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It is well known that polyelectrolyte complexes and coacervates can form on mixing oppositely charged polyelectrolytes in aqueous solutions, due to mainly electrostatic attraction between the oppositely charged polymers. Here, we report the first (to the best of our knowledge) complexation and coacervation of two positively charged polyelectrolytes, which provides a new paradigm for engineering strong, self-healing interactions between polyelectrolytes underwater and a new marine mussel-inspired underwater adhesion mechanism. Unlike the conventional complex coacervate, the like-charged coacervate is aggregated by strong short-range cation-π interactions by overcoming repulsive electrostatic interactions. The resultant phase of the like-charged coacervate comprises a thin and fragile polyelectrolyte framework and round and regular pores, implying a strong electrostatic correlation among the polyelectrolyte frameworks. The like-charged coacervate possesses a very low interfacial tension, which enables this highly positively charged coacervate to be applied to capture, carry, or encapsulate anionic biomolecules and particles with a broad range of applications.polyelectrolyte complexes | complex coacervates | cation-π interaction | like-charged coacervate | surface forces apparatus I t is well known that polyelectrolyte complexes can be formed when oppositely charged polyelectrolytes are mixed in aqueous solutions (1-4). This often leads to fluid-fluid phase separation, the so-called complex coacervation, namely, the appearance of a dense polyelectrolyte-rich liquid phase (coacervate phase) and a more dilute solution phase (aqueous phase, Fig. 1) (3, 4). The formation of polyelectrolyte complexes or coacervate can be impacted by many factors, including structural features of the component polymers (e.g., molecular weight, charge density, functional groups, hydrophilicity and hydrophobicity balance, etc.), mixing ratio and concentration of the oppositely charged polyelectrolytes, and solution and environmental conditions (e.g., pH, ionic strength, temperature, etc.) (3-5).Complex coacervate, which was suggested as "the origin of life" (6), finds application in many engineering and biological systems, such as microencapsulation in food, and in pharmaceutical and cosmetic industries due to the low interfacial energy of the coacervate phase (3,5,(7)(8)(9). Complex coacervate also plays a critical role in the underwater adhesion of many sessile marine organisms such as tubeworms and mussels, which secrete and disperse adhesive proteins to form complex coacervates that facilitate their positioning and spreading over a desired substrate under seawater (10-12).It is believed that polyelectrolyte complexation is driven by mainly electrostatic attraction in long distances between oppositely charged polymer chains in water and by additional molecular recognition driving forces such as chirality, hydrogen bonding, and hydration in short distances, implying that the polyelectrolyte complex is composed of at least one polycation ...

Adhesive systems in many marine organisms are postulated to form complex coacervates (liquid-liquid phase separation) through a process involving oppositely charged polyelectrolytes. Despite this ubiquitous speculation, most well-characterized mussel adhesive proteins are cationic and polyphenolic, and the pursuit of the negatively charged proteins required for bulk complex coacervation formation internally remains elusive. In this study, we provide a clue for unraveling this paradox by showing the bulky fluid/fluid separation of a single cationic recombinant mussel foot protein, rmfp-1, with no additional anionic proteins or artificial molecules, that is triggered by a strong cation-π interaction in natural seawater conditions. With the similar condition of salt concentration at seawater level (>0.7 M), the electrostatic repulsion between positively charged residues of mfp-1 is screened significantly, whereas the strong cation-π interaction remains unaffected, which leads to the macroscopic phase separation (i.e., bulky coacervate formation). The single polyelectrolyte coacervate shows interesting mechanical properties including low friction, which facilitates the secretion process of the mussel. Our findings reveal that the cation-π interaction modulated by salt is a key mechanism in the mussel adhesion process, providing new insights into the basic understanding of wet adhesion, self-assembly processes, and biological phenomena that are mediated by strong short-range attractive forces in water.

At room temperature, meso-hexaaryl-substituted [28]hexaphyrins(1.1.1.1.1.1) in solution exist largely as an equilibrium between planar antiaromatic and distorted Möbius aromatic conformers. As the temperature decreases, the molecular structure changes into the distorted Möbius topology that commonly occurs in [28]hexaphyrins, which gives rise to longer excited singlet and triplet state lifetimes than planar antiaromatic [28]hexaphyrins. Temperature-dependent two-photon absorption measurements of [28]hexaphyrin indicate that the degree of aromaticity of Möbius [28]hexaphyrin is large, comparable to that of Hückel aromatic planar [26]hexaphyrin. Through our spectroscopic investigations, we have demonstrated that a subtle balance between the strains induced by the size of the [28]hexaphyrin macrocyclic ring and the energy stabilization contributed by pi-electron delocalization in the formation of distorted Möbius [28]hexaphyrin leads to the molecular structure change into the Möbius topology as the temperature decreases.

