Inhibition of Methane Hydrate Formation by Ice-Structuring Proteins

Synergistic Effect of Hyperactive Antifreeze Protein on Inhibition of Gas-Hydrate Growth by Hydrophobic and Hydrophilic Groups

Antifreeze proteins (AFPs) are biodegradable inhibitors that effectively prevent the formation of natural gas hydrates that block pipelines. In this study, molecular dynamics simulations were employed to establish a kinetic model of the hyperactive insect antifreeze protein (Tenebrio molitor, TmAFP) and its mutants to inhibit the growth of sI natural methane hydrate. Simulations revealed that the hydrophobic and hydrophilic groups of threonine (Thr) residues at hydrate-binding sites played a synergistic role in binding hydrates. The hydrophobic groups anchored TmAFP to the hydrate surface through residues Thr39-Thr65 by migrating pendant hydrophobic methyl groups to the hydrate semicages. The hydrophilic groups stabilized TmAFP by hydrogen bonding with water molecules and integrating them into a quasi-hydrate structure, which more effectively inhibited hydrate growth. The results suggest that the hydrate growth inhibition is attributed to both the shape complementarity and the flexibility of binding residues. The synergy between hydrophobic and hydrophilic groups provides guidance for the design of more effective hydrate inhibitors.

Adsorption-Inhibition of Clathrate Hydrates by Self-Assembled Nanostructures

The mechanism by which safranine O (SFO), an ice growth inhibitor, halts the growth of single crystal tetrahydrofuran (THF) clathrate hydrates was explored using microfluidics coupled with cold stages and fluorescence microscopy. THF hydrates grown in SFO solutions exhibited morphology changes and were shaped as truncated octahedrons or hexagons. Fluorescence microscopy and microfluidics demonstrated that SFO binds to the surface of THF hydrates on specific crystal planes. Cryo-TEM experiments of aqueous solutions containing millimolar concentrations of SFO exhibited the formation of bilayered lamellae with an average thickness of 4.2±0.2 nm covering several μm² . Altogether, these results indicate that SFO forms supramolecular lamellae in solution, which might bind to the surface of the hydrate and inhibit further growth. As an ice and hydrate inhibitor, SFO may bind to the surface of these crystals via ordered water molecules near its amine and methyl groups, similar to some antifreeze proteins.

Experimental study on the kinetic effect of N-butyl-N-methylpyrrolidinium tetrafluoroborate and poly(N-vinyl-caprolactam) on CH4 hydrate formation

In this work, a series of experiments were carried out to study the kinetic inhibition performance of N-butyl-N-methylpyrrolidinium tetrafluoroborate ([BMP][BF₄]), poly(N-vinylcaprolactam) (PVCap) and compound inhibitor systems on methane hydrate from both macroscopic and microscopic perspectives. In the macroscopic experiments, the influence of the concentration, the ratio of inhibitors, the subcooling on the induction time and gas consumption rate of methane hydrate were studied. The results indicated that [BMP][BF₄] could inhibit the growth rate of CH₄ hydrate, but failed to delay the nucleation. An improved inhibitory effect was observed by combining [BMP][BF₄] and PVCap, and the optimal ratio of the two inhibitors was obtained to gain the best inhibition performance. Furthermore, the microstructure and morphology of methane hydrate crystals formed in different inhibitor systems were investigated through powder X-ray diffraction (PXRD), Raman spectroscopy and scanning electron cryomicroscopy (Cryo-SEM) methods. It was found that [BMP][BF₄] and PVCap had different influences on the large cage occupancy by CH₄ and the morphology of methane hydrate.

The influence of a type III antifreeze protein and its mutants on methane hydrate adsorption-inhibition: a molecular dynamics simulation study

Antifreeze proteins (AFPs) inhibit ice growth in various organisms at subzero temperature. Recently, AFPs as a hydrate inhibitor have been a topic of intense discussion, while the detailed mechanism remains obscure. The present work aims to explore molecular insight into the adsorption and inhibition of an AFP III on methane hydrate. Three polar, hydrophilic, and neutral amino acids (Asn14, Thr18, and Gln44) are mutated to elucidate the molecular mechanism of AFP III antifreeze activity. Another triple mutation is also designed to investigate the effect of the side chain. Atomistic molecular dynamics simulations provide detailed structural and dynamical aspects of protein residues and water molecules at the hydrate/water interface. Initially, it was proposed that the AFP III operates by the adsorption-inhibition mechanism on hydrates, almost similar to that of ice. The exchange of amide and hydroxyl groups by mutagenesis alters the shape of the side chain and the capability of hydrogen bonding and demonstrates that hydrogen bonds are not directly responsible for the AFP III antifreeze activity. Moreover, we deciphered that the length of the pendant group is an important factor in the entrapment of the AFP III on the hydrate cages, which is compatible with van der Waals interactions between the side chains and hydrate surface. The results suggest that this interaction is sensitive to the geometry and shape of the hydrate-binding surface (HBS) of the AFP, which implies that the interface between hydrates and the AFP is relatively rigid.

