Microtubules are dynamic tubulin polymers responsible for many cellular processes, including the capture and segregation of chromosomes during mitosis. In contrast to textbook models of tubulin self-assembly, we have recently demonstrated that microtubules elongate by addition of bent guanosine triphosphate tubulin to the tips of curving protofilaments. Here we explore this mechanism of microtubule growth using Brownian dynamics modeling and electron cryotomography. The previously described flaring shapes of growing microtubule tips are remarkably consistent under various assembly conditions, including different tubulin concentrations, the presence or absence of a polymerization catalyst or tubulin-binding drugs. Simulations indicate that development of substantial forces during microtubule growth and shortening requires a high activation energy barrier in lateral tubulin-tubulin interactions. Modeling offers a mechanism to explain kinetochore coupling to growing microtubule tips under assisting force, and it predicts a load-dependent acceleration of microtubule assembly, providing a role for the flared morphology of growing microtubule ends.

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Photochemical reactions between stabilized furyl‐substituted phosphonium–iodonium ylides and terminal acetylenes led to the formation of new P‐containing heterocyclic compounds. For the heterocyclization of 2‐furyl‐substituted ylides, we observed an unprecedented electrophilic ipso attack onto the α position of the furan ring in a carbocationic intermediate, leading – via a spiro intermediate – to the rearranged product with a 2,3‐shift in the furan ring.

Several years ago the photoinduced reaction of mixed phosphonium-iodonium ylides (1) with acetylenes (2) to give λ 5 -phosphinolines (3) and substituted furans (4) was described. This reaction is one-pot, metal-free synthesis of heterocycles 3 and 4 with the yields of 40% to 80%. The reaction proceeds only in dichloromethane (DCM) at the high ylide concentrations (>0.01 mol/L). The product analysis by 31 P NMR, electrospray ionization mass spectrometry, UVvis spectrophotometry, and the dynamic light scattering study of the self-aggregation of the ylide in DCM showed a dual role of the solvent in the photoinduced reactions of mixed phosphonium-iodonium ylide: (i) at the low ylide concentrations (<0.01 mol/L), the conjugated photoinitiation of the chain reaction in DCM results in the formation of chlorine-containing products and (ii) at the high ylide concentrations (>0.01 mol/L), the photolysis mechanism is determined by self-organization of the ylide molecules to give large stable aggregates in DCM, in which the target heterocycles are synthesized. Two important issues follow from the study. First, the annulation reaction between mixed phosphonium-iodonium ylide and acetylenes occurs only when the reactive intermediates are in close proximity to one other, and, second, DCM is not inert reagent in reactions occurring with participation of radicals and in one form or another can participate in photoinduced radical reactions of various solutes.KEYWORDS annulation with acetylenes, dichloromethane, phosphonium-iodonium ylides, radical reactions, selfassembly This article is dedicated with respect and affection to the memory of the late Prof. Nikolay S. Zefirov.

The preparation and chemistry of novel sulfonyl- and phosphoryl-derived λ(3)-iodanes are reported. These compounds with three different heteroatoms attached to a negatively charged C atom represent potentially useful reagents that combine in one molecule the synthetic advantages of a phosphonium ylide and an iodonium salt. Specifically, they can react with a number of acetylenes, leading to hitherto unknown sulfonyl- and phosphoryl-substituted phosphinolines, phosphininothiophenes, and a novel type of annelated P-containing heterocycle--phosphininopyrazole.