title: Tapping-mode atomic force microscopy produces faithful high-resolution images of protein surfaces.
abstract: Compared to contact-mode atomic force microscopy (CMAFM), tapping-mode atomic force microscopy (TMAFM) has the advantage of allowing imaging surfaces of macromolecules, even when they are only weakly attached to the support. In this study, TMAFM is applied to two different regular protein layers whose structures are known to great detail, the purple membrane from Halobacterium salinarum and the hexagonally packed intermediate (HPI) layer from Deinococcus radiodurans, to assess the faithfulness of high-resolution TMAFM images. Topographs exhibited a lateral resolution between 1.1 and 1. 5 nm and a vertical resolution of approximately 0.1 nm. For all protein surfaces, TMAFM and CMAFM topographs were in excellent agreement. TMAFM was capable of imaging the fragile polypeptide loop connecting the transmembrane alpha-helices E and F of bacteriorhodopsin in its native extended conformation. The standard deviation (SD) of averages calculated from TMAFM topographs exhibited an enhanced minimum (between 0.1 and 0.9 nm) that can be assigned to the higher noise of the raw data. However, the SD difference, indicating the flexibility of protein subunits, exhibited an excellent agreement between the two imaging modes. This demonstrates that the recently invented imaging-mode TMAFM has the ability to faithfully record high-resolution images and has sufficient sensitivity to contour individual peptide loops without detectable deformations.
authors.affiliation: M. E. Müller Institute for Structural Biology, Biozentrum, CH-4056 Basel, Switzerland.
authors.forename: C
authors.initials: C
authors.lastname: Möller
authors.forename: M
authors.initials: M
authors.lastname: Allen
authors.forename: V
authors.initials: V
authors.lastname: Elings
authors.forename: A
authors.initials: A
authors.lastname: Engel
authors.forename: D J
authors.initials: DJ
authors.lastname: Müller
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry / Q000648:ultrastructure; D055592:Biophysical Phenomena; D001703:Biophysics; D005069:Evaluation Studies as Topic; D006095:Gram-Positive Cocci / Q000737:chemistry; D019617:Halobacterium salinarum / Q000737:chemistry; D007091:Image Processing, Computer-Assisted; D046911:Macromolecular Substances; D018625:Microscopy, Atomic Force / Q000379:methods* / Q000706:statistics & numerical data; D011487:Protein Conformation; D011506:Proteins / Q000737:chemistry* / Q000648:ultrastructure*; D018717:Purple Membrane / Q000737:chemistry / Q000648:ultrastructure; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D013499:Surface Properties
chemical_list: D001426:Bacterial Proteins; D046911:Macromolecular Substances; D011506:Proteins