A 3-In-1 Approach to Evaluate Gas Hydrate Inhibitors

With a single apparatus and very short experimentation times, we have assessed phase equilibria, apparent kinetics and morphology of methane gas hydrates in the presence of thermodynamic inhibitors ethane-1,2-diol (MEG) and sodium chloride (NaCl); and kinetic hydrate inhibitor polyvinyl-pyrrolidone (PVP). Tight, local temperature control produced highly repeatable crystal morphologies in constant temperature systems and in systems subject to fixed temperature gradients. Hydrate-Liquid-Vapor (HLV) equilibrium points were obtained with minimal temperature and pressure uncertainties ( u T avg = 0 . 13 K and u p = 0 . 005 MPa). By applying a temperature gradient during hydrate formation, it was possible to study multiple subcoolings with a single experiment. Hydrate growth velocities were determined both under temperature gradients and under constant temperature growth. It was found that both NaCl and MEG act as kinetic inhibitors at the studied concentrations. Finally, insights on the mechanism of action of classical inhibitors are presented.

Adsorption of Kinetic Hydrate Inhibitors on Growing Surfaces: A Molecular Dynamics Study

We investigate the mechanism of a typical kinetic hydrate inhibitor (KHI), polyvinylcaprolactam (PVCap), which has been applied to prevent hydrate plugs from forming in gas pipe lines, using molecular dynamics simulations of crystal growth of ethylene oxide hydrate. Water-soluble ethylene oxide is chosen as a guest species to avoid problems associated with the presence of the gas phase in the simulation cell such as slow crystal growth. A PVCap dodecamer adsorbs irreversibly on the hydrate surface which grows at supercooling of 3 K when the hydrophobic part of two pendent groups are trapped in open cages at the surface. The amide hydrogen bonds make no contribution to the adsorption. PVCap can adsorb on various crystallographic planes of sI hydrate. This is in contrast to antifreeze proteins, each of which prefers a specific plane of ice. The trapped PVCap gives rise to necessarily the concave surface of the hydrate. The crystal growth rate decreases with increasing surface curvature, indicating that the inhibition by PVCap is explained by the Gibbs-Thomson effect.

Antifreeze proteins as gas hydrate inhibitors

Certain organisms survive low temperatures using a range of physiological changes including the production of antifreeze proteins (AFPs), which have evolved to adsorb to ice crystals. Several of these proteins have been purified and shown to also inhibit the crystallization of clathrate hydrates. They have been found to be effective against structure II (sII) hydrates formed from the liquid tetrahydrofuran, sI and sII gas hydrates formed from single gases, as well as sII natural gas hydrates using a mixture of three gases, as assessed using a variety of instrumentation including stirred reactors, differential scanning calorimetry, nuclear magnetic resonance, Raman spectroscopy, and X-ray powder diffraction. For the most part, AFPs are equal to or more effective than the commercial kinetic hydrate inhibitor (KHI) polyvinylpyrolidone, even under field conditions where saline and liquid hydrocarbons are present. Enclathrated gas analysis has revealed that the adsorption of AFPs to the hydrate surface is distinct from tested commercial KHIs and results in properties that should make these proteins more valuable in some field applications. Efforts to overcome the difficulties of recombinant protein production are ongoing, but in silico models of AFP adsorption to hydrates may offer the opportunity to design commercial KHIs for hydrocarbon recovery and transport with all the attributes of these AFP ”green inhibitors”, including their benefits for human and environmental safety.

Xenon hydrate dissociation measurements with model protein systems

Effective long-term storage remains a significant challenge to the use and development of protein pharmaceuticals. We have investigated the interactions between clathrate hydrates and model protein solutions to determine the effects on hydrate formation. Here, the dissociation curve and equilibrium conditions for xenon clathrate hydrate with model lysozyme and lactate dehydrogenase (LDH) protein solutions have been studied using calorimetry measurements at pressures ranging from 3 to 20 bar. Sucrose in solution was shown to exhibit small inhibition effects on xenon hydrate formation, shifting the dissociation curve and decreasing the conversion of water to hydrate by 15-26%. The addition of l-histidine buffer and lysozyme at low concentrations did not substantially inhibit hydrate formation. However, small shifts in the dissociation curve were demonstrated for solutions containing LDH. The presence of lysozyme and LDH in solution did not significantly alter the conversion of water to hydrate, indicating that these and similar proteins do not substantially affect the extent of xenon gas hydrate formation. Preliminary experiments were performed for LDH solutions to assess the impact of xenon hydrate formation and dissociation on enzymatic activity, with samples stored in hydrate systems showing small decreases in activity.