Alex P. Jonke

Electrochemically Controlled Atom by Atom Deposition of Gold to Polyaniline

Polyaniline (PANI) has been an effective matrix for hosting and preserving metal nanoclusters. In the case of gold, the tetrachloroaurate anion (AuCl ― 4 ) has a high affinity for the imine sites of PANI. Upon contact with PANI, AuCl ― 4 is spontaneously reduced to metallic gold, but the size of the formed Au clusters cannot be precisely controlled. Herein, we report on the electrochemical method of controlled deposition of one atom by one atom of gold per imine site of PANI. By controlling the potential, we keep PANI in an oxidized state while exposing it to a solution of AuCl ― 4 to form a PANI * AuCl ― 4 complex, which is reduced to atomic gold by sweeping the potential negative. This frees up the imine sites of PANI again and makes them accessible for the next Au deposition cycle. The repeated deposition of Au atoms follows a cyclic pathway. The amount of gold deposited using this method is consistent for each repeated cycle.

Polyaniline Electrodes Containing Tri-Atomic Au/Pd Clusters: Effect of Ordering

Our atom-by-atom deposition method has been used to prepare hetero-tri-atomic clusters of Au1Pd2 and Au2Pd1 in polyaniline as isolation matrix. The sequence in which individual atoms are added affects the final structure of the cluster. Because of the uniquely defined point of attachment of the cluster to the PANI, isomers such as PANI-Pd–Au–Pd, PANI–Au–Pd–Pd and PANI–Pd–Pd–Au have been created. It is shown that the latter has much higher activity towards electrooxidation of n-propanol in alkaline medium than the first two. In fact, its activity is even higher than that of the most active homo-clusters PANI–Au6 and PANI–Pd2. The observed results are in good agreement with few available theoretical calculations of the HOMO–LUMO gap energy of these structures.Graphical Abstract

Both positional and chemical variables control in vitro proteolytic cleavage of a presenilin ortholog

Mechanistic details of intramembrane aspartyl protease (IAP) chemistry, which is central to many biological and pathogenic processes, remain largely obscure. Here, we investigated the in vitro kinetics of a microbial intramembrane aspartyl protease (mIAP) fortuitously acting on the renin substrate angiotensinogen and the C-terminal transmembrane segment of amyloid precursor protein (C100), which is cleaved by the presenilin subunit of γ-secretase, an Alzheimer disease (AD)-associated IAP. mIAP variants with substitutions in active-site and putative substrate-gating residues generally exhibit impaired, but not abolished, activity toward angiotensinogen and retain the predominant cleavage site (His–Thr). The aromatic ring, but not the hydroxyl substituent, within Tyr of the catalytic Tyr–Asp (YD) motif plays a catalytic role, and the hydrolysis reaction incorporates bulk water as in soluble aspartyl proteases. mIAP hydrolyzes the transmembrane region of C100 at two major presenilin cleavage sites, one corresponding to the AD-associated Aβ42 peptide (Ala–Thr) and the other to the non-pathogenic Aβ48 (Thr–Leu). For the former site, we observed more favorable kinetics in lipid bilayer–mimicking bicelles than in detergent solution, indicating that substrate–lipid and substrate–enzyme interactions both contribute to catalytic rates. High-resolution MS analyses across four substrates support a preference for threonine at the scissile bond. However, results from threonine-scanning mutagenesis of angiotensinogen demonstrate a competing positional preference for cleavage. Our results indicate that IAP cleavage is controlled by both positional and chemical factors, opening up new avenues for selective IAP inhibition for therapeutic interventions.

DRILL: An Electrospray Ionization-Mass Spectrometry Interface for Improved Sensitivity via Inertial Droplet Sorting and Electrohydrodynamic Focusing in a Swirling Flow

We describe the DRILL (dry ion localization and locomotion) device, which is an interface for electrospray ionization (ESI)-mass spectrometry (MS) that exploits a swirling flow to enable the use of inertial separation to prescribe different fates for electrosprayed droplets based on their size. This source adds a new approach to charged droplet trajectory manipulation which, when combined with hydrodynamic drag forces and electric field forces, provides a rich range of possible DRILL operational modes. Here, we experimentally demonstrate sensitivity improvement obtained via vortex-induced inertial sorting of electrosprayed droplets/ions: one possible mode of DRILL operation. In this mode, DRILL removes larger droplets while accelerating the remainder of the ESI plume, producing a high velocity stream of gas-enriched spray with small, highly charged droplets and ions and directing it toward the MS inlet. The improved signal-to-noise ratio (10-fold enhancement) in the detection of angiotensin I is demonstrated using the DRILL interface coupled to ESI-MS along with an improved limit of detection (10-fold enhancement, 100 picomole) in the detection of angiotensin II. The utility of DRILL has also been demonstrated by liquid chromatography (LC)-MS: a stable isotope labeled peptide cocktail was spiked into a complex native tissue extract and quantified by unscheduled multiple reaction monitoring on a TSQ Vantage. DRILL demonstrated improved signal strength (up to a 700-fold) for 8 out of 9 peptides and had no effects on the peak shape of the transitions.