The optical trap formed by a strongly focused IR (l = 1064 nm) laser can be utilized as a force transducer for micromechanical measurements. (Ghislain, Switz, and Webb, Rev. Sci. Instrum. 65: 2762-2767, 1994) In our apparatus, a quadrant photodiode serves as a sensitive monitor of the modulation of the forward scatter of the trapping laser light due to movements of a trapped particle. The calibrated optical trap serves as a force-sensing probe that we are using to investigate forces between individual ligand-receptor pairs, as well as to probe the effect of applied force on dissociation kinetics. The model system currently under investigation is the protein A-IgG interaction, a system which has been studied in bulk assays and has been shown to exhibit bond rupture forces whose magnitude depends on equilibrium dissociation constant (Kuo and Lauffenberger, Biophys. J. 65: 2191-2200, 1993). In these experiments, IgG~s from various species are covalently coupled to 1 micron latex beads with a stoichiometry giving an average of 10 molecules per bead. Protein A molecules from Staphylococcus aureus are covalently coupled to glass coverslips. A bead is brought near the coverslip by the optical trap and scanned along a line parallel to the surface. This geometry results in enhancement of the tangential applied force by a factor as high as 7 due to the leverage resulting when a bound bead rotates around the bond and is pinned to the surface prior to rupture. Thus, the range of forces which can be probed by the optical trap is expanded beyond the 45 pN escape force into the 100-200 pN range, similar to that of the atomic force microscope. Results on this system indicate that 50-70 pN are required to rupture a single protein A-IgG interaction and that the dissociation rate constant is increased logarithmically by applied force.
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