Final answer:
Nuclear magnetic resonance (NMR) is the protein study method limited to proteins below 50 kD.
Step-by-step explanation:
The method used to study proteins that is limited to proteins with a molecular mass of 50 kD or less is nuclear magnetic resonance (NMR). This technique utilizes the magnetic properties of atoms to determine the three-dimensional structure of proteins in solution. However, due to the complex nature of analyzing interactions within larger proteins, NMR is typically constrained to smaller proteins, usually below 50 kilodaltons. It is effective for detecting small amounts of proteins, but it has limitations when it comes to proteins with larger molecular masses.