Electronic movement flashing into view Numerous chemical processes begin with ionization: the ejection of an electron from a molecule. What happens in the immediate aftermath of that event? Kraus et al. explored this question in iodoacetylene by detecting and analyzing the spectrum of emitted high harmonics (see the Perspective by Ueda). They traced the migration of the residual positively charged hole along the molecular axis on a time scale faster than a quadrillionth of a second. They thereby characterized the capacity of a laser field to steer the hole's motion in appropriately oriented molecules. Science , this issue p. 790 ; see also p. 740

Ultrafast, subfemtosecond charge migration in small peptides is discussed on the basis of computational studies and compared with the selective bond dissociation after ionization as observed by Schlag and Weinkauf. The reported relaxation could be probed in real time if the removal of an electron could be achieved on the attosecond time scale. Then the mean field seen by an electron would be changing rapidly enough to initiate the migration. Tyrosine-terminated tetrapeptides have a particularly fast charge migration where in <1 fs the charge arrives at the other end. A femtosecond pulse can be used to observe the somewhat slower relaxation induced by correlation between electrons of different spins. A slower relaxation also is indicated when removing a deeper-lying valence electron. When a chromophoric amino acid is at one end of the peptide, the charge can migrate all along the peptide backbone up to the N end, but site-selective ionization is probably easier to detect for tryptophan than for tyrosine.attosecond lasers ͉ charge transfer ͉ protein mass spectrometry A chemical rearrangement occurs when atoms in a molecule change their specific arrangement. The time scale of chemistry is therefore the time scale for the motion of atoms (1, 2). The forces operating on the atoms determine the path of this motion. In the Born-Oppenheimer approximation, it is the averaging over the much faster motion of the electrons that specifies the potential energy and hence the forces. The physical picture for this approximation is that the electrons instantly adjust to the current position of the nuclei so that the equilibrium arrangement of the bonded atoms is determined as a minimum of the electronic energy. In the Born-Oppenheimer approximation, the system is confined to be in a single stationary electronic state, and if this confinement is strict then there is no electronic time scale that is relevant to chemistry. During a chemical rearrangement, the motion of the atoms can be accompanied by a reorganization of the electronic structure, and recent experimental progress allows for probing this change (3) in real time. However, it is the motion of the nuclei that sets the time scale because the electronic reorganization exactly tracks the shifts in the positions of the nuclei. As the heavier nuclei move, the light electrons immediately adapt.It is recognized that the energy of the same stationary electronic state can have two distinct minima, where the electronic charge distribution has a quite different character. At the immediate vicinity of either of the two configurations of the nuclei, the motion is bounded, but larger-amplitude motion can connect the two hollows. Major intramolecular charge reorganization follows upon a change in nuclear geometry (4-8). The time scale of the charge shuffling is that of the motion of the nuclei relocating between the two configurations.Optical excitation can prepare a nonstationary state, and such a state is often known as the optically bright state. The optically bright state is not sta...

Biological systems that are capable of performing computational operations could be of use in bioengineering and nanomedicine, and DNA and other biomolecules have already been used as active components in biocomputational circuits. There have also been demonstrations of DNA/RNA-enzyme-based automatons, logic control of gene expression, and RNA systems for processing of intracellular information. However, for biocomputational circuits to be useful for applications it will be necessary to develop a library of computing elements, to demonstrate the modular coupling of these elements, and to demonstrate that this approach is scalable. Here, we report the construction of a DNA-based computational platform that uses a library of catalytic nucleic acids (DNAzymes), and their substrates, for the input-guided dynamic assembly of a universal set of logic gates and a half-adder/half-subtractor system. We demonstrate multilayered gate cascades, fan-out gates and parallel logic gate operations. In response to input markers, the system can regulate the controlled expression of anti-sense molecules, or aptamers, that act as inhibitors for enzymes.

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