Remote regulation on the hydration sites of adenine molecules via derivatization.

Hydration, as a ubiquitous and vital phenomenon in nature, has attracted great attention in the field of surface science concerning the fundamental interactions between water and organic molecules. However, the role of functional group derivatization is still elusive in terms of its potential impact on hydration. By the combination of high-resolution scanning tunneling microscopy imaging and density functional theory calculations, the hydration of 9mA molecules was realized on Au(111) in real space, forming 9mA-H2O-9mA structures. In comparison with the hydration of adenine molecules, methyl derivatization is experimentally found to remotely regulate the hydration sites from the imidazole ring to the pyrimidine ring and is further theoretically revealed to allow intramolecular electron redistribution and, therefore, steer the priority of the hydration sites. These results provide sub-molecular understandings of the relationship between derivatization and hydration, which would shed light on the regulation of hydration processes in chemically and biologically related systems.

Determining the optimal cut-off scores for the Chinese version of the Memorial Anxiety Scale for Prostate Cancer (MAX-PC).

PURPOSE: Anxiety is a common emotion experienced by patients with prostate cancer (PCa), and can be exacerbated by testing the prostate-specific antigen (PSA) index. The Memorial Anxiety Scale for Prostate Cancer (MAX-PC) was developed to assess the cancer-specific anxiety of these patients, but lack of appropriate thresholds for this scale limits its use. This study aimed to utilize ROC curve analysis to identify the best cut-off values for the Chinese version of the MAX-PC scale. METHODS: A cross-sectional survey was conducted using the Chinese version of the MAX-PC scale and the Generalized Anxiety Disorder Scale (GAD). ROC curve analysis, Youden index, Kappa consistency test and McNemar test were used for the optimal cutoff points for screening mild, moderate, and severe cancer-specific anxiety levels in patients with PCa, on the Chinese version of the MAX-PC scale. RESULTS: Two hundred eighty-seven patients with PCa completed the survey. The appropriate cut-off values for the MAX-PC scale for screening patients with PCa for cancer-specific anxiety were 20, 28, and 38 for mild, moderate, and severe anxiety, respectively with the highest Youden indices. The Kappa and McNemar's test showed the best level of consistency with values of 0.627, 0.580, and 0.606 for screening mild, moderate, and severe anxiety, respectively. CONCLUSIONS: The scores 20, 28, and 38 are the best cut-off values for the Chinese version of the MAX-PC scale. This scale should be used for screening cancer-specific anxiety for patients with PCa to assess and evaluate their anxiety levels and provide targeted interventions.

Self-Assembly of Glutamic Acid and Serine on Au(111).

Amino acids provide novel and superior performance for two-dimensional materials and bio-based devices. The interaction and adsorption of amino acid molecules on substrates have thus attracted extensive research for exploring the driving forces involved in the formation of nanostructures. Nevertheless, the interactions within amino acid molecules on inert surfaces have not been fully understood. Herein, from the interplay of high-resolution scanning tunneling microscopy imaging and density functional theory calculations, we show the self-assembled structures of Glu and Ser molecules on Au(111), which are dominated by intermolecular hydrogen bonds, and further investigate their most stable structural models at the atomic scale. This study would be of fundamental importance in understanding the formation processes of biologically relevant nanostructures and provide possibilities for chemical modification.

Local Chiral Inversion of Thymine Dimers by Manipulating Single Water Molecules.

Water, as one of the most important and indispensable small molecules in vivo, plays a crucial role in driving biological self-assembly processes. Real-space detection and identification of water-induced organic structures and further capture of dynamic dehydration processes are important yet challenging, which would help to reveal the cooperation and competition mechanisms among water-involved noncovalent interactions. Herein, introduction of water molecules onto the self-assembled thymine (T) structures under ultrahigh vacuum (UHV) conditions results in the hydration of hydrogen-bonded T dimers forming a well-ordered water-involved T structure. Reversibly, a local dehydration process is achieved by in situ scanning tunneling microscopy (STM) manipulation on single water molecules, where the adjacent T dimers connected with water molecules undergo a local chiral inversion process with the hydrogen-bonding configuration preserved. Such a strategy enables real-space identification and detection of the interactions between water and organic molecules, which may also shed light on the understanding of biologically relevant self-assembly processes driven by water.

Interconversion between guanine quartets and triads on the Au(111) surface.

Based on STM imaging and DFT calculations, we show the real-space experimental evidence of the interconversion between G-quartets and G-triads on the Au(111) surface, and further reveal the relative stabilities of these two elementary motifs, which helps to increase the fundamental understanding of the relationship between G-triplex and G-quadruplex DNA structures.

Hydration of iodine adsorbed on the Au(111) surface.

The hydration of halogens has been widely researched because of its close relation with the water desalination and biochemical reactions. In this work, by a combination of scanning tunneling microscopy and X-ray photoelectron spectroscopy, we have explored the hydration process of iodine via the Eley-Rideal process on the Au(111) surface. Moreover, the hydration process of iodine with the presence of the NiPc self-assembled network as a template has also been investigated, where the stepwise hydration of iodine at room temperature can be visualized on Au(111).

