Doping of Green Fluorescent Protein into Superfluid Helium Droplets: Size and Velocity of Doped Droplets

Abstract

We report doping of green fluorescent protein from an electrospray ionization (ESI) source into superfluid helium droplets. From analyses of the time profiles of the doped droplets, we identify two distinct groups of droplets. The faster group has a smaller average size, on the order of 106 helium atoms/droplet, and the slower group is much larger, by at least an order of magnitude. The relative populations of these two groups depend on the temperature of the droplet source: from 11 K to 5 K, the signal intensity of the slower droplet group gradually increases, from near the detection limit to comparable to that of the faster group. We postulate that the smaller droplets are formed via condensation of gaseous helium upon expansion from the pulsed valve, while the larger droplets develop from fragmentation of ejected liquid helium. Our results on the size and velocity of the condensation peak at higher source temperatures (> 7 K) agree with previous reports, but those at lower temperatures (< 7 K) seem to be off. We attribute this discrepancy to the masking effect of the exceedingly large droplets from the fragmentation peak in previous measurements of droplet sizes. Within the temperature range of our investigation, although the expansion condition changes from subcritical to supercritical, there is no abrupt change in either the velocity distribution or the size distribution of the condensation peak, and the most salient effect is in the increasing intensity of the fragmentation peak. The absolute doping efficiency, as expressed by the ratio of ion doped droplets over the total number of ions from the ESI source, is on the order of 10−4, while only hundreds of doped ions have been detected. Further improvements in the ESI source are key to extending the technology for future experiments. On the other hand, the separation of the two groups of droplets in velocity is beneficial for size selection of only the smaller droplets for future experiments of electron diffraction.

Electron diffraction as a structure tool for charged and neutral nanoclusters formed in superfluid helium droplets

“…In fact, our experimental characterization of droplets produced from different types of pulse valves has repeatedly confirmed the bimodal distribution of droplet sizes. 60–63 On the other hand, the ionization and detection scheme can be easily implemented in most laboratories, and hence this knowledge on the conditions of the droplets can be obtained with minimal efforts.…”

Infrared spectroscopy in superfluid helium droplets

“…While resonance multiphoton ionization is only feasible with specific molecules, electrospray ionization (ESI) provides a universal technique for bringing large ions, such as proteins, into the gas phase [195,196,[198][199][200]. However, the low density of ions in electrospray has warranted a considerable modification of the pickup technique which involves ion traps.…”

Chemistry and physics of dopants embedded in helium droplets

“…Similar setups were already realized by the groups of Bierau et al (2010) and Alghamdi et al (2017) and successfully implemented to investigate the pickup conditions of large biomolecular systems as well as to perform UV-vis (Filsinger et al, 2012) and infrared spectroscopy (Florez et al, 2015(Florez et al, , 2016Mucha et al, 2017Mucha et al, , 2018 of mass-selected biomolecular ions. Nevertheless, spectroscopic studies on biomolecules embedded in helium droplets are still in their infancy, but highly desirable to study the intrinsic photophysical properties of biomolecules in a welldefined cryostatic environment, and to deliver benchmark data for high-level quantum chemical calculations.…”