Osteopontin 1-149 is an intrinsically disordered polypeptide, which, under certain conditions, sequesters amorphous calcium phosphate (ACP) forming a type of calcium phosphate nano-cluster (CPN). The CPNs not only play essential regulatory biological roles but also have a number of potential applications, e.g. in the control of mineralisation of prostheses and implants and in the formulation of stable artificial biofluids. A previous SAXS study showed that the OPN-CPN comprised a core of calcium phosphate surrounded by a protein shell. We propose to study how different conditions during OPN-CPN affect CPN structure, by using a full contrast variation study on OPN1-149 purified from bovine milk.