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Conformer-specific dynamics Conformation-dependent dynamics play an important role in organic chemistry syntheses such as electrocyclic reactions, as well as in biological processes such as protein folding. However, current time-resolved experimental methods struggle to distinguish conformers from each other, and conformational isomerism is usually analyzed through reactant and product distributions. Using a combination of mega–electron volt ultrafast electron diffraction and quantum wave packet simulations, Champenois et al . directly followed the photochemical electrocyclic ring opening of the molecule α-phellandrene with femtosecond time resolution and confirmed that the transformation of a specific molecular conformer follows the famous Woodward-Hoffmann rules. The proposed method is potentially a powerful tool to follow conformer specificity in various organic and biological systems in real time. —YS

Shear selectivity Chemical reactions typically proceed by distributing energy statistically among all accessible molecular vibrations. Liu et al. report that external shear forces can sometimes pry open strained carbon rings without dissipating energy into adjacent bond rotations. Through careful design and synthesis of polymer-embedded cyclobutyl rings, the authors showed that certain relative substituent geometries are preserved when sonication induces ring opening. Accompanying simulations support the instigation of “flyby” trajectories that channel energy narrowly to cleave the cyclic sigma bonds and then rapidly form acyclic pi bonds. Science , abi7609, this issue p. 208

Electronic and nuclear dynamics in one Because of the complex, ultrafast interplay between nuclear and electronic degrees of freedom, probing both nuclear and electronic dynamics in excited electronic states within a single time-resolved experiment is a great challenge. Yang et al. used ultrafast electron diffraction in combination with ab initio nonadiabatic molecular dynamics and diffraction simulations to study the relaxation dynamics of isolated pyridine molecules after photoexcitation to the S 1 state (see the Perspective by Domcke and Sobolewski). They showed that electronic state evolution and molecular structural changes can be recorded simultaneously and independently by tracing a transient signal in small-angle inelastic scattering and large-angle elastic diffraction, respectively. Science , this issue p. 885 ; see also p. 820

Forced Open Traditionally, the study of reaction chemistry has relied on random encounters between molecules to initiate the proceedings. Heating and stirring increase the power and frequency of such encounters but provide little finer control. Very recently, chemists have learned how to initiate reactions more directly by embedding precursors in the backbone of a polymer large enough to manipulate with shear forces. Lenhardt et al. (p. 1057 ) applied this technique to a cyclopropyl ring-opening reaction. When the strained triangular carbon rings were embedded within a polymer, shear force applied by sonication ruptured their bonds as the polymer backbone stretched. The taut polymer then redirected the ring-opened intermediates toward a product differently arranged from that generated by simple heating.

In our Perspective, we argue that the earliest Americans followed a coastal migration route. Potter et al. counter that an inland, ice-free corridor route was more likely. The genetic and archaeological evidence that Potter et al. discuss support both the timing and coastal route for the peopling of

We have used adenosine diphosphate analogs containing electron paramagnetic resonance (EPR) spin moieties and EPR spectroscopy to show that the nucleotide-binding site of kinesin-family motors closes when the motor·diphosphate complex binds to microtubules. Structural analyses demonstrate that a domain movement in the switch 1 region at the nucleotide site, homologous to domain movements in the switch 1 region in the G proteins [heterotrimeric guanine nucleotide–binding proteins], explains the EPR data. The switch movement primes the motor both for the free energy–yielding nucleotide hydrolysis reaction and for subsequent conformational changes that are crucial for the generation of force and directed motion along the microtubule.

Large segmental duplications cover much of the Arabidopsis thaliana genome. Little is known about their origins. We show that they are primarily due to at least four different large-scale duplication events that occurred 100 to 200 million years ago, a formative period in the diversification of the angiosperms. A better understanding of the complex structural history of angiosperm genomes is necessary to make full use of Arabidopsis as a genetic model for other plant species.

We sequenced and annotated the genome of the filamentous fungus Fusarium graminearum , a major pathogen of cultivated cereals. Very few repetitive sequences were detected, and the process of repeat-induced point mutation, in which duplicated sequences are subject to extensive mutation, may partially account for the reduced repeat content and apparent low number of paralogous (ancestrally duplicated) genes. A second strain of F. graminearum contained more than 10,000 single-nucleotide polymorphisms, which were frequently located near telomeres and within other discrete chromosomal segments. Many highly polymorphic regions contained sets of genes implicated in plant-fungus interactions and were unusually divergent, with higher rates of recombination. These regions of genome innovation may result from selection due to interactions of F. graminearum with its plant hosts.

Depending on environmental conditions, the aquatic angiosperm Callitriche heterophylia can develop two different leaf types with distinctive morphological characteristics. Cellular turgor pressure seems to act as the biophysical mechanism responsible for the selection of leaf form in control conditions designed to mimic the natural habitat of this plant. The experimental induction of leaf form involves the ability of various treatments to mediate cell expansion through their effects on turgor pressure or wall extensibility.