Jun-ya Kohno

Ejection mechanism of molecules and neutral clusters from liquid beam under irradiation of IR laser

Abstract Resorcinol molecules and those solvated with solvent water molecules were isolated in the gas phase from a liquid beam of an aqueous solution of resorcinol by resonant vibrational excitation of solvent water molecules under IR-laser irradiation. The spatial distribution of the ejected species at various delay times from the IR-laser irradiation indicates that two different isolation mechanisms operate: One dominates in a time range shorter than ∼1 μs (early-time domain), and the other in a time range longer than ∼1 μs (late-time domain). A time-dependent measurement of the liquid-beam profile by optical diffraction shows that the beam has a smooth surface in the early-time domain, whereas in the late-time domain the surface roughness overweighs the wavelength of the illumination laser.

Simultaneous detection of images and Raman spectra of colliding droplets: composition analysis of protrusions emerging during collisions of ethanol and water droplets.

Processes involved between colliding droplets were investigated using simultaneous analysis of spectra and images of Raman-scattered light emitted by irradiation with a pulsed laser. This enabled spatially and temporally resolved Raman spectra of the colliding droplets to be obtained. Colliding droplets of ethanol and water produce a characteristic protrusion from the contact point to the antipode of the water droplet in the course of interaction. From its Raman spectrum, the protrusion is seen to be composed of water. This result supports our surface-tension release model previously proposed to describe the mechanism of protrusion formation because the protrusion is the result of positive interference of a capillary wave propagating over the surface of the water droplet in this model.

Microcrystal delivery by pulsed liquid droplet for serial femtosecond crystallography

A liquid-droplet injector has been developed that delivers pristine microcrystals to an X-ray irradiation area for conducting serial femtosecond crystallography (SFX) with an X-ray free-electron laser (XFEL). By finely tuning the pulsed liquid droplets in time and space, a high hit rate of the XFEL pulses to microcrystals in the droplets was achieved for measurements using 5 µm tetragonal lysozyme crystals, which produced 4265 indexable diffraction images in about 30 min. The structure was determined at a resolution of 2.3 Å from <0.3 mg of protein. With further improvements such as reduction of the droplet size, liquid droplets have considerable potential as a crystal carrier for SFX with low sample consumption.

Mechanism of ion ejection from a liquid beam following laser photoionization

An anisole—ethanol solution was introduced into vacuum as a continuous liquid flow (liquid beam), and the molecules in the liquid beam were ionized by laser two-photon ionization. Ions ejected from the liquid beam were extracted by applying a pulsed electric field for the measurement of time-of-flight mass spectra of the ions. The intensities and peak profiles of the ions were measured by varying the delay time from the laser ionization to the pulse extraction of the ions at different laser powers. All the ions observed have almost the same velocity (≈ 700 m s−1) and need ≈ 1 μs to leave the liquid beam after laser irradiation. This finding implies that each photoion forming a solvation structure with almost the same number of solvent molecules is expelled from the liquid surface by Coulomb ejection and is dissociated into a cluster ion outside the influence of the Coulomb potential. The rate constants for ion ejection were determined.

Charge State of Lysozyme Molecules in the Gas Phase Produced by IR-Laser Ablation of Droplet Beam

Molecules exhibit their intrinsic properties in their isolated forms. Investigations of isolated large biomolecules require an understanding of the detailed mechanisms for their emergence in the gas phase because these properties may depend on the isolation process. In this study, we apply droplet-beam laser-ablation mass spectrometry to isolate protein molecules in the gas phase by IR-laser ablation of aqueous protein solutions, and we discuss the isolation mechanism. Multiply charged hydrated lysozyme clusters were produced by irradiation of the IR laser onto a droplet beam of aqueous lysozyme solutions with various pH values prepared by addition of sodium hydroxide to the solution. The ions produced in the gas phase show significantly low abundance and have a lower number of charges on them than those in the aqueous solutions, which we explained using a nanodroplet model. This study gives quantitative support for the nanodroplet model, which will serve as a fundamental basis for further studies of biomolecules in the gas phase.

Ejection of Clusters from Liquid Beam Surface by IR Laser Irradiation

A continuous liquid flow of an aqueous solution of phenol (Phe) in a vacuum (a liquid beam) was irradiated with an IR laser at 2.85 μm, which is resonant to the vibrational absorption band of the liquid water. The Phe (H 2 O) N , ejected from the liquid beam surface into the gas phase, was ionized by a UV laser at 270 nm into hydrated phenol cluster ions, Phe + (H 2 O) n (n = 0 - 30), and analyzed by a time-of-flight mass spectrometer. The velocity distributions of the product cluster ions were derived from the spatial distributions measured at different elapsed times after the IR laser irradiation. The results and the analysis show that dense neutral clusters are ejected from surface regions locally heated by the intense IR laser.

Mechanism of Protein Molecule Isolation by IR Laser Ablation of Droplet Beam

Gas-phase isolation of bovine serum albumin (BSA) from aqueous solutions is performed by IR laser ablation of a droplet beam. Multiply charged BSA ions (positive and negative) were produced by the IR laser irradiation onto a droplet beam of aqueous BSA solutions with various pH values prepared by addition of hydrochloric acid or sodium hydroxide to the solution. The isolation mechanism was discussed based on the charge state of the isolated BSA ions. A nanodroplet model explains the gas-phase charge distribution of the BSA ions. This study provides a fundamental basis for further studies of a wide variety of biomolecules in the gas phase isolated directly from solution.

Coloring Rate of Phenolphthalein by Reaction with Alkaline Solution Observed by Liquid-Droplet Collision.

Many important chemical reactions are induced by mixing two solutions. This paper presents a new way to measure rates of rapid chemical reactions induced by mixing two reactant solutions using a liquid-droplet collision. The coloring reaction of phenolphthalein (H2PP) by a reaction with NaOH is investigated kinetically. Liquid droplets of H2PP/ethanol and NaOH/H2O solutions are made to collide, which induces a reaction that transforms H2PP into a deprotonated form (PP(2-)). The concentration of PP(2-) is evaluated from the RGB values of pixels in the colored droplet images, and is measured as a function of the elapsed time from the collision. The obtained rate constant is (2.2 ± 0.7) × 10(3) M(-1) s(-1), which is the rate constant for the rate-determining step of the coloring reaction of H2PP. This method was shown to be applicable to determine rate constants of rapid chemical reactions between two solutions.

Ionization-induced nucleation above liquid beam of aqueous solution of phenol after IR laser irradiation

Abstract A continuous liquid flow of an aqueous solution of phenol (Ph) in a vacuum (a liquid beam) was irradiated with a pulsed IR laser at 3 μm, which was resonant to the OH-stretching vibration of the solvent water molecules. Phenol molecules ejected from the liquid beam were selectively ionized at about 0.5 mm above it by a pulsed UV laser (270–280 nm). The photoions thus produced were extracted in a pulsed electric field with a given residence time after the photoionization for mass analysis. It was shown that photoions, Ph+, were solvated into Ph+(H2O)n in a dense cloud of water vapor ejected from the liquid beam by IR irradiation.

Nanosecond pump–probe device for time-resolved serial femtosecond crystallography developed at SACLA

A nanosecond pump–probe device for time-resolved serial femtosecond crystallography has been developed at SACLA.