Theoretical and Experimental Study of the Adsorption of Neutral Glycine on Silica from the Gas Phase

Abstract

The adsorption of neutral glycine onto amorphous silica was investigated both theoretically and experimentally. DFT calculations were performed at the BLYP-631++G** level using a cluster approach. Several possible configurations involving the formation of H-bonds between glycine and one, two or three silanols, (SiOH) were considered. The most favorable bonding of glycine with one silanol (45 kJ/mol) occurs through the COOH moiety, forming a cycle in which the CO group is H-bond acceptor whereas the acidic OH group is H-bond donor. With two or three silanols, additional H-bonds are formed between the amine moiety and the silanols, leading to an increased adsorption energy (70 and 80 kJ/mol for two and three silanols respectively). Calculated ν CO , δ HNH and δ ΗCH are sensitive to the adsorption mode. A bathochromic shift of νCO as compared to νCO of free glycine (calculated in the 1755-1790 cm -1 range) is found for glycine in interaction with silanol(s). The more H-bonds of COOH with silanol groups, the higher the bathochromic shift. For δHNH, no shift is found for glycine adsorbed on one and two silanols (where the amine is either not bound or H bond donor), whereas a bathochromic shift is calculated with three silanols when the amine moiety is H-bond acceptor.Experimental FTIR spectra performed at RT for glycine adsorbed at 160°C on Aerosil amorphous silica exhibit bands at 1371, 1423, 1630 and 1699 cm -1 . The experimental/calculated frequencies have their best correspondance for glycine adsorbed on two silanols. It is important to note that the forms giving the best correspondence to experimental frequencies are the most stable ones. TITLE RUNNING HEAD : Adsorption of Glycine on Silica

Silica Surface Features and Their Role in the Adsorption of Biomolecules: Computational Modeling and Experiments

“…In contrast, Lomenech et al 223 did not observe any band at this region. The three works assigned (C=O) bands corresponding to…”

“…Moreover, these results provide a plausible explanation of why Basiuk et al 647 observed the formation of the silica SMA whereas Lomenech et al 223 did not. That is, in Basiuk's surfaces, after pretreatment at 700 ºC, some S2R defects were probably present, which reacted with vapors of Gly during the CVD experiments to form the SMA bond; in contrast, Lomenech's silica surface was pretreated at 100 ºC so that such strained defects were probably not produced and consequently SMAs were not able to form.…”

“…Remarkably, the computed Gly/silica complexes that exhibit these two H-bond patterns were found to be the most stable adducts. Differences between the experimental results of Basiuk et al 647 and those of Lomenech et al 223 can be related to the different pretreatment of the silica surfaces. The latter authors pretreated the silica sample at 100ºC overnight, while the former ones outgassed the silica sample at about 700 ºC.…”

“…644 The works of Groenewegen et al 646 and Basiuk et al 647 are two seminal studies focused on the interaction of Gly vapors with silica surfaces outgassed at 650-700 ºC using IR spectroscopy measurements. More recently, Lomenech et al 223 reported the FTIR spectrum of the adsorption of…”

“…Drawn from data of Refs. 74,223 The value of 1699 cm -1 represents the maximum bathochromic shift (-85 cm -1 ) of the (C=O) fingerprint mode and results from the H-bonding with two different terminal silanol groups (see "double H-bonding" interaction pattern in Figure 62). Theoretical results indicate that the interaction of Gly with silica surfaces may also take place with only one SiOH group through the "single H-bonding" interaction pattern (see Figure 62).…”