The proton/electron transfer reactions between cysteine residue (Cys) and tyrosinyl radical (Tyr(•)) are an important step for many enzyme-catalyzed processes. On the basis of the statistical analysis of protein data bank, we designed three representative models to explore the possible proton/electron transfer mechanisms from Cys to Tyr(•) in proteins. Our ab initio calculations on simplified models and quantum mechanical/molecular mechanical (QM/MM) calculations on real protein environment reveal that the direct electron transfer between Cys and Tyr(•) is difficult to occur, but an inserted water molecule can greatly promote the proton/electron transfer reactions by a double-proton-coupled electron transfer (dPCET) mechanism. The inserted H2O plays two assistant roles in these reactions. The first one is to bridge the side chains of Tyr(•) and Cys via two hydrogen bonds, which act as the proton pathway, and the other one is to enhance the electron overlap between the lone-pair orbital of sulfur atom and the π-orbital of phenol moiety and to function as electron transfer pathway. This water-mediated dPCET mechanism may offer great help to understand the detailed electron transfer processes between Tyr and Cys residues in proteins, such as the electron transfer from Cys439 to Tyr730(•) in the class I ribonucleotide reductase.

The ab initio calculations predict that the side chains of four aromatic amino acids (Phe, His, Tyr, and Trp residues) may promote methionine and cystine residues to participate in the protein electron hole transport by the formation of special multicenter, three-electron bonds (S∴π) between the Satoms and the aromatic rings. The formations of S∴π bonds can efficiently lower the local ionization energies, which drive the electron hole moving to the close side chains of S-containing and aromatic residues in proteins. Additionally, the proper binding energies for the S∴π bonds imply that the self-movement of proteins can dissociate these three-electron bonds and promote electron hole relay.

Layered sodium silicates β-Na2Si2O5 and kanemite were synthesized via facile methods under mild conditions. The tribological properties of β-Na2Si2O5 and kanemite utilized as additives in lithium grease were evaluated with a four-ball tester under different experimental conditions. The maximum nonseizure load value of 5.0 wt % β-Na2Si2O5 grease jumped from 353 N (the base grease) to 1568 N. However, 5.0 wt % MoS2 grease could only reach 617 N under the same conditions. The SEM and EDS results confirm that a protective film mainly composed of sodium silicates was formed on the worn surface during the friction process. The structural stability of β-Na2Si2O5 and kanemite after the wear test was studied by XRD. It was found that a loss of interlamellar water causes the layer structure of kanemite to collapse during a long-duration wear test. The layered structure of β-Na2Si2O5 is stable, and its tribological properties are better than those of kanemite.

The paper is of relevance to weak interactions between two parallel rings of close aromatic amino acids, which may participate in electron hole transport in proteins. The ab initio calculations reveal the possibility for the formation of the π∴π three-electron bond between two parallel aromatic rings, facilitating electron hole transport in proteins as the effective relay stations. The relay functionality of these special structures comes from their lower local ionization energies and proper binding energies, which vary with the different aromatic amino acids and the arrangements of the same aromatic rings according to the local microsurroundings in proteins.

No abstract