Current Status and Development Trend of Research on Polymer-Based Kinetic Inhibitors for Natural Gas Hydrates

As the understanding of natural gas hydrates as a vast potential resource deepens, their importance as a future clean energy source becomes increasingly evident. However, natural gas hydrates trend towards secondary generation during extraction and transportation, leading to safety issues such as pipeline blockages. Consequently, developing new and efficient natural gas hydrate inhibitors has become a focal point in hydrate research. Kinetic hydrate inhibitors (KHIs) offer an effective solution by disrupting the nucleation and growth processes of hydrates without altering their thermodynamic equilibrium conditions. This paper systematically reviews the latest research progress and development trends in KHIs for natural gas hydrates, covering their development history, classification, and inhibition mechanisms. It particularly focuses on the chemical properties, inhibition effects, and mechanisms of polymer inhibitors such as polyvinylpyrrolidone (PVP) and polyvinylcaprolactam (PVCap). Studies indicate that these polymer inhibitors provide an economical and efficient solution due to their low dosage and environmental friendliness. Additionally, this paper explores the environmental impact and biodegradability of these inhibitors, offering guidance for future research, including the development, optimization, and environmental assessment of new inhibitors. Through a comprehensive analysis of existing research, this work aims to provide a theoretical foundation and technical reference for the commercial development of natural gas hydrates, promoting their safe and efficient use as a clean energy resource.

Molecular basis for inhibition of methane clathrate growth by a deep subsurface bacterial protein

Methane clathrates on continental margins contain the largest stores of hydrocarbons on Earth, yet the role of biomolecules in clathrate formation and stability remains almost completely unknown. Here, we report new methane clathrate-binding proteins (CbpAs) of bacterial origin discovered in metagenomes from gas clathrate-bearing ocean sediments. CbpAs show similar suppression of methane clathrate growth as the commercial gas clathrate inhibitor polyvinylpyrrolidone and inhibit clathrate growth at lower concentrations than antifreeze proteins (AFPs) previously tested. Unlike AFPs, CbpAs are selective for clathrate over ice. CbpA3 adopts a nonglobular, extended structure with an exposed hydrophobic surface, and, unexpectedly, its TxxxAxxxAxx motif common to AFPs is buried and not involved in clathrate binding. Instead, simulations and mutagenesis suggest a bipartite interaction of CbpAs with methane clathrate, with the pyrrolidine ring of a highly conserved proline residue mediating binding by filling empty clathrate cages. The discovery that CbpAs exert such potent control on methane clathrate properties implies that biomolecules from native sediment bacteria may be important for clathrate stability and habitability.

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.

Advances in the Study of Gas Hydrates by Dielectric Spectroscopy

The influence of kinetic hydrate inhibitors on the process of natural gas hydrate nucleation was studied using the method of dielectric spectroscopy. The processes of gas hydrate formation and decomposition were monitored using the temperature dependence of the real component of the dielectric constant ε′(T). Analysis of the relaxation times τ and activation energy ΔE of the dielectric relaxation process revealed the inhibitor was involved in hydrogen bonding and the disruption of the local structures of water molecules.

Surviving in a Frozen Forest: the Physiology of Eastern Spruce Budworm Overwintering

The eastern spruce budworm, Choristoneura fumiferana, is one of North America's most destructive forest insects. It survives the harsh winters by deploying both a sophisticated diapause program and a complex suite of cryoprotective molecules. The spruce budworm's cryoprotective biochemistry could revolutionize organ storage and transplants. Here we review the latest in C. fumiferana overwintering physiology and identify emerging theoretical and practical questions that are open for exploration.

Mainly on the Plane: Deep Subsurface Bacterial Proteins Bind and Alter Clathrate Structure

Gas clathrates are both a resource and a hindrance. They store massive quantities of natural gas but also can clog natural gas pipelines, with disastrous consequences. Eco-friendly technologies for controlling and modulating gas clathrate growth are needed. Type I Antifreeze Proteins (AFPs) from cold-water fish have been shown to bind to gas clathrates via repeating motifs of threonine and alanine. We tested whether proteins encoded in the genomes of bacteria native to natural gas clathrates bind to and alter clathrate morphology. We identified putative clathrate-binding proteins (CBPs) with multiple threonine/alanine motifs in a putative operon (cbp) in metagenomes from natural clathrate deposits. We recombinantly expressed and purified five CbpA proteins, four of which were stable, and experimentally confirmed that CbpAs bound to tetrahydrofuran (THF) clathrate, a low-pressure analogue for structure II gas clathrate. When grown in the presence of CbpAs, the THF clathrate was polycrystalline and platelike instead of forming single, octahedral crystals. Two CbpAs yielded branching clathrate crystals, similar to the effect of Type I AFP, while the other two produced hexagonal crystals parallel to the [1 1 1] plane, suggesting two distinct binding modes. Bacterial CBPs may find future utility in industry, such as maintaining a platelike structure during gas clathrate transportation.

