The roles of functional groups of antifreeze protein in inhibition of hydrate growth

The search for green and efficient inhibitors to solve the hydrate blockage problem is a widespread concern in natural gas hydrate exploitation and oil/gas transportation. Tenebrio molitor antifreeze protein (TmAFP) as a biodegradable substance with a high value of thermal hysteresis activity has an excellent inhibition effect on ice growth. Considering that gas hydrates are ice-like solids, we performed molecular dynamics simulations on TmAFP to study its inhibition behavior on methane hydrate growth. Our results show that TmAFP adsorbs on hydrate growth surface by providing several functional groups as “central gas” to be trapped in half hydrate cages where the methyl groups of threonine residues have the highest chance. Moreover, the amide groups of amino acids can also be engaged in half cages. Since these amino acids are uncharged and polar, we infer that they interact with the half-cage with mainly hydrogen bonding. Surprisingly, all adsorption sites were non-ice binding sites and TmAFP can disrupt surrounding water molecules before adsorption, unlike the behavior of previously well-studied fish antifreeze proteins. However, functional groups of the backbone of the ice-binding surface on the β-sheet do not act as the adsorption sites. The reason is that the backbone is not easily deformable and the amino acid group spacing in this part does not match the adjacent hydrate cage spacing. This implies that the structure of the antifreeze protein has a crucial role in hydrate growth inhibition. When seeking efficient antifreeze proteins for hydrate growth inhibition, functional groups as well as their relative positions should be simultaneously considered. These studies guide the development of green and efficient kinetic hydrate growth inhibitors.