Cover picture: Probing the electrostatic potential of transmembrane channels. The collage shows an atomic force microscopy (AFM) probe (gray) detecting the electrostatic potential generated by OmpF porin from Escherichia coli (pink to purple). The background (yellow-brown) shows individual OmpF trimers imaged in buffer solution using AFM. See the article by Philippsen et al. on p. 1667.