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A very precise measurement of the magnetic moment of a single electron bound to a carbon nucleus, combined with a state-of-the-art calculation in the framework of bound-state quantum electrodynamics, gives a new value of the atomic mass of the electron that is more precise than the currently accepted one by a factor of 13. Electron mass to unprecedented precision The atomic mass of the electron is a key parameter for fundamental physics. A precise determination is a challenge because the mass is so low. Sven Sturm and colleagues report on a new determination of the electron's mass in atomic units. The authors measured the magnetic moment of a single electron bound to a reference ion (a bare nucleus of carbon-12). The results were analysed using state-of-the-art quantum electrodynamics theory to yield a mass value with a precision that exceeds the current literature value by more than an order of magnitude. The quest for the value of the electron’s atomic mass has been the subject of continuing efforts over the past few decades 1 , 2 , 3 , 4 . Among the seemingly fundamental constants that parameterize the Standard Model of physics 5 and which are thus responsible for its predictive power, the electron mass m e is prominent, being responsible for the structure and properties of atoms and molecules. It is closely linked to other fundamental constants, such as the Rydberg constant R ∞ and the fine-structure constant α (ref. 6 ). However, the low mass of the electron considerably complicates its precise determination. Here we combine a very precise measurement of the magnetic moment of a single electron bound to a carbon nucleus with a state-of-the-art calculation in the framework of bound-state quantum electrodynamics. The precision of the resulting value for the atomic mass of the electron surpasses the current literature value of the Committee on Data for Science and Technology (CODATA 6 ) by a factor of 13. This result lays the foundation for future fundamental physics experiments 7 , 8 and precision tests of the Standard Model 9 , 10 , 11 .

Recent years have witnessed a remarkable improvement in the theoretical description of bound-electron g factors, paralleled with a quantum jump in the experimental accuracy in the investigation of these quantities. In the present article we give a brief summary of the latest developments, with emphasis on the influence of quantum electrodynamic and nuclear effects on the g factor of few-electron highly charged ions, and on the possible determination of fundamental constants.

The determination of the electron mass from Penning-trap measurements with \\(^{12}\\)C\\(^{5+}\\) ions and from theoretical results for the bound-electron \\(g\\) factor is described in detail. Some recently calculated contributions slightly shift the extracted mass value. Prospects of a further improvement of the electron mass are discussed both from the experimental and from the theoretical point of view. Measurements with \\(^4\\)He\\(^+\\) ions will enable a consistency check of the electron mass value, and in future an improvement of the \\(^4\\)He nuclear mass and a determination of the fine-structure constant.

The nuclear shape correction to the g factor of a bound electron in 1S-state is calculated for a number of nuclei in the range of charge numbers from Z=6 up to Z=92. The leading relativistic deformation correction has been derived analytically and also its influence on one-loop quantum electrodynamic terms has been evaluated. We show the leading corrections to become significant for mid-Z ions and for very heavy elements to even reach the 10^(-6) level.

The crystal structure of the core domain of the hepatitis C virus surface glycoprotein E2 has been solved; the structure shows that, contrary to expectation, E2 is unlikely to be the viral fusion protein. Hepatitis C virus envelope structure There is currently no vaccine against hepatitis C virus, so it is important to learn more about the processes by which the virus establishes infection. Joseph Marcotrigiano and colleagues solve the crystal structure of the core domain of the hepatitis C virus surface glycoprotein E2. The structure shows that, contrary to expectation, E2 is unlikely to be the viral fusion protein. This work helps to clarify the role of E2 and the mechanism of hepatitis C virus entry. Hepatitis C virus (HCV) is a significant public health concern with approximately 160 million people infected worldwide 1 . HCV infection often results in chronic hepatitis, liver cirrhosis and hepatocellular carcinoma. No vaccine is available and current therapies are effective against some, but not all, genotypes. HCV is an enveloped virus with two surface glycoproteins (E1 and E2). E2 binds to the host cell through interactions with scavenger receptor class B type I (SR-BI) and CD81, and serves as a target for neutralizing antibodies 2 , 3 , 4 . Little is known about the molecular mechanism that mediates cell entry and membrane fusion, although E2 is predicted to be a class II viral fusion protein. Here we describe the structure of the E2 core domain in complex with an antigen-binding fragment (Fab) at 2.4 Å resolution. The E2 core has a compact, globular domain structure, consisting mostly of β-strands and random coil with two small α-helices. The strands are arranged in two, perpendicular sheets (A and B), which are held together by an extensive hydrophobic core and disulphide bonds. Sheet A has an IgG-like fold that is commonly found in viral and cellular proteins, whereas sheet B represents a novel fold. Solution-based studies demonstrate that the full-length E2 ectodomain has a similar globular architecture and does not undergo significant conformational or oligomeric rearrangements on exposure to low pH. Thus, the IgG-like fold is the only feature that E2 shares with class II membrane fusion proteins. These results provide unprecedented insights into HCV entry and will assist in developing an HCV vaccine and new inhibitors.