Mineralized collagen fibrils are the building blocks of bone, and the mineralization of collagen fibrils is generally regulated by noncollagenous proteins (NCPs). However, the functions of NCPs are difficult to investigate in vivo. Here, we use poly(acrylic acid) (PAA) with different molecular weights (5, 50, 450, and 4000 kDa) as analogs of NCPs and explore their effects on collagen mineralization in vitro. All the PAA molecules can promote the intrafibrillar mineralization of calcium carbonate (CaCO