Biochemical Science Division, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA. Anne.plant@nist.gov|Biochemical Science Division, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA. Anne.plant@nist.gov|Biochemical Science Division, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA. Anne.plant@nist.gov