Cyclic or Linear? Parameters Determining the Outcome of Glycine Polymerization in Silica Surface Prebiotic Scenarios

Abstract

The parameters that determine the formation of linear peptides and cyclic dimers (diketopiperazine, DKP) on silica surfaces of different surface area, silanol and siloxane ring populations, controlled by thermal treatments, are investigated upon glycine deposition from gas and liquid phases. The formed products were characterized by infrared and Raman spectroscopies, X‐ray diffraction and thermogravimetric analysis. The results reveal the importance of “nearly‐free” silanols to form ester centers as primers for the formation of linear peptides over DKP, on surfaces with medium silanol density (1.4 to 2.7 nm−2). Quenched reactivity is seen on isolated silanols (density≤0.7 nm−2), while silanols involved in hydrogen bonding (density of 4.5 nm−2) weakly interact with Gly resulting in its cyclization to DKP. Deposition of glycine from liquid phase may also form both DKP and linear polymers, depending on its loading and silica surface. These conclusions demonstrate the complexity of glycine surface chemistry in the polymerization reaction and highlight the interest of a surface science approach to evaluate geochemical prebiotic scenarios.

Amino Acid Polymerization on Silica Surfaces

“…[44][45][46][47][48][49][50][51] Values as high as 7.7 OH nm À 2 have been reported for precipitated silicas, [52] while values around 1.5 OH nm À 2 or lower have been associated to dehydroxylated silicas as a result of a high temperature (> 600 °C) treatment. [44,53] Depending on their concentration and distribution on the surface, silanol groups represented in Figure 2 can be described as: [25] i) isolated: silanol groups separated by more than ~6 Å from their nearest neighbors, which are not able to establish hydrogen bonding with other silanols; ii) vicinal: silanol groups on tetrahedra sharing a common vertex, separated by less than ~3 Å, which are involved in strong hydrogen bonding interactions with each other; iii) interacting, associated or H-bonded: even if they do not belong to the same tetrahedra, they are distant by less than ~3 Å and are involved in mutual interactions; iv) geminal: these are silanol groups linked to the same Si surface atom (HOÀ SiÀ OH); even though they are separated by a short distance, they cannot form H-bonds with each other because of geometric constraint. Due to the different behaviors of the silanol groups, the hydrophilicity of the silica surface depends on their relative concentration: isolated silanols are not hydrophilic, while water shows a strong affinity for hydrogen-bonded patches.…”

“…In addition, AA adsorption (through hydrogen bonding and/or electrostatic interactions) often occurs in concurrence with the precipitation of AAs as bulk crystals, increasing the complexity of the study of the adsorption process. [20,22,24,53,66,73,83] Solid-state NMR is useful to discriminate between these two phenomena, since it can detect the formation of crystalline forms even when these are below the detection limit of X-ray diffraction. The saturation coverage depends on the AA, and on the type of silica; it can correspond to very low loadings and to surface coverages much smaller than the silanols density.…”

“…Some variations of the method consist in the pretreatment of the silica powder before the sublimation step, which was dehydrated at 100 °C overnight [86] or outgassed at room temperature directly in the U-shaped cell. [53] This methodology presents several key differences from aqueous phase deposition ones. First, the speciation of the AAs is different: in the gas phase, they are present in the canonical form (Figure 1A) rather than the zwitterion (Figure 1B) or ionic forms found in high water activity conditions.…”

“…A majority of the experimental studies of the interaction of AAs with silica are based on deposition procedures from the liquid phase. [15,22,33,53,[62][63][64][65][66][67][68][69][70][71][72] In a common procedure, a small amount of silica is dispersed in an aqueous solution of the AAs, the solution is stirred for few hours, the solid is separated by centrifugation or filtration and dried (Figure 3A). Bouchoucha et al referred to this methodology as selective adsorption (SA), a term taken from the field of catalyst preparation.…”

“…It is quite customary to transform these surface coverages to fractions of a physical "monolayer", the latter being calculated on the basis of the area of the AA molecule, assumed to be 'lying flat' on the surface. [53] Monolayer coverage may be useful as a point of reference, but in the case of site adsorption, the saturation coverage has no reason to be equal to the monolayer coverage. In addition, AA adsorption (through hydrogen bonding and/or electrostatic interactions) often occurs in concurrence with the precipitation of AAs as bulk crystals, increasing the complexity of the study of the adsorption process.…”