Theoretical Study of Pyridoxine (Vitamin B6) Photolysis

A regioselective etherification of pyridoxine via an ortho-pyridinone methide intermediate

The catalyst-free, regioselective synthesis of 4'-O-substituted pyridoxine derivatives under solventless conditions is described. The methodology relies on the highly regioselective formation of the ortho-pyridinone methide from pyridoxine and subsequent oxa-Michael addition of alcohol nucleophiles. This methodology provides good to excellent yields for primary and secondary alcohols and moderate yields for tertiary alcohols.

Photochemical reaction mechanism of UV-B-induced monomerization of UVR8 dimers as the first signaling event in UV-B-regulated gene expression in plants

The Arabidopsis thaliana UV RESISTANCE LOCUS8 (UVR8) protein has been identified to specifically mediate photomorphogenic UV-B responses by acting as a UV-B photoreceptor. The dimeric structure of the UVR8 protein dissociates into signaling-active monomers upon UV-B exposure, and the monomers rapidly interact with downstream signaling components to regulate gene expression. UVR8 monomers revert to dimers in the absence of UV-B radiation, thereby reversing transcription activation. UVR8 amino acid residues W233 and W285 have been identified to play critical roles in the UVR8 dimer for the response to UV-B irradiation. In the present work, the photoreaction mechanism for UVR8 monomerization is explored with quantum chemical cluster calculations and evaluated by molecular dynamics simulations using the wild-type UVR8 dimer and novel force field parameters developed for intermediate radicals formed in the photochemical process. Three different models are investigated, which show that the preferred mechanism for UVR8 monomerization involves electron transfer from residue W233 to W285 and onward to R338 initiated by UV-B irradiation, coupled to simultaneous proton transfer from W233 to D129 leading to the formation of protonated D129, a deprotonated W233 radical, and a neutral R338 radical. Due to the formation of the neutral R338 radical, salt bridges involving this residue are disrupted together with the concomitant interruption of several other key salt bridges R286-D96, R286-D107, R338-D44, R354-E43, and R354-E53. The resulting large decrease in protein-protein interaction energy arising from this sequence of events leads to the monomerization of the UVR8 dimer. The mechanism presented is in accord with all experimental data available to date.

UV-B exposure, ROS, and stress: inseparable companions or loosely linked associates?

Ultraviolet-B (UV-B) radiation has long been perceived as a stressor. However, a conceptual U-turn has taken place, and UV-B damage is now considered rare. We question whether UV-stress and UV-B-induced reactive oxygen species (ROS) are still relevant concepts, and if ROS-mediated signaling contributes to UV-B acclimation. Measurements of antioxidants and of antioxidant genes show that both low and high UV-B doses alter ROS metabolism. Yet, there is no evidence that ROS control gene expression under low UV-B. Instead, expression of antioxidant genes is linked to the UV RESISTANCE LOCUS 8 pathway. We hypothesize that low UV-B doses cause 'eustress' (good stress) and that stimuli-specific signaling pathways pre-dispose plants to a state of low alert that includes activation of antioxidant defenses.