Density functional theory study of the interaction of vinyl radical, ethyne, and ethene with benzene, aimed to define an affordable computational level to investigate stability trends in large van der Waals complexes

siRNA Seed Region Is Divided into Two Functionally Different Domains in RNA Interference in Response to 2'-OMe Modifications

In RNA interference (RNAi), small interfering RNA (siRNA) functions to suppress the expression of its target mRNA with perfect sequence complementarity. In a mechanism different from above, siRNA also suppresses unintended mRNAs with partial sequence complementarities, mainly to the siRNA seed region (nucleotides 2-8). This mechanism is largely utilized by microRNAs (miRNAs) and results in siRNA-mediated off-target effects. Thus, the siRNA seed region is considered to be involved in both RNAi and off-target effects. In this study, we revealed that the impact of 2'-O-methyl (2'-OMe) modification is different according to the nucleotide positions. The 2'-OMe modifications of nucleotides 2-5 inhibited off-target effects without affecting on-target RNAi activities. In contrast, 2'-OMe modifications of nucleotides 6-8 increased both RNAi and off-target activities. The computational simulation revealed that the structural change induced by 2'-OMe modifications interrupts base pairing between siRNA and target/off-target mRNAs at nucleotides 2-5 but enhances at nucleotides 6-8. Thus, our results suggest that siRNA seed region consists of two functionally different domains in response to 2'-OMe modifications: nucleotides 2-5 are essential for avoiding off-target effects, and nucleotides 6-8 are involved in the enhancement of both RNAi and off-target activities.

Benchmarking of Density Functionals for the Accurate Description of Thiol-Disulfide Exchange

A set of 92 density functionals was employed to accurately characterize thiol-disulfide exchange. The properties we have benchmarked throughout the study include the geometry of a 15 atoms model system, the potential energy surface, the activation barrier, and the energy of reaction for thiol-disulfide exchange. Reference energies were determined at the CCSD(T)/CBS//MP2/aug-cc-pVDZ level of theory, and reference geometries were calculated at the MP2/aug-cc-pVTZ level. M11-L, M06-2X, M06-HF, N12-SX, PBE1PBE, PBEh1PBE, and OHSE2PBE described better the geometry of the model system, with average deviations of 0.06 Å in bond lengths (0.06 Å in bond-breaking lengths) and 1.9° in bond angles. On the other hand, the potential energy surface and its gradient were more accurately described by the hybrid density functional BHandH, closely followed by mPW1N, mPW1K, and mPWB1K. The barrier height and energy of reaction were better reproduced by the BMK and M06-2X functionals (deviations of 0.17 and 0.07 kcal·mol(-1), respectively) for a set of 10 Pople's basis sets. MN12-SX and M11-L showed very good results for the widely used 6-311++G(2d,2p) basis set, with deviations of 0.02 and 0.05 kcal·mol(-1), respectively. We studied the effect of the split-valence, diffuse, and polarized functions in the activation barrier of thiol-disulfide exchange, for a set of 10 Pople's basis sets. While increasing the splitting and polarization may increase the activation barrier in approximately 1 kcal·mol(-1), diffuse functions generally contribute to decreasing it no more than 0.10 kcal·mol(-1). In general, 13 functionals provided energies within 1 kcal·mol(-1) of the reference value. The BB1K density functional is one of the best density functionals to characterize thiol-disulfide exchange reactions; however, several density functionals with modified Perdew-Wang exchange and about 40% Hartree-Fock exchange, such as mPW1K, mPW1N, and mPWB1K, show a good performance, too.

Molecular growth of PAH-like systems induced by oxygen species: experimental and theoretical study of the reaction of naphthalene with HO (2Π3/2), O (3P), and O2 (3Σ−g)

To assess if reactions with oxygen species can induce a mass increase of polycyclic aromatic hydrocarbons, we exposed naphthalene molecules to an oxidative gas flow containing the radicals H and HO (²Π_3/2) and the diradicals O (³P) and O₂ (³Σ−g).