2D nanomaterials have been found to show surface‐dominant phenomena and understanding this behavior is crucial for establishing a relationship between a material's structure and its properties. Here, the transition of molybdenum disulfide (MoS2) from a diffusion‐controlled intercalation to an emergent surface redox capacitive behavior is demonstrated. The ultrafast pseudocapacitive behavior of MoS2 becomes more prominent when the layered MoS2 is downscaled into nanometric sheets and hybridized with reduced graphene oxide (RGO). This extrinsic behavior of the 2D hybrid is promoted by the fast Faradaic charge‐transfer kinetics at the interface. The heterostructure of the 2D hybrid, as observed via high‐angle annular dark field–scanning transmission electron microscopy and Raman mapping, with a 1T MoS2 phase at the interface and a 2H phase in the bulk is associated with the synergizing capacitive performance. This 1T phase is stabilized by the interactions with the RGO. These results provide fundamental insights into the surface effects of 2D hetero‐nanosheets on emergent electrochemical properties.

Attenuated total reflection (ATR)/infrared (IR) and Raman spectra were measured at room temperature for -lactoglobulin (BLG) in phosphate buffer (pH 6.6) solutions over a concentration range of 1-5 wt %. Twodimensional (2D) IR and 2D Raman correlation spectra in the amide III region were generated from the concentration-dependent spectral variations of the BLG solutions to investigate band assignments in the region and to explore concentration-induced conformational changes in BLG. The great resolution enhancement yielded by the 2D IR and 2D Raman spectra enabled us to propose very detailed band assignments for the amide III region. Moreover, the basis of the sign of the asynchronous cross-peaks, we revealed the sequence order of the secondary structure changes induced by the protein association; the changes in the random coil structure exposed to water occur first, and then those in other secondary structure elements follow. 2D IRRaman heterospectral analysis was also attempted for the same IR and Raman data. The heterospectral correlation maps elucidated the correlation between IR and Raman bands in the amide III region, confirming their band assignments. IntroductionGeneralized two-dimensional (2D) correlation spectroscopy, 1-3 which is an extension of the original 2D correlation spectroscopy, 4 has recently been applied extensively to protein research through studies on secondary structure, denaturation, protein unfolding, and hydration. [5][6][7][8][9][10][11][12][13][14][15] The new method allows one to employ a variety of perturbations to generate 2D correlation spectra of proteins, including not only time but also any other physical variables such as temperature, pressure, concentration, and composition. 1,2 By use of 2D correlation spectroscopy, it is possible to enhance the resolution of individual component bands in the vibrational spectra of proteins. Moreover, this method permits one to probe a series of events occurring during denaturation, adsorption, formation of protein aggregates, hydrogen-deuterium exchange, and so on. Thus far, however, all of the 2D correlation studies of proteins have been concerned with infrared (IR) or near-infrared (NIR) spectroscopy, and there is no 2D Raman study of proteins. [5][6][7][8][9][10][11][12][13][14][15] In addition, most of the 2D IR studies focus on the amide I band region.The purpose of the present study is to apply 2D correlation spectroscopy to the concentration-dependent Raman spectral variations of -lactoglobulin (BLG) in buffer solutions and to explore the possibility of IR-Raman heterospectral analysis for protein research. This study is strongly related to the study reported in the preceding paper, 15 in which we discussed 2D attenuated total reflection (ATR)/IR correlation spectra of adsorbtion-induced and concentration-dependent spectral variations of BLG in aqueous solutions. We focused solely on the amide I region in the preceding paper.The idea of making a correlation between the two kinds of spectroscopy is not new, but it was very difficult t...

As a new analytical technology, surface-enhanced Raman scattering (SERS) has received increasing attention, and researchers have discovered the importance of SERS-active materials. Considerable effort has been made by researchers to develop multiperformance and multipurpose SERS-active substrates ranging from coinage metals to transition metals and semiconductor materials. SERS-active substrates are critical for obtaining accurate and reproducible spectral information. Among all the substrate materials, semiconductors are considered one of the most promising materials, as they exhibit high chemical stability, good biocompatibility, high carrier mobility, and good controllability during fabrication. Here, we provide an overview of SERS enhancement mechanisms based on semiconductor materials, such as inorganic semiconductors, metal/semiconductor composites, and organic semiconductors.

To investigate the mechanism of the electrochemical insertion of lithium into the CoO electrode in a Li/Li y CoO cell, we applied a two-dimensional (2D) correlation analysis to the spectra of the Li y CoO system during the first insertion−extraction reaction as obtained by lithium concentration-dependent X-ray absorption spectroscopy (XAS) and Raman spectroscopy. 2D XAS and 2D Raman spectra yield great resolution enhancement and show that the insertion of lithium into the CoO electrode leads to Li2O formation. Moreover, by analyzing the sign of the asynchronous cross peaks, we reveal the sequence of the first insertion process:  first, the intensity of the band due to CoO decreases and then that of the band due to Li2O increases. A 2D heterospectral XAS−Raman correlation analysis was also undertaken of the same XAS and Raman spectra. This heterospectral 2D correlation analysis has elucidated not only the sequence of events between XAS and Raman signals from the same species but also the correlation between the XAS and Raman bands, confirming their band assignments.