CO₂‑Induced Reversible Dispersion of Graphene by a Melamine Derivative

Smart graphene with stimuli-responsive dispersity has great potential for applications in medical and biochemical fields. Nevertheless, reversible dispersion/aggregation of graphene in water with biocompatible and removable trigger still represents a crucial challenge. Here, we report CO₂-induced reversible graphene dispersion by noncovalent functionalization of reduced graphene oxide with <i>N</i>²,<i>N</i>⁴,<i>N</i>⁶-tris­(3-(dimethyl­amino)­propyl)-1,3,5-triazine-2,4,6-triamine (MET). It was demonstrated that MET can be strongly adsorbed on graphene surface through van der Waals interaction to facilitate dispersing graphene in water. Moreover, reversible aggregation/dispersion of graphene can be achieved simply by alternately bubbling CO₂ and N₂ to control the desorption/adsorption of MET on graphene surface

Insights into the Relationship between CO₂ Switchability and Basicity: Examples of Melamine and Its Derivatives

Owing to its wide availability, nontoxicity, and low cost, CO₂ working as a trigger to reversibly switch material properties, including polarity, ionic strength, hydrophilicity, viscosity, surface charge, and degree of polymerization or cross-linking, has attracted an increasing attention in recent years. However, a quantitative correlation between basicity of these materials and their CO₂ switchability has been less documented though it is of great importance for fabricating switchable system. In this work, the “switch-on” and “switch-off” abilities of melamine and its amino-substituted derivatives by introducing and removing CO₂ are studied, and then their quantitative relationship with basicity is established, so that performances of other organobases can be quantitatively predicted. These findings are beneficial for forecasting the CO₂ stimuli-responsive behavior of other organobases and the design of CO₂-switchable materials

The ATP binding procedure.

<p><b>(A,B)</b> The bottom and side views of the binding pocket residue and the ATP plus Mg²⁺ (the sphere colored in blue) complex. The residue T433 (the fixed end), G431 (the hinge) and D429 (the gate) form the ATP gate <b>(B)</b>. Lock 1 is illustrated as twin-headed arrow with solid line, Lock 2 is illustrated as dashed line and Lock 3 is represented as dotted line. The bottom two rows of figures represent the views from the bottom and the side of the ATP binding pocket, showing the four key snapshots during ATP binding process, namely the Apo state, WS, TS and ATP bound state. The oxygen atoms near the gate are illustrated as red balls in the ATP bound state.</p

The movement at the sub-domain level.

<p><b>(A)</b> The C-α displacement of the tip residue H513 on the β hairpin. <b>(B)</b> The C-α displacement of the tip residue D455 on the central channel of the C-terminal. <b>(C)</b> The average folding angle of D2/D3, represented by the angle between C-α of LYS331, ASN366 and HIS513. <b>(D)</b> The average channel radius on the top (black), middle (red) and bottom section (blue). <b>(E)</b> The angle that can be changed between two domains (domain folding angle) is indicated (red arrow), C-α positions of the top K331, middle H513 and bottom D455 are shown in the helicase. The H513 and D455 displacements are shown in grey arrows.</p

The structural features of LTag helicase and its nucleotide pocket.

<p><b>(A)</b> The side view of the hexamer structure of LTag helicase. <b>(B)</b> The side view of LTag monomeric structure. <b>(C)</b> The structure of the LTag helicase binding pocket viewing from C-terminal end (bottom). The <i>cis</i>-residues are in copper and the <i>trans</i>-residues are in blue. The ATPs are painted in yellow in the middle figure. The ATP is colored by element type in the left mini-view. The nitrogen, carbon, oxygen and phosphate atoms are painted in blue, cyan read and gold. The N1-C4′-C5′-PB dihedral angle and the N1-C4′-PB bending angle are used to represent the conformational change in the ATP binding procedure.</p

The interaction energy profile between the ATP-Mg²⁺ complex and the binding pocket.

<p>The negative time slot represents the conformation before the docking stage. The top profile <b>(A)</b> is generated with ε=20, and the bottom profile <b>(B)</b> is generated with ε=40.</p

The timeline of the major hydrogen bonds formed in the binding pocket during the ATP binding procedure.

<p>The hydrogen bonds formed between the ATP and the <i>cis</i>-residues are plotted in black lines, between ATP and <i>trans</i>-residues are plotted in grey lines, between the binding lock residues are plotted in red lines, and between apical water and the coordinated residues are plotted in blue lines.</p

Domain-scale conformational change.

<p><b>(A)</b> A side view of a monomer in the context of a LTag hexamer, viewing from the outside. The yellow, blue and cyan helices are the alpha-helices H15, H6 and H8 respectively. <b>(B)</b> Bottom view of the monomer. The dotted line with two round ends is the axis, along which, the D2/3 part moves around. The circle with a cross inside indicates the position of the central channel. The red and blue residues are <i>cis</i> and <i>trans</i> residues respectively. <b>(C)</b> A side view of the monomer in the context of the LTag hexamer, viewing from inside of the hexamer. The movement of the tip residue of the β hairpin (H513) is illustrated in a series of red dots. The moving trajectory is about 15° to the axis of the central channel. <b>(D)</b> Side view of the monomer perpendicular to the rotation axis. The D2/D3 movement is illustrated by a series of tip residues, such as H513 and D455. The green, cyan, yellow residues correspond to the position of WS,TS and ATP bound state. The circles in yellow, dark blue and cyan represent the axis position of H15, H6 and H8, respectively. <b>(E)</b> The cooperative iris movement of the D2/D3 domain from the bottom view. <b>(F)</b> The cooperative upwards movement of the β-hairpin along the central channel in a screw manner. The upward arrows represent the H513 movement on the tips of the β-hairpin. The curved arrows illustrate the domain folding movement of D2/D3 along the axes in solid line.</p

The ATP conformational change and the cross locking system in ATP binding to the pocket.

<p><b>(A)</b> The binding direction of the ATP-Mg²⁺ complex. In the PDB file, the PA, PB and PG represent the αPi, βPi, and γPi respectively. The black, red, green and blue lines represent the distance from the αPi, βPi, γPi and ribose to the relative stable ASP474 Ca in the inner side of the binding pocket. <b>(B)</b> The dihedral angle of ATP; <b>(C)</b> The bending angle of ATP; <b>(D)</b> Lock1, the distance plot between ATP ribose O4′ and the LYS419 CE. <b>(E)</b> Lock2, the distance plot between ASP429 CG and the LYS419/419 CE. <b>(F)</b> Lock3, the distance plot between ASP474 CG and the ARG540 CZ.</p

Study sites and their ¹⁴C ages.

<p>Study sites and their ¹⁴C ages.</p