BackgroundAmong the 20 natural amino acids histidine is the most active and versatile member that plays the multiple roles in protein interactions, often the key residue in enzyme catalytic reactions. A theoretical and comprehensive study on the structural features and interaction properties of histidine is certainly helpful.ResultsFour interaction types of histidine are quantitatively calculated, including: (1) Cation-π interactions, in which the histidine acts as the aromatic π-motif in neutral form (His), or plays the cation role in protonated form (His+); (2) π-π stacking interactions between histidine and other aromatic amino acids; (3) Hydrogen-π interactions between histidine and other aromatic amino acids; (4) Coordinate interactions between histidine and metallic cations. The energies of π-π stacking interactions and hydrogen-π interactions are calculated using CCSD/6-31+G(d,p). The energies of cation-π interactions and coordinate interactions are calculated using B3LYP/6-31+G(d,p) method and adjusted by empirical method for dispersion energy. ConclusionsThe coordinate interactions between histidine and metallic cations are the strongest one acting in broad range, followed by the cation-π, hydrogen-π, and π-π stacking interactions. When the histidine is in neutral form, the cation-π interactions are attractive; when it is protonated (His+), the interactions turn to repulsive. The two protonation forms (and pKa values) of histidine are reversibly switched by the attractive and repulsive cation-π interactions. In proteins the π-π stacking interaction between neutral histidine and aromatic amino acids (Phe, Tyr, Trp) are in the range from -3.0 to -4.0 kcal/mol, significantly larger than the van der Waals energies.

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We performed an unbiased structure search for low-lying energetic minima of neutral and charged palladium PdnQ (n = 2–20, Q = 0, + 1 and –1) clusters using CALYPSO method in combination with density functional theory (DFT) calculations. The main candidates for the lowest energy neutral, cationic and anionic clusters are identified, and several new candidate structures for the cationic and anionic ground states are obtained. It is found that the ground state structures of small palladium clusters are more sensitive to the charge states. For the medium size Pdn0/+/– (n = 16–20) clusters, a fcc-like growth behavior is found. The structural transition from bilayer-like structures to cage-like structures is likely to occur at n = 14 for the neutral and cationic clusters. In contrast, for the anionic counterparts, the structural transition occurs at Pd13–. The photoelectron spectra (PES) of palladium clusters are simulated based on the time-dependent density functional theory (TD-DFT) method and compared with the experimental data. The good agreement between the experimental PES and simulated spectra provides us unequivocal structural information to fully solve the global minimum structures, allowing for new molecular insights into the chemical interactions in the Pd cages.

Flexible pressure sensors have received wide attention because of their potential applications in wearable electronics and electronic skins (e-skins). However, the high performance of the pressure sensors relies principally on the introduction of complex surface microstructures, which often involves either complicated procedures or costly microfabrication methods. Moreover, these devices predominantly use synthetic polymers as flexible substrates, which are generally nonbiodegradable or not ecofriendly. Here, we report a facile and scalable processing strategy to convert naturally rigid wood into reduced graphene oxide (rGO)-modified flexible wood (FW/rGO) via saw cutting, chemical treatment, and rGO coating, resulting in high-performance wood-based flexible piezoresistive pressure sensors. Benefiting from the largely deformable ribbon-like surface microstructures, the obtained wood-based pressure sensor displayed a high sensitivity of 1.85 kPa −1 over a broad linear range up to 60 kPa and showed high stability over 10 000 cyclic pressings. The favorable sensing performance of the pressure sensor allows for accurate recognition of finger movements, acoustic vibrations, and real-time pulse waves. Moreover, a large-area pressure sensor array has been successfully assembled on one piece of flexible wood for spatial pressure mapping. The proposed strategy of directly using natural wood for high-performance flexible pressure sensors is simple, low-cost, sustainable, and scalable, opening up a new avenue for the development of next-generation wearable electronics and e-skins.