HR: 0800h
AN: B21B-0871    [Abstracts]
TI: Aspartate Chain Length Controls Calcite Step Morphology
AU: * Elhadj, S
EM: elhadj@vt.edu
AF: Dept. of Geosciences, Virgnia Tech, Derring Hall 4044, Blacksburg, VA 24061 United States
AU: Han, N
EM: nhan@vt.edu
AF: Dept. of Geosciences, Virgnia Tech, Derring Hall 4044, Blacksburg, VA 24061 United States
AU: Dove, P M
EM: pdove@vt.edu
AF: Dept. of Geosciences, Virgnia Tech, Derring Hall 4044, Blacksburg, VA 24061 United States
AU: Salter, E
EM: asalter@jaguar1.usouthal.edu
AF: Dept. of Chemistry, University of South Alabama, 307 University Blvd. N. , Mobile, AL 36688 United States
AU: Wierzbicki, A
EM: awierzbi@jaguar1.usouthal.edu
AF: Dept. of Chemistry, University of South Alabama, 307 University Blvd. N. , Mobile, AL 36688 United States
AU: De Yoreo, J J
EM: deyoreo1@llnl.gov
AF: Dept. of Chem. and Mat. Sci. Lawrence Livermore National Labs, 7000 East Ave., P.O. Box 808, Livermore, CA 94551 United States
AB: Most controlled crystallization is believed to be achieved under the direction of macromolecular protein-based templates with specific affinities for accumulating and assembling the desired growth units from solution. This reliance upon cellular proteins with specific sequences and structures suggests that the chemistry and stereochemistry of the amino acids forming the proteins are essential in conferring targeted activity in controlled biosynthesis. Calcium carbonate is a key biogenic mineral and model system to study the molecular mechanisms by which amino acids and, thus, polypeptides interact with mineral surfaces. Investigations of molecularly resolved crystal-peptide interactions are essential for a first order understanding of how biomolecules achieve regulated growth during biomineralization. Our previous studies have focused on the role of specific single and dipeptides species known to constitute a significant fraction of the proteins involved in biomineralization processes, in particular, acidic amino acids and aspartate. In this study we extend our investigation to the role of chain length of polyaspartates since they represent an integral part of the active site of these biomolecules' sequences. In particular, the interactions of aspartates (Asp$_{n}$, n=1,2,4,5,6) were investigated to measure their effects on the kinetics and thermodynamics of growth in parallel with theoretical calculations. Using in situ Atomic Force Microscopy (AFM) and carefully characterized solution compositions, our experimental measurements of growth kinetics and direct observations of the calcite hillock morphology at the nanoscale show that step directions with acute and obtuse geometries are affected differently for all aspartate derivatives. Short chain aspartates (n$>$=2) roughen acute steps more strongly than obtuse steps. This situation is exactly reversed for longer chain aspartates (n$>$2). Further, the critical Asp$_{n}$ concentration required for the roughening transition to occur decreases with increasing length of the Asp peptide. Circular Dichroism (CD) measurements of solution aspartate conformations were performed to determine the impact of conformation on stereochemical and steric interactions, and used as input in our molecular modeling studies. We examine the molecular origin for these experimental results using molecular modeling. Not unexpectedly, due to cooperative binding, the concentrations required to affect step edge morphology in general, decreased exponentially with a linear increase in aspartate chain length. This increased binding with chain length correlates with inhibition of step flow velocity measurements by AFM, and was explained using molecular modeling relating chain length to calcite steps binding energies. Our findings suggest that there is a qualitative change in aspartate-calcite steps interactions with a quantitative change in aspartate chain length, in addition to an exponential increase in the degree of aspartates interactions. This chain length dependent differential binding to opposing calcite steps may be an additional mechanism by which biomineralizing proteins can achieve calcite growth regulation.
DE: 0400 Biogeosciences
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting