We show that standard silicon nitride cantilevers can be used for tapping mode atomic force microscopy (AFM) in air, provided that the energy of the oscillating cantilever is sufficiently high to overcome the adhesion of the water layer. The same cantilevers are successfully used for tapping mode AFhif in liquid. Acoustic modes in the liquid excite the canti1eve.r. On soft samples, e.g., biological material, this tapping mode AFM is much more gentle than the regular contact mode AFM. Not only is the destructive infuence of the lateral forces minimized, but more important, the intrinsic viscoelastic properties of the sample itself are effectively used to "harden" the soft sample.

Specific molecular recognition events, detected by atomic force microscopy (AFM), so far lack the detailed topographical information that is usually observed in AFM. We have modified our AFM such that, in combination with a recently developed method to measure antibody-antigen recognition on the single molecular level (Hinterdorfer, P., W. Baumgartner, H. J. Gruber, K. Schilcher, and H. Schindler, Proc. Natl. Acad. Sci. USA 93:3477-3481 (1996)), it allows imaging of a submonolayer of intercellular adhesion molecule-1 (ICAM-1) in adhesion mode. We demonstrate that for the first time the resolution of the topographical image in adhesion mode is only limited by tip convolution and thus comparable to tapping mode images. This is demonstrated by imaging of individual ICAM-1 antigens in both the tapping mode and the adhesion mode. The contrast in the adhesion image that was measured simultaneously with the topography is caused by recognition between individual antibody-antigen pairs. By comparing the high-resolution height image with the adhesion image, it is possible to show that specific molecular recognition is highly correlated with topography. The stability of the improved microscope enabled imaging with forces as low as 100 pN and ultrafast scan speed of 22 force curves per second. The analysis of force curves showed that reproducible unbinding events on subsequent scan lines could be measured.

Atomic force microscopy (AFM) is nowadays frequently applied to determine interaction forces between biological molecules. Starting with the detection of the first discrete unbinding forces between ligands and receptors by AFM only several years ago, measurements have become more and more quantitative. At the same time, theories have been developed to describe and understand the dynamics of the unbinding process and experimental techniques have been refined to verify this theory. In addition, the detection of molecular recognition forces has been exploited to map and image the location of binding sites. In this review we discuss the important contributions that have led to the development of this field. In addition, we emphasize the potential of chemically well-defined surface modification techniques to further improve reproducible measurements by AFM. This increased reproducibility will pave the way for a better understanding of molecular interactions in cell biology.

Single chromatin fibers were assembled directly in the flow cell of an optical tweezers setup. A single lambda phage DNA molecule, suspended between two polystyrene beads, was exposed to a Xenopus laevis egg extract, leading to chromatin assembly with concomitant apparent shortening of the DNA molecule. Assembly was force-dependent and could not take place at forces exceeding 10 pN. The assembled single chromatin fiber was subjected to stretching by controlled movement of one of the beads with the force generated in the molecule continuously monitored with the second bead trapped in the optical trap. The force displayed discrete, sudden drops upon fiber stretching, reflecting discrete opening events in fiber structure. These opening events were quantized at increments in fiber length of approximately 65 nm and are attributed to unwrapping of the DNA from around individual histone octamers. Repeated stretching and relaxing of the fiber in the absence of egg extract showed that the loss of histone octamers was irreversible. The forces measured for individual nucleosome disruptions are in the range of 20-40 pN, comparable to forces reported for RNA- and DNA-polymerases.

By using optical tweezers and a specially designed flow cell with an integrated glass micropipette, we constructed a setup similar to that of Smith et al. (Science 271:795–799, 1996) in which an individual double‐stranded DNA (dsDNA) molecule can be captured between two polystyrene beads. The first bead is immobilized by the optical tweezers and the second by the micropipette. Movement of the micropipette allows manipulation and stretching of the DNA molecule, and the force exerted on it can be monitored simultaneously with the optical tweezers. We used this setup to study elongation of dsDNA by RecA protein and YOYO‐1 dye molecules. We found that the stability of the different DNA–ligand complexes and their binding kinetics were quite different. The length of the DNA molecule was extended by 45% when RecA protein was added. Interestingly, the speed of elongation was dependent on the external force applied to the DNA molecule. In experiments in which YOYO‐1 was added, a 10–20% extension of the DNA molecule length was observed. Moreover, these experiments showed that a change in the applied external force results in a time‐dependent structural change of the DNA–YOYO‐1 complex, with a time constant of approximately 35 s (1/e2). Because the setup provides an oriented DNA molecule, we determined the orientation of the transition dipole moment of YOYO‐1 within DNA by using fluorescence polarization. The angle of the transition dipole moment with respect to the helical axis of the DNA molecule was 69° ± 3. Cytometry 36:200–208, 1999. © 1999 Wiley‐Liss, Inc.

A Michelson interferometer and an optical beam deflection configuration (both shot noise and diffraction limited) are compared for application in an atomic force microscope. The comparison shows that the optical beam deflection method and the interferometer have essentially the same sensitivity. This remarkable result is explained by indicating the physical equivalence of both methods. Furthermore, various configurations using optical beam deflection are discussed. All the setups are capable of detecting the cantilever displacements with atomic resolution in a 10 kHz bandwidth.-')To whom correspondence should be directed.

Restricted expression of activated leukocyte cell adhesion molecule (ALCAM) by hematopoietic cells suggests an important role in the immune system and hematopoiesis. To get insight into the mechanisms that control ALCAM-mediated adhesion we have investigated homotypic ALCAM-ALCAM interactions. Here, we demonstrate that the cytoskeleton regulates ALCAM-mediated cell adhesion because inhibition of actin polymerization by cytochalasin D (CytD) strongly induces homotypic ALCAM-ALCAM interactions. This induction of cell adhesion is likely due to clustering of ALCAM at the cell surface, which is observed after CytD treatment. Single-particle tracking demonstrated that the lateral mobility of ALCAM in the cell membrane is increased 30-fold after CytD treatment. In contrast, both surface distribution and adhesion of a glycosylphosphatidylinositol (GPI)-anchored ALCAM mutant are insensitive to CytD, despite the increase in lateral mobility of GPI-ALCAM upon CytD treatment. This demonstrates that clustering of ALCAM is essential for cell adhesion, whereas enhanced diffusion of ALCAM alone is not sufficient for cluster formation. In addition, upon ligand binding, both free diffusion and the freely dragged distance of wild-type ALCAM, but not of GPI-ALCAM, are reduced over time, suggesting strengthening of the cytoskeleton linkage. From these findings we conclude that activation of ALCAM-mediated adhesion is dynamically regulated through actin cytoskeleton-dependent clustering.

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