In biominerals, proteins are key elements in the controlled nucleation and growth of the mineral phase. We report here on the coupled evolution of the organic and inorganic structures during the nucleation and growth of CaCO3 under a monolayer of acidic beta-sheet forming peptides that mimic the natural proteins found in nacre. The investigation is carried out using in situ analytical techniques (X-ray diffraction and IR spectroscopy) to provide molecular scale structural information over the whole course of the mineralization process. Mineralization is shown to coexist with beta-sheet order while inducing other conformational changes to the peptide assembly. Peptides promote the growth of unoriented vaterite crystals; no templating effect of the beta-sheet order is observed.