We study the structural dynamics of photoexcited ½CoðterpyÞ 2 2þ in an aqueous solution with ultrafast x-ray diffuse scattering experiments conducted at the Linac Coherent Light Source. Through direct comparisons with density functional theory calculations, our analysis shows that the photoexcitation event leads to elongation of the Co-N bonds, followed by coherent Co-N bond length oscillations arising from the impulsive excitation of a vibrational mode dominated by the symmetrical stretch of all six Co-N bonds. This mode has a period of 0.33 ps and decays on a subpicosecond time scale. We find that the equilibrium bond-elongated structure of the high spin state is established on a single-picosecond time scale and that this state has a lifetime of ∼7 ps. DOI: 10.1103/PhysRevLett.117.013002 Several Co(II) compounds are known to transition between their low spin (LS) and high spin (HS) electronic states [1][2][3]. Such transitions can be induced by temperature increase, excitation by light, or high magnetic fields [4], and they are accompanied by distinct changes in magnetic and structural properties that may be exploited in the design of display and memory devices [5,6] and in single-molecule spintronic applications [7]. The realization of exploitable spin-state transitions (SSTs) in Co(II) compounds is more challenging than in the corresponding Fe(II) complexes, which have been investigated in great detail during the last decades [8][9][10][11][12][13][14][15]. These challenges stem from the partial occupation of the antibonding e à g orbitals in the ground state, which leads to smaller structural changes arising from the SST phenomenon; the corresponding smaller energy barriers between the potential surfaces of the HS and LS Co(II) states result in faster dynamics [1], as well as a high sensitivity to the crystalline environment or to the solvent properties [2]. The key structural parameters for the SSTs are the Co-N bond lengths [8], but the time scales and the dynamics of the LS-HS transitions have remained unclear for Co compounds. Time-resolved x-ray scattering can be used to monitor such structural changes and dynamics if the time resolution of the experiment is sufficiently high. X-ray free electron lasers (XFELs) provide ultrashort (∼30 fs) x-ray pulses and high flux allowing the nuclear dynamics following photoexcitation to be recorded at the required femtosecond time scales [16,17]. Here, we report, for the first time, direct measurements of the excited-state structure and the ultrafast structural dynamics of a solvated Co(II) complex upon a photoinduced SST. Figure 1 shows the molecular structure of ½CoðterpyÞ 2 2þ ðterpy ¼ 2; 2 0 ∶6 0 ; 2 00 − terpyridineÞ. In this six-coordinated complex, the d 7 Co center can be either a LS doublet state or a HS quartet state [2,18]. In solid-state samples, the relative populations of both spin states depend strongly on the In this work, we utilized x-ray diffuse scattering (XDS) laser pump-x-ray probe experiments to study the formation, structure, and decay of...

Microtubules are protein polymers that form "molecular highways" for long-range transport within living cells. Molecular motors actively step along microtubules to shuttle cellular materials between the nucleus and the cell periphery; this transport is critical for the survival and health of all eukaryotic cells. Structural defects in microtubules exist, but whether these defects impact molecular motor-based transport remains unknown. Here, we report a new, to our knowledge, approach that allowed us to directly investigate the impact of such defects. Using a modified optical-trapping method, we examined the group function of a major molecular motor, conventional kinesin, when transporting cargos along individual microtubules. We found that microtubule defects influence kinesin-based transport in vitro. The effects depend on motor number: cargos driven by a few motors tended to unbind prematurely from the microtubule, whereas cargos driven by more motors tended to pause. To our knowledge, our study provides the first direct link between microtubule defects and kinesin function. The effects uncovered in our study may have physiological relevance in vivo.

The structure of the microtubule is tightly regulated in cells via a number of microtubule associated proteins and enzymes. Microtubules accumulate structural defects during polymerization, and defect size can further increase under mechanical stresses. Intriguingly, microtubule defects have been shown to be targeted for removal via severing enzymes or self-repair. The cell’s control in defect removal suggests that defects can impact microtubule-based processes, including molecular motor-based intracellular transport. We previously demonstrated that microtubule defects influence cargo transport by multiple kinesin motors. However, mechanistic investigations of the observed effects remained challenging, since defects occur randomly during polymerization and are not directly observable in current motility assays. To overcome this challenge, we used end-to-end annealing to generate defects that are directly observable using standard epi-fluorescence microscopy. We demonstrate that the annealed sites recapitulate the effects of polymerization-derived defects on multiple-motor transport, and thus represent a simple and appropriate model for naturally-occurring defects. We found that single kinesins undergo premature dissociation, but not preferential pausing, at the annealed sites. Our findings provide the first mechanistic insight to how defects impact kinesin-based transport. Preferential dissociation on the single-molecule level has the potential to impair cargo delivery at locations of microtubule defect sites in vivo.

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