Engineering Peptides to Catalyze and Control Stabilization of Gas Hydrates: Learning From Nature

Clathrate hydrates are nonstoichiometric crystalline inclusion compounds. Water acts as a "host lattice" and traps small guest molecules in stable cavities. One example, methane hydrates, are especially prevalent in situ at the seafloor. Although microorganism-produced proteins and polypeptides, including marine methylotroph porin proteins, can accelerate methane hydrate formation under conditions simulating their natural occurrence at the seafloor, the role that particular peptide sequences play in biocatalytic hydrate kinetics enhancement is unclear, especially the underlying molecular-level mechanisms. Here, we reveal the peptide-focused regulation of microorganisms' role in managing marine hydrates via an approximation mechanism of enzymatic catalysis accelerating hydrate formation. Aside from control of hydrate kinetics per se, we speculate that this peptide-centric mechanistic understanding could lead to a re-evaluation of the extent and geological importance of bioregulation of methane turnover in the biosphere.

Computational Study of Differences between Antifreeze Activity of Type-III Antifreeze Protein from Ocean Pout and Its Mutant

The antifreeze activity of a type-III antifreeze protein (AFP) expressed in ocean pout (Zoarces americanus) is compared with that of a specific mutant (T18N) using all-atom molecular dynamics simulations. The antifreeze activity of the mutant is only 10% of the wild-type AFP. The results from this simulation study revealed the following insights into the mechanism of antifreeze action by type-III AFPs. The AFP gets adsorbed to the advancing ice front due to its hydrophobic nature. A part of the hydrophobicity is caused by the presence of clathrate structure of water molecules near the ice-binding surface (IBS). The mutation in the AFP disrupts this structure and thereby reduces the ability of the mutant to adsorb to the ice-water interface leading to the loss of antifreeze activity. The mutation, however, has no effect on the ability of the adsorbed protein to bind to the growing ice phase. Simulations also revealed that all surfaces of the protein can bind to the ice phase, although the IBS is the preferred surface.

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.

Hydrophobic Hydration and the Effect of NaCl Salt in the Adsorption of Hydrocarbons and Surfactants on Clathrate Hydrates

Adsorption of functional molecules on the surface of hydrates is key in the understanding of hydrate inhibitors. We investigate the adsorption of a hydrocarbon chain, nonionic and ionic surfactants, and ions at the hydrate-aqueous interface. Our results suggest a strong connection between the water ordering around solutes in bulk and the affinity for the hydrates surface. We distinguish two types of water ordering around solutes: (i) hydrophobic hydration where water molecules form a hydrogen bond network similar to clathrate hydrates, and (ii) ionic hydration where water molecules align according to the polarity of an ionic group. The nonionic surfactant and the hydrocarbon chain induce hydrophobic hydration and are favorably adsorbed on the hydrate surface. Adsorption of ions and the ionic headgroups on the hydrate surface is not favorable because ionic hydration and the hydrogen bond structure of hydrates are incompatible. The nonionic surfactant is adsorbed by the headgroup and tail while adsorption of the ionic surfactants is not favorable through the head. Water ordering is analyzed using the hydrogen bond and tetrahedral density profiles as a function of the distance to the chemical groups. The adsorption of solutes is studied through the free energy profiles as a function of the distance to the hydrate surface. Salt lowers the melting temperature of hydrates, disrupts hydrophobic hydration, reduces the solubility of solutes in the aqueous solution, and increases the propensity of solutes to be adsorbed on hydrate surfaces. Our studies are performed by the unbiased and steered molecular dynamics simulations. The results are in line with experiments on the effect of salt and alkanes in hydrate antiagglomeration.