Water-Induced Chiral Separation on a Au(111) Surface.

Facing the scientific question of the origin of chirality in life, water is considered to play a crucial role in driving many biologically relevant processes in vivo. Water has been demonstrated in vitro to be related to chiral generation, amplification, and inversion, while the underlying mechanism is still not fully understood. Real-space evidence at the single-molecule level is thus urgently required to understand the role of water molecules in biomolecular chirality related issues. Herein, we choose one of the RNA bases, the biomolecule uracil (U), which self-assembles into racemic hydrogen-bonded structures. Upon water exposure, surprisingly, racemic structures could be transformed to homochiral water-involved structures, resulting in an unexpected chiral separation on the surface. The origin of chiral separation is due to preferential binding between water and the specific site of U molecules, which leads to the formation of the energetically most favorable homochiral (U-H2O-U)2 cluster as seed for subsequent chiral amplification. Such a water-driven self-assembly process may also be extended to other biologically relevant systems such as amino acids and sugars, which would provide general insights into the role that water molecules may play in the origin of homochirality in vivo.

On-Surface Fabrication of Bimetallic Metal-Organic Frameworks through the Synergy and Competition among Noncovalent Interactions.

Two-dimensional metal-organic frameworks (2D-MOFs) are attracting more attention due to their unique properties. Various 2D-MOF structures have been fabricated on surfaces in which either only one kind of metal was incorporated or only one kind of noncovalent interaction was involved in a bimetallic network. However, 2D-MOFs involving different kinds of noncovalent interactions and multiple metal components are more complex and less predictable. Here, we choose the uracil (U) molecule together with alkali metals [sodium (Na) and cesium (Cs)] and a transition metal [iron (Fe)] as model systems and successfully construct two kinds of bimetallic 2D-MOFs through the synergy and competition among noncovalent interactions, which is revealed by the high-resolution scanning tunneling microscopy imaging and density functional theory calculations. Such a systematic study may help to improve our fundamental understanding of the interaction mechanism of noncovalent bonds and, moreover, lead to further investigations of the unprecedented functions of surface-supported 2D-MOF structures.

Selectively Scissoring Hydrogen-Bonded Cytosine Dimer Structures Catalyzed by Water Molecules.

A single-molecule-level understanding of the activity of solvating water molecules in hydrogen-bonded assemblies would provide insights into the properties of the first hydration shells. Herein, we investigate the solvation of one of the DNA bases, cytosine, whose glassy-state network formed on Au(111) contains diverse types of hydrogen-bonded dimer configurations with hierarchical strengths. Upon water exposure, a global structural transformation from interwoven chain segments to extended chains was identified by scanning tunneling microscopy and atomic force microscopy. Density functional theory calculation and coarse-grained molecular dynamics simulation indicate that water molecules selectively break the weak-hydrogen-bonded dimers at T-junctions, while the stable ones within chains remain intact. The resulting hydrated chain segments further self-assemble into molecular chains by forming strong hydrogen bonds and spontaneously releasing water molecules. Such an intriguing transformation cannot be realized by thermal annealing, indicating the dynamic nature of water molecules in the regulation of hydrogen bonds in a catalytic manner.

Linear array of cesium atoms assisted by uracil molecules on Au(111).

Based on STM imaging and DFT calculations, we demonstrate that it is feasible to construct a series of metal-organic U + Cs structures composed of configuration-adjustable multiple-metal-center motifs, which is the consequence of the synergy and competition between hydrogen bonds and ionic bonds. Moreover, assisted by U molecules, the central Cs cations tend to form linear arrays.

Dissolution of Sodium Halides by Confined Water on Au(111) via Langmuir-Hinshelwood Process.

Salt dissolution is generally encountered in widespread phenomena in nature. As a typical case, the dissolution of NaCl has been widely investigated in aqueous environment, while the process and mechanism of the on-surface dissolution may differ from that in solution and remain to be explored. Herein, we model a NaCl dissolution process on the Au(111) surface with confined water at room temperature (RT) and above. With the assistance of adenine molecules as water reservoir and carrier, the dissolution of NaCl is achieved above RT via the Langmuir-Hinshelwood mechanism rather than the Eley-Rideal one, along with the selective formation of stable Cl- hydrates, which desorb from the surface in the next step. To explore the generality, such a strategy has been extended to other sodium halide systems ( e. g., NaBr and NaI), and expectedly, the dissolution of sodium halides is also achieved by forming stable Br- and I- hydrates via the Langmuir-Hinshelwood process.

Chlorine-assisted fabrication of hybrid supramolecular structures via electrostatic interactions.

Supramolecular self-assembly is a spontaneous process relying on non-covalent intermolecular interactions. Among them, electrostatic interactions generated by organic molecules interacting with alkali metals and/or halogens play an important role in structural formation. Herein, we choose cytosine and NaCl as a model system and, from the interplay of STM imaging and DFT calculations, a hybrid network composed of both metal-organic and pure organic motifs interlinked by Cl ions via electrostatic interactions is observed on the Au(111) surface. Moreover, the occasionally missing Cl ions in connections are accompanied by the absence of adjacent organic motifs resulting in defects of the network. This study successfully demonstrates the generality of salt providing both cations and anions simultaneously in the modulation of the structure and provides fundamental knowledge on the formation of hybrid structures as well as the function of halogens in affecting the self-assembly process.

