1. Department of Biochemistry and Molecular Biophysics, California Institute of Technology, Pasadena, CA 91125, USA.
2. Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
3. Ronald and Maxine Linde Center for Global Environmental Science, California Institute of Technology, Pasadena, CA 91125, USA.
4. Departments of Applied Physics and Bioengineering, California Institute of Technology, Pasadena, CA 91125, USA.