Water is the most abundant liquid on earth and also the substance with the largest number of anomalies in its properties. It is a prerequisite for life and as such a most important subject of current research in chemical physics and physical chemistry. In spite of its simplicity as a liquid, it has an enormously rich phase diagram where different types of ices, amorphous phases, and anomalies disclose a path that points to unique thermodynamics of its supercooled liquid state that still hides many unraveled secrets. In this review we describe the behavior of water in the regime from ambient conditions to the deeply supercooled region. The review describes simulations and experiments on this anomalous liquid. Several scenarios have been proposed to explain the anomalous properties that become strongly enhanced in the supercooled region. Among those, the second critical-point scenario has been investigated extensively, and at present most experimental evidence point to this scenario. Starting from very low temperatures, a coexistence line between a high-density amorphous phase and a low-density amorphous phase would continue in a coexistence line between a high-density and a low-density liquid phase terminating in a liquid–liquid critical point, LLCP. On approaching this LLCP from the one-phase region, a crossover in thermodynamics and dynamics can be found. This is discussed based on a picture of a temperature-dependent balance between a high-density liquid and a low-density liquid favored by, respectively, entropy and enthalpy, leading to a consistent picture of the thermodynamics of bulk water. Ice nucleation is also discussed, since this is what severely impedes experimental investigation of the vicinity of the proposed LLCP. Experimental investigation of stretched water, i.e., water at negative pressure, gives access to a different regime of the complex water diagram. Different ways to inhibit crystallization through confinement and aqueous solutions are discussed through results from experiments and simulations using the most sophisticated and advanced techniques. These findings represent tiles of a global picture that still needs to be completed. Some of the possible experimental lines of research that are essential to complete this picture are explored.

One Protein, Two Probes: A central challenge in the use of x-ray diffraction to characterize macromolecular structure is the propensity of the high-energy radiation to damage the sample during data collection. Recently, a powerful accelerator-based, ultrafast x-ray laser source has been used to determine the geometric structures of small protein crystals too fragile for conventional diffraction techniques. Kern et al. (p. 491 , published online 14 February) now pair this method with concurrent x-ray emission spectroscopy to probe electronic structure, as well as geometry, and were able to characterize the metal oxidation states in the oxygen-evolving complex within photosystem II crystals, while simultaneously verifying the surrounding protein structure.

Pointing to a second critical point: One explanation for the divergence of many of the thermodynamic properties of water is that there is a critical point in deeply supercooled water at some positive pressure. For bulk water samples, these conditions are described as “no man's land,” because ice nucleates before such temperatures can be reached. Kim et al. used femtosecond x-ray laser pulses to probe micrometer-sized water droplets cooled to 227 K (see the Perspective by Gallo and Stanley). The temperature dependence of the isothermal compressibility and correlation length extracted from x-ray scattering functions showed maxima at 229 K for H ₂ O and 233 K for D ₂ O, rather than diverging to infinity. These results point to the existence of the Widom line, a locus of maximum correlation lengths emanating from a critical point in the supercooled regime. Science , this issue p. 1589 ; see also p. 1543

Catching CO oxidation: Details of the transition state that forms as carbon monoxide (CO) adsorbed on a ruthenium surface is oxidized to CO ₂ have been revealed by ultrafast excitation and probe methods. Öström et al. initiated the reaction between CO and adsorbed oxygen atoms with laser pulses that rapidly heated the surface and then probed the changes in electronic structure with oxygen x-ray absorption spectroscopy. They observed transition-state configurations that are consistent with density functional theory and a quantum oscillator model. Science , this issue p. 978

An electrospun liquid microjet has been developed that delivers protein microcrystal suspensions at flow rates of 0.14–3.1 µl min⁻¹to perform serial femtosecond crystallography (SFX) studies with X-ray lasers. Thermolysin microcrystals flowed at 0.17 µl min⁻¹and diffracted to beyond 4 Å resolution, producing 14 000 indexable diffraction patterns, or four per second, from 140 µg of protein. Nanoflow electrospinning extends SFX to biological samples that necessitate minimal sample consumption.