Ion mobility mass spectrometry studies on Aun-Lyz adducts showed gradual unfolding of the protein structure during binding of Au+ to the protein. The change of the charge state envelope in Aun-Lyz from that of Lyz in ESI MS data confirmed the relaxation of the protein structure. This Au+ binding occurs at cysteine sites through the breakage of disulfide bonds and this ruptures the H-bonded folded network structure of the protein leading to ∼30% change in helicity. Nearly 15% loss in the total H-bonding occurred during the attachment of 8 Au to the protein as calculated by a molecular dynamics simulation. Different Aun-Lyz structures were simulated, which confirmed significant unfolding of the protein. The structural insights were used to understand similar unfolding in the solution state as seen via circular dichroism (CD) and Fourier transform infrared (FTIR) spectroscopy. This open structure is indeed necessary to accommodate a cluster core inside a protein cavity during luminescent cluster synthesis. These studies unambiguously establish noble metal binding-induced conformational changes of protein structures to accommodate the clusters. © 2017 American Chemical Society.

Pradeep Thalappil

Department Of Chemistry

Debasmita Ghosh

Ananya Baksi

Abhijit Nag

Mohd Azhardin Ganayee

Amino acids

Binding sites

Bins

Circular dichroism spectroscopy

Covalent bonds

Dichroism

Fourier transform infrared spectroscopy

Gold compounds

Hydrogen bonds

Luminescence

Mass spectrometry

molecular dynamics

Proteins

cluster synthesis

Conformational change

Disulfide bonds

ION MOBILITY-MASS SPECTROMETRY

Molecular dynamics simulations

NETWORK STRUCTURES

Protein structures

Structural insights

Biosynthesis

IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

Journal of Physical Chemistry C

A new synthetic protocol is introduced which preserves the secondary structure of protecting proteins encapsulating a luminescent atomically-precise silver cluster. This was achieved by using a preformed triphenylphosphine (TPP)-protected silver cluster as the precursor forming bovine serum albumin (BSA)- and human serum albumin (HSA)-protected Ag18 clusters. This is the first example of the formation of luminescent protein-protected clusters in a neutral medium, without using any reducing agent, which results in minimal alteration of the protein structure during cluster growth. The cluster formed showed exceptional stability, unlike other silver clusters of this class. The formation of these red luminescent clusters was visualized by UV-vis and photoluminescence spectroscopy. The identification of Ag18 core was made through matrix-assisted laser desorption ionization mass spectrometry (MALDI MS), and a plausible mechanism of the formation was identified by monitoring the systematic growth of the cluster core by time-dependent MALDI MS experiments and electrospray ionization mass spectrometry (ESI MS) of the reaction mixture. The cluster was successfully employed as a luminescent probe for cancer cell imaging. Retention of protein conformation in the clusters was confirmed through circular dichroism (CD) spectroscopy, and the same was reflected in the retention of 89% of the esterase activity of BSA in the Ag18@BSA clusters synthesized by this method, compared to only 28.7% for AgQC@BSA clusters synthesized using previous protocols, conducted in basic medium. © 2019 American Chemical Society.

Internalization of a Preformed Atomically Precise Silver Cluster in Proteins by Multistep Events and Emergence of Luminescent Counterparts Retaining Bioactivity

Hydrogen/deuterium exchange mass spectrometry was employed to probe the conformational changes in lysozyme (Lyz) during the course of formation of a protein-protected atomically precise Au8 cluster. MALDI MS showed the protein, Lyz, to be present in a denatured state in the cluster. Detailed ESI MS analysis of the Au-attached Lyz adducts, an intermediate of cluster formation, confirmed that these conformational changes are brought about by Au-S bond formation and a similar conformation is retained in the final cluster. These results were supported by computational results, which showed an increase in solvent accessible surface area upon the formation of the adducts. Infrared spectroscopy established that change in the rate as well as the extent of the hydrogen/deuterium exchange observed in the cluster was due to the change in the amide II region of the encapsulating protein. Hydrogen/deuterium exchange ESI MS of Cu adducts of Lyz showed a lower degree of denaturation than their Au counterparts. XPS analysis revealed that Cu binds differently to Lyz than it does with Au, which is likely because of the stronger soft-soft Au-S interaction. Alkali metal ion binding, on the other hand, does not affect the protein conformation because such ions do not affect the disulfide bonds. © 2019 American Chemical Society.

Conformational Changes of Protein upon Encapsulation of Noble Metal Clusters: An Investigation by Hydrogen/Deuterium Exchange Mass Spectrometry

A detailed examination of collision cross sections (CCSs) coupled with computational methods has revealed new insights into some of the key questions centered around curcumin, one of the most intensively studied natural therapeutic agents. In this study, we have distinguished the structures and conformers of the well-known enol and the far more elusive keto form of curcumin by using ion mobility mass spectrometry (IM MS). The values of the theoretically predicted isomers were compared with the experimental CCS values to confirm their structures. We have identified a bent structure for the keto form and the degree of bending was estimated. Using IM MS, we have also shown that ESI MS reflects the solution phase structures and their relative populations, in this case. Piperine, a naturally occurring heterocyclic compound, is known to increase the bioavailability of curcumin. However, it is still not clearly understood which tautomeric form of curcumin is better stabilized by it. We have identified preferential stabilization of the enol form in the presence of piperine using IM MS. Cyclodextrins (CDs) are used as well-known carriers in the pharmaceutical industry for increasing the stability, solubility, bioavailability, and tolerability of curcumin. However, the crystal structures of supramolecular complexes of curcumin∩CD are unknown. We have determined the structures of different isomers of curcumin∩CD (α- and β-CD) complexes by comparing the CCSs of theoretically predicted structures with the experimentally obtained CCSs, which will further help in understanding the specific role of the structures involved in different biological activities. © 2018 American Chemical Society.

