Building stable centromeres: Each of our chromosomes has a single centromere, seen as a constriction during cell division, which is required for accurate chromosome segregation to daughter cells. Falk et al. show that the special histone protein known as CENP-A, which forms part of the nucleosomes at centromeres, makes the chromatin at these constrictions very stable and long-lived. This stability is conferred by a protein-binding partner, CENP-C, recruited to the centromere by the CENP-A histone. Binding of CENP-C to CENP-A–containing nucleosomes alters the behavior of the macromolecular centromere complex to help it maintain its identity Science , this issue p. 699

Photogenerated spin-correlated radical pairs (SCRPs) in electron donor–bridge–acceptor (D–B–A) molecules can act as molecular qubits and inherently spin qubit pairs. SCRPs can take singlet and triplet spin states, comprising the quantum superposition state. Their synthetic accessibility and well-defined structures, together with their ability to be prepared in an initially pure, entangled spin state and optical addressability, make them one of the promising avenues for advancing quantum information science. Coherence between two spin states and spin selective electron transfer reactions form the foundation of using SCRPs as qubits for sensing. We can exploit the unique sensitivity of the spin dynamics of SCRPs to external magnetic fields for sensing applications including resolution-enhanced imaging, magnetometers, and magnetic switch. Molecular quantum sensors, if realized, can provide new technological developments beyond what is possible with classical counterparts. While the community of spin chemistry has actively investigated magnetic field effects on chemical reactions via SCRPs for several decades, we have not yet fully exploited the synthetic tunability of molecular systems to our advantage. This review offers an introduction to the photogenerated SCRPs-based molecular qubits for quantum sensing, aiming to lay the foundation for researchers new to the field and provide a basic reference for researchers active in the field. We focus on the basic principles necessary to construct molecular qubits based on SCRPs and the examples in quantum sensing explored to date from the perspective of the experimentalist.

Modulation of water exchange in lanthanide(III)–DOTA type complexes has drawn considerable attention over the past two decades, particularly because of their application as contrast agents for magnetic resonance imaging. LnDOTA–tetraamide complexes display unusually slow water exchange kinetics and this chemical property offers an opportunity to use these complexes as a new type of contrast agent based upon the chemical exchange saturation transfer (CEST) mechanism. Six new DOTA–tetraamide ligands having side‐chain amide arms with varying hydrophobicity and polarity were prepared and the water exchange characteristic of complexes formed with europium(III) complexes were investigated. The results show that introduction of steric bulk into the amide side‐chain arms of the europium(III) complexes not only favors formation of the mono‐capped twisted square antiprism coordination isomers, the isomer that is generally less favourable for CEST, but also accelerates water exchange in the mono‐capped square antiprism isomers. However, converting single methyl groups on these bulky arms to carboxyl or carboxyl ethyl esters results in a rather dramatic decrease in water exchange rates, about 50‐fold. Thus, steric bulk, polarity and hydrophobicity of the amide side‐chains each contribute to organization of water molecules in the second hydration sphere of the europium(III) ion and this in turn controls water exchange in these complexes. Copyright © 2009 John Wiley & Sons, Ltd.