Acceleration of reaction in charged microdroplets

Abstract

Using high-resolution mass spectrometry, we have studied the synthesis of isoquinoline in a charged electrospray droplet and the complexation between cytochrome c and maltose in a fused droplet to investigate the feasibility of droplets to drive reactions (both covalent and noncovalent interactions) at a faster rate than that observed in conventional bulk solution. In both the cases we found marked acceleration of reaction, by a factor of a million or more in the former and a factor of a thousand or more in the latter. We believe that carrying out reactions in microdroplets (about 1-15 μm in diameter corresponding to 0·5 pl - 2 nl) is a general method for increasing reaction rates. The mechanism is not presently established but droplet evaporation and droplet confinement of reagents appear to be two important factors among others. In the case of fused water droplets, evaporation has been shown to be almost negligible during the flight time from where droplet fusion occurs and the droplets enter the heated capillary inlet of the mass spectrometer. This suggests that (1) evaporation is not responsible for the acceleration process in aqueous droplet fusion and (2) the droplet-air interface may play a significant role in accelerating the reaction. We argue that this 'microdroplet chemistry' could be a remarkable alternative to accelerate slow and difficult reactions, and in conjunction with mass spectrometry, it may provide a new arena to study chemical and biochemical reactions in a confined environment.

Keywords: Electospray ionization; cytochrome c; droplet fusion; isoquinoline; kinetics; maltose.

Fig. 1

Schematic diagrams of the experimental setup used in our study of ‘microdroplet chemistry’. The upper panel (a) shows the electrospray assisted synthesis of isoquinoline, and the lower panel (b) shows the droplet fusion mass spectrometry to study the complexation between maltose and cytochrome c.

Fig. 2

Pomeranz–Fritsch synthesis of isoquinoline in the charged droplet produced by electrospray process. The left panel shows the two step synthesis of isoquinoline that we followed in the present study. In the first step, the conventional bulk reaction method was used to synthesize the precursor imine C. Then in the second step, the precursor C was injected from methanolic solution through an on-axis electrospray source, in positive ion mode, to form charged droplets encapsulating the precursor C, which was then converted into isoquinoline (E) inside the charged droplet via intermediate D. Each protonated species (precursor C, intermediate D, and product E) were detected and characterized by a high resolution orbitrap mass spectrometer (see the spectra in the right panel; solvent: methanol). The theoretical values of m/z (see left panel) are in good agreement with that experimentally observed (see the right panel).

Fig. 3

ESI-mass spectra of (a) cytochrome c (100 μM) and (b) cytochrome c (100 μM) incubated with maltose (100 mM) for 20 min. The subscript n in PLn denotes the number of bound maltose to cytochrome c (square denotes +8 charge state and circle denotes +7 charge state).

Fig. 4

Kinetics of the binding of cytochrome c and maltose. (a) Deconvoluted mass spectra at different distances (x) with cytochrome c (100 μM) in one droplet source and maltose (100 mM) in the other source. The subscript n in PLn denotes the number of maltose bound to cytochrome c. (b) Normalized relative abundances of cytochrome c with different number of bound maltose (green square: PL1, red circle: PL₆, blue triangle up: PL₁₁, magenta triangle down: PL₁₈). The normalized factor for each plot for PL₁, PL₆, PL₁₁, and PL₁₈ is ×1, ×4·7, ×11·7, and ×17·4, respectively. (c) Average number of bound maltose to cytochrome c as a function of distance x and reaction time. The axes on top of (a) and (c) show the converted reaction time from the corresponding distance.

Fig. 5

Average diameter of pure-water droplets in the microdroplet fusion mass spectrometry as a function of the distance (x). Few noticeable differences were observed in the average size of microdroplets up to the distance of about 7 mm from the droplet fusion center. All kinetic measurements shown in Fig. 4 are performed at distances of 4 mm or less.

Scheme 1

The plausible mechanism of Pomeranz–Fritsch reaction.