Many proteins from the nacre and prismatic layers of mollusk shells have been identified and sequenced, yet the mutual molecular interactions, organization, and rearrangements that occur when proteins interact with calcium carbonate minerals during biomineralization are unknown. Examining the organic-mineral interface (OMI) is central to understanding these processes further. We prepared model biomineral systems, consisting of calcium carbonate crystals grown in the presence of model biomineral peptides (AP7N, AP24N, n16N) and a protein (LSM34) and used X-ray absorption near edge structure (XANES) spectromicroscopy to probe the electronic structure of both the calcium carbonate crystals and the peptides, to detect changing bonds at the OMI in crystals grown in the presence of peptides. All model biomineral systems gave similar results: disruption of CO bonds in calcite and enhancement of the peaks associated with C-H bonds in peptides. We also show that the observed changes do not occur when Asp and Glu are replaced in the n16N sequence with Asn and Gln, respectively, indicating that carboxyl groups in Asp and Glu are important to the organic-mineral interaction at the molecular level .