title: Synthesis and nonenzymatic template-directed polymerization of 2'-amino-2'-deoxythreose nucleotides.
abstract: Threose nucleic acid (TNA) is a potential alternative genetic material that may have played a role in the early evolution of life. We have developed a novel synthesis of 2'-amino modified TNA nucleosides (2'-NH2-TNA) based on a cycloaddition reaction between a glycal and an azodicarboxylate, followed by direct nucleosidation of the cycloadduct. Using this route, we synthesized the thymine and guanine 2'-NH2-TNA nucleosides in seven steps with 24% and 12% overall yield, respectively. We then phosphorylated the guanine nucleoside on the 3'-hydroxyl, activated the phosphate as the 2-methylimidazolide, and tested the ability of the activated nucleotide to copy C4 RNA, DNA, and TNA templates by nonenzymatic primer extension. We measured pseudo-first-order rate constants for the first nucleotide addition step of 1.5, 0.97, and 0.57 h(-1) on RNA, DNA, and TNA templates, respectively, at pH 7.5 and 4 °C with 150 mM NaCl, 100 mM N-(hydroxylethyl)imidazole catalyst, and 5 mM activated nucleotide. The activated nucleotide hydrolyzed with a rate constant of 0.39 h(-1), causing the polymerization reaction to stall before complete template copying could be achieved. These extension rates are more than 1 order of magnitude slower than those for amino-sugar ribonucleotides under the same conditions, and copying of the TNA template, which best represented a true self-copying reaction, was the slowest of all. The poor kinetics of 2'-NH2-TNA template copying could give insight into why TNA was ultimately not used as a genetic material by biological systems.
authors.affiliation: Howard Hughes Medical Institute and Department of Molecular Biology and Center for Computational and Integrative Biology, Massachusetts General Hospital, 185 Cambridge Street, Boston, Massachusetts 02114, United States.
authors.forename: J Craig
authors.initials: JC
authors.lastname: Blain
authors.forename: Alonso
authors.initials: A
authors.lastname: Ricardo
authors.forename: Jack W
authors.initials: JW
authors.lastname: Szostak
mesh_terms: D018360:Crystallography, X-Ray; D061565:Cycloaddition Reaction; D003853:Deoxyribonucleosides / Q000138:chemical synthesis* / Q000737:chemistry / Q000235:genetics; D006868:Hydrolysis; D015394:Molecular Structure; D013698:Templates, Genetic; D013780:Tetroses / Q000737:chemistry* / Q000235:genetics
chemical_list: D003853:Deoxyribonucleosides; D013780:Tetroses; C073321:erythrose