Analytical Chemistry

Bent Keto Form of Curcumin, Preferential Stabilization of Enol by Piperine, and Isomers of Curcumin∩Cyclodextrin Complexes: Insights from Ion Mobility Mass Spectrometry

In this paper is reported the synthesis and detailed mass spectrometric and spectroscopic characterization of highly luminescent Au and Ag clusters protected with mixed proteins. Taking advantage of the aggregation tendency of the protein, lysozyme (Lyz), an inter-protein conjugate was made from a physical mixture of two proteins, bovine serum albumin (BSA) and Lyz. Based on matrix-assisted laser desorption/ionization mass spectrometry data, the new cluster is assigned as ∼Au<inf>36</inf>@BSA-Lyz. This specific system showed a very high red luminescence and the calculated quantum yield was 42% which is the highest to date for such cluster systems. A similar study on an Ag system showed the formation of ∼Ag<inf>35</inf>@BSA-Lyz when a similar metal and protein concentration was used. By varying the concentration of the Ag precursor, different compositions of the cluster protected by the mixed protein were achieved. Such a system with a high quantum yield can be used for various applications such as sensors for ultralow levels of analytes, fluorescent tags and for tracking biomolecules in real systems. © The Royal Society of Chemistry 2015.

Fulltext

RSC Advances

Noble metal clusters protected with mixed proteins exhibit intense photoluminescence

We report a combined small-angle X-ray scattering (SAXS) and mass spectrometric (MS) study of the growth of gold clusters within proteins, in the solution state. Two different proteins, namely, lysozyme (Lyz) and bovine serum albumin (BSA), were used for this study. SAXS study of clusters grown in Lyz shows the presence of a 0.8 nm gold core, which is in agreement with the Au10 cluster observed in MS. Dynamic light scattering suggests the size of the cluster core to be 1.2 nm. For BSA, however, a bigger core size was observed, comparable to the Au33 core obtained in MS. Concentration- and time-dependent data do not show much change in the core size in both SAXS and MS investigations. When metal-protein adducts were incubated for longer time in solution, nanoparticles were formed and protein size decreased, possibly due to the fragmentation of the latter during nanoparticle formation. The data are in agreement with dynamic light scattering studies. This work helps to directly visualize cluster growth within protein templates in solution. (Graph Presented). © 2014 American Chemical Society.

Size evolution of protein-protected gold clusters in solution: A combined SAXS-MS investigation

We probed the initial growth kinetics of luminescent quantum clusters of silver (AgQCs) within two albumin family proteins, bovine serum albumin (BSA) and ovalbumin (Ova). Shorter time scale (seconds to minutes) growth of AgQCs monitored using real time photoluminescence spectroscopy has shown that, at lower concentrations of Ag+, only unstable QCs were formed. The major role of basic pH in the synthesis was not only to facilitate Ag+-BSA conjugation but also to provide well dispersed medium for controlled nucleation of QCs. Increase in the concentration of NaBH<inf>4</inf> affects growth kinetics greatly and leads to increase in the growth rate of AgQCs; but for NaBH<inf>4</inf> concentrations higher than the optimum value, growth rate becomes constant. Precise measurements have shown that excitation and emission of AgQCs exhibit linear red-shift with the increasing concentration of NaBH<inf>4</inf> whereas protein excitation remains constant. Similar results were observed for both the proteins, Ova and BSA. We believe that various insights provided by this study will be helpful for further improvements in the synthetic methodology and applications of protein protected AgQCs. © 2015 American Chemical Society.

Initial growth kinetics of luminescent quantum clusters of silver within albumin family protein templates

Although various complex, bulky ligands have been used to functionalize plasmonic gold nanoparticles, introducing them to small, atomically precise gold clusters is not trivial. Here, we demonstrate a simple one-pot procedure to synthesize fluorescent magic number Au25 clusters carrying controlled amounts of bulky calix[4]arene functionalities. These clusters are obtained from a synthesis feed containing binary mixtures of tetrathiolated calix[4]arene and 1-butanethiol. By systematic variation of the molar ratio of ligands, clusters carrying one to eight calixarene moieties were obtained. Structural characterization reveals unexpected binding of the calix[4]arenes to the Au 25 cluster surface with two or four thiolates per moiety. © 2014 American Chemical Society.

Journal of Physical Chemistry Letters

Mixed-monolayer-protected Au25 clusters with bulky calix[4]arene functionalities

Sub-nanometer-sized metal clusters, having dimensions between metal atoms and nanoparticles, have attracted tremendous attention in the recent past due to their unique physical and chemical properties. As properties of such materials depend strongly on size, development of synthetic routes that allows precise tuning of the cluster cores with high monodispersity and purity is an area of intense research. Such materials are also interesting owing to their wide variety of applications. Novel sensing strategies based on these materials are emerging. Owing to their extremely small size, low toxicity, and biocompatibility, they are widely studied for biomedical applications. Primary focus of this review is to provide an account of the recent advances in their applications in areas such as environment, energy, and biology. With further experimental and theoretical advances aimed at understanding their novel properties and solving challenges in their synthesis, an almost unlimited field of applications can be foreseen. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Particle and Particle Systems Characterization

Noble metal clusters: Applications in energy, environment, and biology

Gold-Induced Unfolding of Lysozyme: Toward the Formation of Luminescent Clusters

Journal	Data powered by TypesetJournal of Physical Chemistry C
Publisher	Data powered by TypesetAmerican Chemical Society
ISSN	19327447
Open Access	No