1Johns Hopkins Univ, Sch Med, Dept Biol Chem, Baltimore, MD 21205, USA and Bruker Instruments Inc, Fremont, CA 94539, USA
2Johns Hopkins Univ, Sch Med, Dept Biophys & Biophys Chem, 217 Traylor Bldg, 720 Rutland Ave, Baltimore, MD 21205, USA
3Johns Hopkins Univ, Sch Med, Dept Radiol, Baltimore, MD 21205, USA