The spectroscopic analysis of large biomolecules is important in applications such as biomedical diagnostics and pathogen detection(1,2), and spectroscopic techniques can detect such molecules at the nanogram level or lower. However, spectroscopic techniques have not been able to probe the structure of large biomolecules with similar levels of sensitivity. Here, we show that superchiral electromagnetic fields(3), generated by the optical excitation of plasmonic planar chiral metamaterials(4,5), are highly sensitive probes of chiral supramolecular structure. The differences in the effective refractive indices of chiral samples exposed to left- and right-handed superchiral fields are found to be up to 10(6) times greater than those observed in optical polarimetry measurements, thus allowing picogram quantities of adsorbed molecules to be characterized. The largest differences are observed for biomolecules that have chiral planar sheets, such as proteins with high beta-sheet content, which suggests that this approach could form the basis for assaying technologies capable of detecting amyloid diseases and certain types of viruses.