²⁹Si NMR Chemical Shifts and Energetics of Silica-Serine and Silica-Polyalcohol Complexes as Indicators of Silica Biomineralization Mechanisms

Abstract

Serine- and polysaccharide-enriched organic matrix is associated with biogenic silica such as diatom tests, sponge spicules, and phytoliths. We have used molecular orbital theory to determine the relative stability and ²⁹Si NMR shifts of direct Si-O-C ester-like bonds versus hydrogen bonds between the monomeric silicic acid and the alcohol group on aliphatic organics such as serine and threitol (a polyacohol as proxy for polysaccharides). Preliminary results suggest that at neutral pHs, H-bonds and ester-bonds of four-fold coordinated silicon are of comparable stability. Formation of ester-like bonds with five-fold coordinated silicon is endothermic at neutral pHs but is stabilized at higher pHs. ²⁹Si shifts of the H-bonded and ester-bonded complexes of four-fold coordinated silicon range from -55 to -73 ppm similar to monomeric inorganic silicic acid but far more positive than the -92, -102, and -110 ppm values observed experimentally in biogenic silicas. The five-coordinated silicon complexes yield shifts of -96 to -107 ppm. The latter range is within the range of inorganic, polymerized silica. If five-fold coordinated Si with direct Si-O-C bonds is present as a precursor or intermediate or stable species in biogenic silica, it could have escaped detection due to overlap with inorganic polymerized silica. Thus, ²⁹Si NMR shifts are not necessarily diagnostic of the presence or absence of Si-O-C bonds in biogenic silica.