  Protein fibril accumulation at interfaces is an important step in many
physiological processes and neurodegenerative diseases as well as in designing
materials. Here we show, using $\beta$-lactoglobulin fibrils as a model, that
semiflexible fibrils exposed to a surface do not possess the Gaussian
distribution of curvatures characteristic for wormlike chains, but instead
exhibit a spontaneous curvature, which can even lead to ring-like
conformations. The long-lived presence of such rings is confirmed by atomic
force microscopy, cryogenic scanning electron microscopy and passive probe
particle tracking at air- and oil-water interfaces. We reason that this
spontaneous curvature is governed by structural characteristics on the
molecular level and is to be expected when a chiral and polar fibril is placed
in an inhomogeneous environment such as an interface. By testing
$\beta$-lactoglobulin fibrils with varying average thicknesses, we conclude
that fibril thickness plays a determining role in the propensity to form rings.
