HR: 0800h
AN: B31D-0624    [Abstracts]
TI: New Origins of the Vital Effect in Calcites: Mg-Enhancing Influence of Biomolecules
AU: * Stephenson, A E
EM: aestephe@vt.edu
AF: Department of Geosciences, Virginia Tech, Blacksburg, VA 24061, United States
AU: Wu, L
EM: wu22@llnl.gov
AF: Lawrence Livermore National Lab, Chemistry and Materials Science Directorate, Livermore, CA 94550, United States
AU: Wu, K J
EM: wu17@llnl.gov
AF: Lawrence Livermore National Lab, Chemistry and Materials Science Directorate, Livermore, CA 94550, United States
AU: DeYoreo, J J
EM: deyoreo1@llnl.gov
AF: Lawrence Livermore National Lab, Chemistry and Materials Science Directorate, Livermore, CA 94550, United States
AU: Dove, P M
EM: dove@vt.edu
AF: Department of Geosciences, Virginia Tech, Blacksburg, VA 24061, United States
AB: Owing to the intense interest in the compositional signatures of biominerals, the mechanistic basis for vital effects and their roles in modifying or masking impurity contents are receiving increasing scrutiny. To date, much of the effort has been focused on the influence of physical environment and inorganic chemical factors. In a recent study that investigated the effects of acidic proteins on calcite growth, our research group found that nanomolar concentratios of acidic amino acids, peptides, and full proteins accelerate the rate of mineral formation by a relationship that correlates with the acidity (hydrophilicity) of the biomolecule (Elhadj et al., 2006, PNAS). Experimental and theoretical evidence suggest that the measured rate-enhancing effect (up to 25X) arises from weak interactions of the biomolecule with the calcite surface to alter the local solvation environment. This relation suggests that the acidic macromolecules that have been isolated from diverse calcifying taxa may have yet unrecognized effects on mineralization. Because Mg has a strong hydration shell relative to Ca, we hypothesized that the presence of these rate-modifying peptides in growth solutions would also lower the barrier to incorporating Mg, and thereby increase the MgCO3 content of calcite overgrowths. To test this idea, measurements of calcite growth rate were made using Atomic Force Microscopy, and in the presence or absence of acidic, hydrophilic 27-mer peptides. The peptide increased the growth rate of obtuse flanks (42% faster, on average, and up to 92% faster) and acute flanks (17% faster on average; up to 54% faster). The calcite overgrowths from AFM experiments were then analyzed for corresponding MgCO3 compositions by Time-of-Flight Secondary Ion Mass Spectrometry. The data yield an inorganic baseline that quantifies the relation between Mg content and the solution concentration. Comparisons of the baseline Mg content to that of peptide-enriched overgrowths show the MgCO3 composition is enhanced by 50 to 70% (in acute and obtuse flanks respectively) in calcite grown in the presence of peptides. Comparisons of these measurements to the MgCO3 compositions reported by Mucci (1987, GCA) for 5-40∞C synthetic seawater show that these differences are equivalent to the offset induced by a temperature change of several degrees. Two possible explanations for the enhanced Mg content in the presence of peptide are 1) step roughening increasing the kink density and 2) partial desolvation of the hydrated Mg ion, which lowers the energy barrier to incorporation.
DE: 0419 Biomineralization
DE: 0424 Biosignatures and proxies
DE: 1042 Mineral and crystal chemistry (3620)
SC: Biogeosciences [B]
MN: 2007 Fall Meeting