1. Department of Mechanical Engineering and Materials Science, Yale University, 9 Hillhouse Avenue, New Haven, Connecticut 06520-8286, United States
2. Department of Cellular and Molecular Biology, Centro Nacional de Biotecnología (CNB), CSIC, Darwin 3, 28049 Madrid, Spain
3. Department of Macromolecular Structures, Centro Nacional de Biotecnología (CNB), CSIC, Darwin 3, 28049 Madrid, Spain