Uncovering the Pathway of Serine Octamer Magic Number Cluster Formation during Electrospray Ionization: Experiments and Simulations

Electrospray ionization (ESI) of serine (Ser) solution generates Ser8H+ as an abundant magic number cluster. ESI clustering of most other solutes yields nonspecific stoichiometries. It is unclear why Ser8H+ dominates in the case of Ser, and how Ser8H+ forms during ESI. Even the location of Ser8H+ formation is contentious (in solution, in ESI droplets, or elsewhere). Here we unravel key aspects of the l-Ser8H+ formation pathway. Harsh ion sampling conditions promote the collision-induced dissociation (CID) of regular ESI analytes. Unexpectedly, Ser8H+ was seemingly resistant against CID during ion sampling, despite its extremely low tandem mass spectrometry (MS/MS) stability. This unusual behavior reveals that Ser8H+ forms during ion sampling. We propose the following pathway: (1) Nonspecific Ser clusters are released when ESI droplets evaporate to dryness. These initial clusters cover a wide size range, from a few Ser to hundreds or thousands of monomers. (2) The clusters undergo dissociation during ion sampling, mostly via successive loss of neutral monomers. For any source activation voltage, there is a subpopulation of clusters for which this CID cascade tends to terminate at the octamer level, culminating in Ser8H+-dominated product distributions. Mobile proton molecular dynamics simulations were used to model the entire pathway. Ser8H+ structures formed in these simulations were consistent with ion mobility experiments. The most compact structures resembled the model of [Scutelnic, V. J. Am. Chem. Soc. 2018, 140, 7554-7560], with numerous intermolecular salt bridges and H-bonds. Our findings illustrate how the interplay of association and dissociation reactions across phase boundaries can culminate in magic number clusters.

Theoretical investigation of amino-acid adsorption on hydroxylated quartz surfaces: dispersion can determine enantioselectivity

Chiral mineral surfaces, such as quartz, are attractive substrates for use in enantioselective separation and may have contributed to the origin of biological homochirality. In this work, we apply density-functional theory and the exchange-hole dipole moment (XDM) dispersion model to study the adsorption of 5 amino acids (glycine, serine, alanine, valine, and phenylalanine) on a hydroxylated α-quartz (0001) surface. It is demonstrated that London dispersion is responsible for 30-50% of the total adsorption energies and its inclusion or omission can reverse predictions of enantioselectivity. Differing dispersion stabilization, caused by the opposing side-chain placements relative to the quartz surface, lead to differences of 1.0 and 1.8 kcal mol^-1 in the adsorption energies of the alanine and phenylalanine enantiomers, respectively. These results are consistent with a 3-point model, with the hydrogen-bonding sites conserved and variations in the dispersion interactions determining enantioselectivity.

Surfaces with Controllable Topography and Chemistry Used as a Template for Protein Crystallization

Surfaces with controllable topography and chemistry were prepared to act as substrates for protein crystallization, in order to investigate the influence of these surface properties on the protein crystallization outcome. Three different methods were investigated to deposit 1,3,5-tris(10-carboxydecyloxy)benzene (TCDB) on a muscovite mica substrate to find the best route for controlled topography. Of these three, sublimation worked best. Contact angle measurements revealed that the surfaces with short exposure to the TCDB vapor (20 min or less) are hydrophilic, while surfaces exposed for 30 min or longer are hydrophobic. The hydrophilic surfaces are flat with low steps, while the hydrophobic surfaces contain macrosteps. Four model proteins were used for crystallization on the surfaces with controlled topography and chemistry. Hen egg white lysozyme crystals were less numerous on the surface with macrosteps than on smoother surfaces. On the other hand, insulin nucleated faster on the hydrophobic surfaces with macrosteps, and therefore, the crystals were more abundant and smaller. Bovine serum albumin and talin protein crystals were more numerous on all TCDB functionalized surfaces, compared to the reference clean muscovite mica surfaces. Overall, this shows that surface topography and chemistry is an important factor that partly determines the outcome in a protein crystallization experiment.

Amino-acid-based chiral nanoparticles for enantioselective crystallization

Chiral polymer nanoparticles based on amino acids are prepared by miniemulsion polymerization and are demonstrated to serve as nucleating agents for the enantioselective crystallization of racemic mixtures of amino acids. The synthesized chiral nanoparticles are suited for the development of enantioselective processes and also contribute to a better understanding of chiral recognition on polymer surfaces.