Exploring the Self-Assembly of a Short Aromatic Aβ(16–24) Peptide

The use of self-assembling peptides as scaffolds for creating biomaterials has prompted the scientific community to carry out studies on short peptides as model systems. Short peptides help in dissecting contributions from different interactions, unlike large peptides, where multiple interactions make it difficult to dissect the contributions of individual interactions. This opens avenues for fine tuning peptides to carry out a wide range of physical or chemical properties. In this line of study Aβ­(16–24) is a versatile building block not only as a scaffold for creating biomaterials but also because it forms the active core in the protein that forms amyloid plaques. In this study, we probe the self-assembly of peptide Aβ(16–24) using fluorescence spectroscopy, circular dichroism, isothermal titration calorimetry, transmission electron microscopy, and atomic force microscopy. The process of self-assembly is dictated by the burial of phenyl alanines in the hydrophobic core and guided by nonbonding interactions and H-bonding. The process of fibril formation is enthalpically driven, and the fibrils showed blue and green luminescence without the addition of any external agent or sensitizer. Because these short peptides are known to bind with fully formed amyloid fibrils, this opens a route to the study of amyloid systems in vitro or isolated from patients suffering from Alzheimer’s disease

Microheterogeneity and Microviscosity of F127 Micelle: The Counter Effects of Urea and Temperature

F127 is the most widely studied triblock copolymer and due to the presence of very long polypropylene oxide (PPO) and polyethylene oxide (PEO) groups, F127 micelle has different microenvironments clearly separated into core, corona, and peripheral regions. Urea has been known to have adverse effects on the micellar properties and causes demicellization and solvation; on the other hand, rise in temperature causes micellization and solvent evacuation from the core and corona regions. In the present study, we have investigated the microheterogeneity of the core, corona, and peripheral regions of the F127 micelle using red edge excitation shift (REES) at different temperatures and urea concentrations and correlated the effect of both on the micellar system. It was found that the temperature counteracts the effect of urea and also that the counteraction is more prominent in the core region with respect to corona, and the peripheral region is least affected. Also, the core and corona regions are very much heterogeneous, while the peripheral region is more of a homogeneous nature. Using time-resolved fluorescence anisotropy, we found that the microviscosity within the micelles vary in the order of core > corona > peripheral region, and urea has a general tendency to reduce the microviscosity, especially for core and corona regions. On the other hand, rise in temperature initially increases and then decreases the microviscosity throughout, and at elevated temperatures the effect of urea is being dominated by the effect of temperature, thereby establishing the counter effects of temperature and urea on the F127 micellar system

Kinetic Aspects of Enzyme-Mediated Evolution of Highly Luminescent Meta Silver Nanoclusters

The proteolysis of human serum albumin (HSA) templated Ag nanoclusters (NCs) induced by trypsin digestion reveals that photoluminescence (PL) intensity of the blue emitting Ag:HSA NCs exhibits a decremental behavior. The unique aspect of this study is the gradual evolution of a new red emission band characterized by an enhanced quantum yield (QY) of ∼28% and a lifetime ∼80 ns in contrast to the parent blue-emitting Ag:HSA NCs (QY = 16% and lifetime ∼7.4 ns). The decay and growth PL kinetics of Ag NCs upon trypsin digestion have been used to unravel the process of destabilization of the Ag:HSA NCs and the simultaneous formation of the meta AgTp NCs as evidenced from MALDI-TOF spectra. Our fluorescence correlation spectroscopy (FCS) data substantiate the evolution of larger sized Ag NCs due to enzymatic digestion which thereby results in a slower translational diffusion rate as a consequence of their increasing hydrodynamic diameters

Toggling Between Blue- and Red-Emitting Fluorescent Silver Nanoclusters

A very efficient protocol for synthesizing highly fluorescent, protein-templated silver nanoclusters (Ag/NCs) has been discussed. Two types of Ag/NCs (Ag₉/HSA and Ag₁₄/HSA), although showing significant differences in their photophysical properties, can be interconverted at will, which makes this study unique. The Ag/HSA NCs have been quantified by several spectroscopic techniques, and they find tremendous applications as photoluminescent markers. Besides their rather easy synthetic methodology, our Ag/HSA NCs show two-photon excitation properties that enable them to be used in bioimaging

Photostable Copper Nanoclusters: Compatible Förster Resonance Energy-Transfer Assays and a Nanothermometer

To address the concern of material chemists over the issue of stability and photoluminescent (PL) characteristics of Cu nanoclusters (NCs), herein we present an efficient protocol discussing PL Cu NCs (Cu/HSA) having blue emission and high photostability. These PL NCs were illustrated as efficient probes for Förster resonance energy transfer (FRET) with a compatible fluorophore (Coumarin 153). Our spectroscopic results were well complemented by our molecular docking calculations, which also favored our proposed mechanism for Cu NC formation. The beneficial aspect and uniqueness of these nontoxic Cu/HSA NCs highlights their temperature-dependent PL reversibility as well as the reversible FRET with Coumarin 153, which enables them to be used as a nanothermometer and a PL marker for sensitive biological samples

Temperature Induced Morphological Transitions from Native to Unfolded Aggregated States of Human Serum Albumin

The circulatory protein, human serum albumin (HSA), is known to have two melting point temperatures, 56 and 62 °C. In this present manuscript, we investigate the interaction of HSA with a synthesized bioactive molecule 3-pyrazolyl 2-pyrazoline (PZ). The sole tryptophan amino acid residue (Trp214) of HSA and PZ forms an excellent FRET pair and has been used to monitor the conformational dynamics in HSA as a function of temperature. Molecular docking studies reveal that the PZ binds to a site which is in the immediate vicinity of Trp214, and such data are also supported by time-resolved FRET studies. Steady-state and time-resolved anisotropy of PZ conclusively proved that the structural and morphological changes in HSA mainly occur beyond its first melting temperature. Although the protein undergoes thermal denaturation at elevated temperatures, the Trp214 gets buried inside the protein scaffolds; this fact has been substantiated by acrylamide quenching studies. Finally, we have used atomic force microscopy to establish that at around 70 °C, HSA undergoes self-assembly to form fibrillar structures. Such an observation may be attributed to the loss of α-helical content of the protein and a subsequent rise in β-sheet structure