Two-dimensional self-assembled nanostructures of nucleobases and their related derivatives on Au(111).

The construction of two-dimensional (2D) self-assembled nanostructures has been one of the considerably interesting areas of on-surface chemistry in the past few decades, and has benefited from the rapid development and improvement of scanning probe microscopy techniques. In this research field, many attempts have been made in the controllable fabrication of well-ordered and multifunctional surface nanostructures, which attracted interest because of the prospect for artificial design of functional molecular nanodevices. DNA and RNA are considered to be programmable self-assembly systems and it is possible to use their base sequences to encode instructions for assembly in a predetermined fashion at the nanometer scale. As important constituents of nucleic acids, nucleobases, with intrinsic functional groups for hydrogen bonding, coordination bonding, and electrostatic interactions, can be employed as a potential system for the versatile construction of various biomolecular nanostructures, which may be used to structure the self-assembly of DNA-based artificial molecular constructions and play an important role in novel biosensors based on surface functionalization. In this article, we will review the recent progress of on-surface self-assembly of nucleobases and their derivatives together with different reactants (e.g., metals, halogens, salts and water), and as a result, various 2D surface nanostructures are summarized.

Real-space evidence of Watson-Crick and Hoogsteen adenine-uracil base pairs on Au(111).

From the interplay of high-resolution scanning tunneling microscopy imaging and density functional theory calculations, we show the real-space evidence of the formation of Watson-Crick and Hoogsteen adenine-uracil base pairs on an Au(111) surface with the employment of base derivatives, and further investigate the relative stability of the two types of adenine-uracil base pairs.

Scission and stitching of adenine structures by water molecules.

Based on high-resolution STM imaging/manipulations and DFT calculations, we display the dynamic hydration process of adenine networks on Au(111) in real space, which results in controllable scission and stitching of adenine structures by water molecules.

Hierarchical formation of Fe-9eG supramolecular networks via flexible coordination bonds.

From the interplay between high-resolution scanning tunneling microscopy imaging/manipulations and density functional theory calculations, we display the hierarchical formation of supramolecular networks by codeposition of 9eG molecules and Fe atoms on Au(111) based on the flexible coordination bonds (the adaptability and versatility in the coordination modes). In the first step, homochiral islands composed of homochiral G4Fe2 motifs are formed; and then in the second step, thermal treatment results in the transformation into the porous networks composed of heterochiral G4Fe2 motifs with the ratio of the components being constant. In situ STM manipulations and the coexistence of some other heterochiral G4Fe2 motifs and clusters also show the flexibility of the coordination bonds involved. These studies may provide a fundamental understanding of the regulations of multilevel supramolecular structures and shed light on the formation of designed supramolecular nanostructures.

On-Surface Synthesis of Adenine Oligomers via Ullmann Reaction.

Despite the fact that DNA bases have been well-studied on surface, the on-surface synthesis of one-dimensional DNA analogs through in situ reactions is still an interesting topic to be investigated. Herein, from the interplay of high-resolution scanning tunneling microscopy (STM) imaging and density functional theory (DFT) calculations, we have delicately designed a halogenated derivative of adenine as precursor to realize the combination of DNA bases and Ullmann reaction, and then successfully synthesized adenine oligomers on Au(111) via Ullmann coupling. This model system provides a possible bottom-up strategy of fabricating adenine oligomers on surface, which may further give access to man-made DNA strands with multiple bases.

Real-space evidence of the formation of the GCGC tetrad and its competition with the G-quartet on the Au(111) surface.

From the interplay of high-resolution scanning tunneling microscopy (STM) imaging and density functional theory (DFT) calculations, we show the first real-space evidence of the formation of GCGC tetrad on an Au(111) surface, and further investigate its competition with the well-known G-quartet with the aid of NaCl under ultrahigh vacuum (UHV) conditions.

Structural diversity of metal-organic self-assembly assisted by chlorine.

From the combination of STM imaging and DFT calculations, we show that both alkali metal and halogens interact with different sites of the target molecules resulting in structural formation in a synergistic way. The elementary metal-organic motifs are connected by Cl in a variety of fashions demonstrating structural diversity.

Structural Transformation and Stabilization of Metal-Organic Motifs Induced by Halogen Doping.

The structural transformation of supramolecular nanostructures with constitutional diversity and adaptability by dynamic coordination chemistry would be of fundamental importance for potential applications in molecular switching devices. The role of halogen doping in the formation of elementary metal-organic motifs on surfaces has not been reported. Now, the 9-ethylguanine molecule (G) and Ni atom, as a model system, are used for the structural transformation and stabilization of metal-organic motifs induced by iodine doping on Au(111). The iodine atoms are homogeneously located at particular hydrogen-rich locations enclosed by G molecules by electrostatic interactions, which would be the key for such an unexpected stabilizing effect. The generality and robustness of this approach are demonstrated in different metal-organic systems (G/Fe) and also by chlorine and bromine.