Abstract: The present invention generally relates to Magnetic Resonance Imaging (MRI) analysis of the location, presence, and/or quantity of a particular molecule or material. In some embodiments, the invention relates to compositions and methods for determination of an analyte, wherein a compositions may produce an MRI signal that can be affected by the presence of a particular composition (e.g., analyte). In a particular embodiment, the composition comprises a protein. The invention also provides related nucleic acid molecules, polypeptides, and fragments thereof. Compositions of the invention may exhibit reduced toxicity and may be readily delivered in vivo. Various embodiments of the invention may be useful as sensors, diagnostic tools, and the like.