - "the boron-phosphorus elemental exchange strategy" as well as the superiority of boron groups over other commonly employed phosphate bioisosteres. Boron phosphate-mimetics have been utilized for the development of enzyme inhibitors as well as novel borononucleotides. Both the boron functionalities described in this review-boronic acids and benzoxaboroles-contain a boron connected to two oxygens and one carbon atom. The boron atom of these functional groups coordinates with a water molecule in the enzyme site forming a tetrahedral molecule which mimics the phosphate structure. Although boron phosphate-mimetic molecules - FDA-approved Crisaborole and phase II/III clinical candidate Acoziborole are products of the boron-phosphorus bioisosteric elemental exchange strategy, this technique is still in its infancy. The review aims to promote the use of this strategy in future medicinal chemistry projects.

Applications of the Reversible Boronic Acids/Boronate Switch to Nucleic Acids

Over the last decades, boron and nucleic acids chemistries have gained a lot of attention for biological, medicinal and analytical applications. Our laboratory has a long-standing interest in both chemistries and owing to the ability of boronic acids to react with cis-diol function in aqueous media we developed over the years a variety of applications ranging from molecular recognition and sensing to the development of reversible dynamic systems in which the natural phosphodiester linkage was replaced by a boronate. In this account, we summarize research results from our group from our preliminary studies on molecular recognition of ribonucleosides to the dynamic assembly of functional DNAzymes. In particular, the various parameters influencing the dynamic nature of these reversible covalent bonds able to respond to external stimuli are discussed. Finally, current challenges and opportunities for boron-based nucleic acids are also addressed.

Nuclease stability of boron-modified nucleic acids: application to label-free mismatch detection

5'-End boronic acid-modified oligonucleotides were evaluated against various nucleases at single and double stranded levels. The results show that these modifications induce a high resistance to degradation by calf-spleen and snake venom phosphodiesterases. More importantly, this eventually led to the development of a new label-free enzyme-assisted fluorescence-based method for single mismatch detection.

DNA-templated borononucleic acid self assembly: a study of minimal complexity

The minimal degree of sequence complexity needed for DNA-templated self-assembly of bifunctional oligonucleotides able to form internucleosidic boronate linkages has been studied.

pH-controlled DNA- and RNA-templated assembly of short oligomers

In the area of artificial genetics the development of non-enzymatic self-organization of synthetic building blocks is critical for both providing biopolymers with extended functions and understanding prebiotic processes. While reversibility is believed to have played a major role in early functional genetic materials, we previously reported an efficient DNA-templated ligation of suitably designed 5'-end boronic acid and 3'-end ribonucleosidic half-sequences. Here, we report the enzyme-free and activation-free DNA- and RNA-templated assembly of bifunctional hexamers. The reversible assembly was found to be regio- and sequence specific and the stabilities of the resulting duplexes were compared to their nicked counterparts. To go further with our understanding of this unprecedented process we also examined an auto-templated duplex self-assembly representing a key step toward the evolution of sequence-defined synthetic polymers.