Inhibition of Tetrahydrofuran Hydrate Formation in the Presence of Polyol-Modified Glass Surfaces

Glycerol was conjugated to glass test tube surfaces in four configurations by employing two different silane spacers, covalent attachment to glycerol at either the 1- or the 2-position, and with a succinic acid spacer. The resulting surfaces were tested for their ability to inhibit the nucleation of tetrahydrofuran hydrate (THF hydrate) in comparison with polyvinylpyrrolidone (PVP), a known polymeric inhibitor of THF hydrate formation. Contact angle measurements were used as an indication of surface modification throughout the glass derivatization steps. Of the four final surfaces modified with glycerol, only the coating with (3-aminopropyl)triethoxysilane (APTES) and glycerol coupled at the 1-position (leaving a free 1,2-diol) showed significant inhibition of the formation of THF hydrate. The corresponding N-[3-(trimethoxysilyl)propyl]-ethylenediamine (AEAPTMS) coating with glycerol coupled at the 1-position did not show a significant difference over the untreated test tubes. Attachment of glycerol at the 2-position yielded a coating with no benefit over the untreated test tubes regardless of the silane used, and a surface modified with APTES and succinic acid alone enhanced the formation of THF hydrate. The ability to inhibit THF hydrate formation using a polyol-modified surface is a first step in the development of a coating that, alone or in combination with known gas hydrate inhibitors, could be used to prevent gas hydrates from plugging pipelines in field applications.

PicW orthologs from spruce with differential freezing tolerance expressed in Escherichia coli

Spruce can grow at an extra low temperature (LT), and is inferred with important antifreezing gene resources. The research here identified 4 different spruce varieties, named as PicW1, PicW2, PicM and PicK. Sequence alignment showed base-substitution and deficiency mutations among them with sequence identity between 97.61% and 99.25%. Each gene was transferred into E. coli, where protein was induced by IPTG (isopropyl-β-d-thiogalactoside). Strains cultured at -5°C showed the lethal dose 50% (LD-50) between 53h and 57h for the transgenic strains, but 35h for the control. Strains cultivated at -20°C showed the LD-50 between 38h and 44h for the transgenic strains, but 25h for the control. Further, the soluble gene proteins were extracted and purified for Differential Scanning Calorimeter (DSC) test, which showed characteristic thermal hysteresis (TH) value of 0.77°C (PicW1), 0.78°C (PicW2), 0.72°C (PicM), and 0.86°C (PicK) respectively, significantly higher than the value of 0.05°C of the control (BSA). Summarily, four homologous proteins showed good antifreeze property with the range from high to low as PicK>PicW2>PicW1>PicM. It suggested that they can be used as resources for genetic engineering of plant cold tolerance.

Ice-Binding Proteins in Plants

Sub-zero temperatures put plants at risk of damage associated with the formation of ice crystals in the apoplast. Some freeze-tolerant plants mitigate this risk by expressing ice-binding proteins (IBPs), that adsorb to ice crystals and modify their growth. IBPs are found across several biological kingdoms, with their ice-binding activity and function uniquely suited to the lifestyle they have evolved to protect, be it in fishes, insects or plants. While IBPs from freeze-avoidant species significantly depress the freezing point, plant IBPs typically have a reduced ability to lower the freezing temperature. Nevertheless, they have a superior ability to inhibit the recrystallization of formed ice. This latter activity prevents ice crystals from growing larger at temperatures close to melting. Attempts to engineer frost-hardy plants by the controlled transfer of IBPs from freeze-avoiding fish and insects have been largely unsuccessful. In contrast, the expression of recombinant IBP sequences from freeze-tolerant plants significantly reduced electrolyte leakage and enhanced freezing survival in freeze-sensitive plants. These promising results have spurred additional investigations into plant IBP localization and post-translational modifications, as well as a re-evaluation of IBPs as part of the anti-stress and anti-pathogen axis of freeze-tolerant plants. Here we present an overview of plant freezing stress and adaptation mechanisms and discuss the potential utility of IBPs for the generation of freeze-tolerant crops.

Structural Basis for the Inhibition of Gas Hydrates by α-Helical Antifreeze Proteins

Kinetic hydrate inhibitors (KHIs) are used commercially to inhibit gas hydrate formation and growth in pipelines. However, improvement of these polymers has been constrained by the lack of verified molecular models. Since antifreeze proteins (AFPs) act as KHIs, we have used their solved x-ray crystallographic structures in molecular modeling to explore gas hydrate inhibition. The internal clathrate water network of the fish AFP Maxi, which extends to the protein's outer surface, is remarkably similar to the {100} planes of structure type II (sII) gas hydrate. The crystal structure of this water web has facilitated the construction of in silico models for Maxi and type I AFP binding to sII hydrates. Here, we have substantiated our models with experimental evidence of Maxi binding to the tetrahydrofuran sII model hydrate. Both in silico and experimental evidence support the absorbance-inhibition mechanism proposed for KHI binding to gas hydrates. Based on the Maxi crystal structure we suggest that the inhibitor adsorbs to the gas hydrate lattice through the same anchored clathrate water mechanism used to bind ice. These results will facilitate the rational design of a next generation of effective green KHIs for the petroleum industry to ensure safe and efficient hydrocarbon flow.