title: Microdroplet fusion mass spectrometry for fast reaction kinetics.
abstract: We investigated the fusion of high-speed liquid droplets as a way to record the kinetics of liquid-phase chemical reactions on the order of microseconds. Two streams of micrometer-size droplets collide with one another. The droplets that fused (13 μm in diameter) at the intersection of the two streams entered the heated capillary inlet of a mass spectrometer. The mass spectrum was recorded as a function of the distance x between the mass spectrometer inlet and the droplet fusion center. Fused droplet trajectories were imaged with a high-speed camera, revealing that the droplet fusion occurred approximately within a 500-μm radius from the droplet fusion center and both the size and the speed of the fused droplets remained relatively constant as they traveled from the droplet fusion center to the mass spectrometer inlet. Evidence is presented that the reaction effectively stops upon entering the heated inlet of the mass spectrometer. Thus, the reaction time was proportional to x and could be measured and manipulated by controlling the distance x. Kinetic studies were carried out in fused water droplets for acid-induced unfolding of cytochrome c and hydrogen-deuterium exchange in bradykinin. The kinetics of the former revealed the slowing of the unfolding rates at the early stage of the reaction within 50 μs. The hydrogen-deuterium exchange revealed the existence of two distinct populations with fast and slow exchange rates. These studies demonstrated the power of this technique to detect reaction intermediates in fused liquid droplets with microsecond temporal resolution.
authors.affiliation: Department of Chemistry, Stanford University, Stanford, CA 94305; Center for Plant Aging Research, Institute for Basic Science, Daegu 711-873, Republic of Korea; and.
authors.forename: Jae Kyoo
authors.initials: JK
authors.lastname: Lee
authors.affiliation: Department of Chemistry, Stanford University, Stanford, CA 94305; Center for Plant Aging Research, Institute for Basic Science, Daegu 711-873, Republic of Korea; and.
authors.forename: Samuel
authors.initials: S
authors.lastname: Kim
authors.affiliation: Center for Plant Aging Research, Institute for Basic Science, Daegu 711-873, Republic of Korea; and Department of New Biology, Daegu Gyeongbuk Institute of Science and Technology (DGIST), Daegu 711-873, Republic of Korea zare@stanford.edu nam@dgist.ac.kr.
authors.forename: Hong Gil
authors.initials: HG
authors.lastname: Nam
authors.affiliation: Department of Chemistry, Stanford University, Stanford, CA 94305; zare@stanford.edu nam@dgist.ac.kr.
authors.forename: Richard N
authors.initials: RN
authors.lastname: Zare
keywords: hydrogen–deuterium isotope exchange; liquid microdroplets; mass spectrometry; protein unfolding; reaction kinetics