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the neutral base must approach closer to the Sn2 center than the charged base for the same extent of CH3F distortion, C-F (leaving) charge buildup and bond weakening, and C-base bond formation.

Throughout molecular evolution, organisms create assorted chemicals in response to varying ecological niches. Catalytic landscapes underlie metabolic evolution, wherein mutational steps alter the biosynthetic properties of enzymes. We report the first systematic quantitative characterization of a catalytic landscape underlying the evolution of sesquiterpene chemical diversity. Based on our previous discovery of a set of 9 naturally occurring amino acid substitutions that functionally inter-converted orthologous sesquiterpene synthases from Nicotiana tabaccum and Hyoscyamus muticus, we created a library of all possible residue combinations (29 = 512) in the N. tabaccum parent. The product spectra of 418 active enzymes to reveal a rugged landscape where several minimal combinations of the 9 mutations encode convergent solutions to the inter-conversions of parental activities. Quantitative comparisons indicate context dependence for mutational effects - epistasis - in product specificity and promiscuity. These results provide a measure of the mutational accessibility of phenotypic variability among a diverging lineage of terpene synthases.

The recent isolation of isofulminic acid has opened the door for a comparative study of all the four possible CHNO isomers: isocyanic acid, cyanic acid, fulminic acid, and isofulminic acid. Their infrared spectra have been measured in an argon matrix at 13 K and are compared with the results of ab initio calculations.

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