B. Andes Hess

Quantitative exploration of the catalytic landscape separating divergent plant sesquiterpene synthases.

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.

On the aromaticity of heterocycles containing the amine nitrogen or the ether oxygen

A new method of predicting aromaticity [B. A. Hess, Jr. and L. J. Schaad, J. Am. Chem. Soc. 93, 305 (1971)] is extended to conjugated heterocyclic systems containing the amine nitrogen or the ether oxygen. Coulomb and resonance integrals for these atoms were determined using experimental heats of atomization. Empirical π bond energies for the carbon-heteroatom bonds were determined from heteroatom containing acyclic molecules. These were in turn used to calculate the HMO π resonance energies of a wide variety of heterocyclic compounds. An excellent correlation between calculated resonance energy and experimental aromaticity was found.

Propargylene: A C3H2 isomer with unusual bonding

Propargylene was identified in a matrix as a product of photolysis of cyclopropenylidene and diazopropyne. The molecule is a triplet. The optimum geometry predicted by ab initio calculations corresponds to a structure HC≡C–CH. The transition structure in the interconversion HC≡C–CH⇄HĊ=C=ĊH⇄HC–C≡CH is very low in energy and close to the energy of the vibrational ground state. Owing to this nonrigidity, computed infrared (IR) frequencies based on a harmonic treatment do not match the experimental spectrum. When this nonrigidity is taken into account by using a nonharmonic approximation calculated UMP2/6‐31G** IR spectra are in good agreement with the observed spectra of HCCCH and DCCCD.

Ab initio second-order Møller-Plesset calculation of the vibrational spectra of C4 clusters☆

Abstract Second-order Moller-Plesset calculations on the C 4 potential surface yielded three isomers, a linear triplet and rhombic and tetrahedral singlets. A large discrepancy between observed and calculated frequencies of the Σ u + vibration of linear 12 C 4 is outside the expected range of error. It is tentatively suggested that an observed absorption at 1544 cm −1 previously assigned to C 5 might belong instead to C 4 .

Concerted, highly asynchronous, enzyme-catalyzed [4 + 2] cycloaddition in the biosynthesis of spinosyn A; computational evidence

A theoretical study has been carried out on model systems to study a recently reported, (Nature, 2011, 473, 109) biosynthetic, [4 + 2] cycloaddition catalyzed by a stand-alone enzyme (the cyclase SpnF). It was suggested in this paper that SpnF is the first known example of a Diels-Alderase (DA). In the present study, for a model system of the substrate a transition structure was found with density functional calculations (DFT). In addition, the intrinsic reaction coordinate calculations indicated that the transition structure is that of a concerted, but highly asynchronous, DA reaction. Based on the DFT and Møller-Plesset second order calculations the activation energy was estimated to be about 15 kcal mol(-1). The results of a natural population analysis indicated that there is significant charge transfer in the transition state, and it is proposed that possibly the enzyme plays a dual role of not only folding the substrate into the proper conformation for the DA reaction to occur, but also lowering its activation energy by stabilization of the highly polarized transition structure.

On the aromaticity of annulenones

Abstract Application of our method of predicting aromaticity 1 shows that the behavior of the annulenones is intermediate between that of the fulvenes (a nonaromatic series) and the annulenes (a series with strong alternation between aromatic and antiaromatic).

Photoisomerisierung von tetrachlormethan in einer argon-matrix

Abstract Irradiation of tetrachloromethane (3) in argon at 12 K leads to a new species, which - from a comparison of the calculated and experimental IR spectra - is best described as an iso-tetrachloromethane with structure 5. The reverse reaction 5 → 3 can also be induced photochemically.

Compelling computational evidence for the concerted cyclization of the ABC rings of hopene from protonated squalene.

The long-standing question of what is the nature of the cyclization of squalene to form tetracyclic and pentacyclic triterpenes has been addressed computationally. Using the DFT method with an intrinsic reaction coordinate calculation, we find that the first three rings of protonated squalene were formed without the intermediacy of mono- or bicyclic carbocations. The cyclization, calculated in the gas phase, proceeds in a highly asynchronous, concerted reaction to yield the tricyclic, tertiary carbocation with a 5-membered C ring. The fourth double bond of squalene is not properly oriented for the ring expansion of the C ring in concert with the formation of the 5-membered ring.

Third order abinitio calculations of the f↔f transition amplitudes for ions across the lanthanide series

The behavior of various contributions to the f↔f transition amplitude defined up to the third order in perturbation theory is analyzed. Main attention is paid to the third‐order contributions which are due to electron correlation inside the rare earth ion and arise from the static and dynamic models. The discussion is based on the numerical results of exact ab initio calculations performed for Pr+3, Nd+3, Eu+3, Gd+3, Tb+3, and Tm+3 ions.

An ab initio study of the structure and electronic spectrum of 1,2-dithiete and 1,2-dithiin

Abstract The ground-state geometries of 1,2-dithiete and 1,2-dithiin were optimized at both the Hartree/3-Fock and MP2 levels with the 6-31G* basis set. In contrast to 1,2-dithiete, 1,2-dithiin is nonplanar with C 2 symmetry. Subsequent calculations on the first excited singlet states of each molecule were carried out using the variation-perturbation CIPSI method, in order to elucidate the main features of the electronic absorption spectrum.