  Imaging single proteins has been a long-standing ambition for advancing
various fields in natural science, as for instance structural biology,
biophysics and molecular nanotechnology. In particular, revealing the distinct
conformations of an individual protein is of utmost importance. Here, we show
the imaging of individual proteins and protein complexes by low-energy electron
holography. Samples of individual proteins and protein complexes on ultraclean
freestanding graphene were prepared by soft-landing electrospray ion beam
deposition, which allows chemical- and conformational-specific selection and
gentle deposition. Low-energy electrons do not induce radiation damage, which
enables acquiring sub-nanometer resolution images of individual proteins
(cytochrome C and bovine serum albumin) as well as of protein complexes
(hemoglobin), which are not the result of an averaging process.

Biological Physics;Soft Condensed Matter;Instrumentation and Detectors;Quantitative Methods