4shows a plot of the measured root mean square deviation of the particle from the tether point. to the surface, we attach a streptavidin-coated polystyrene bead and measure force-versus-extension using an optical trap. We show that our method allows a tethered DNA molecule to be pulled through its overstretching transition (>60 pN) multiple times. We anticipate this simple yet powerful method will be useful for many researchers. Keywords:DNA, Surface attachment, Covalent bond, Single-molecule, Silane-PEG-NHS == 1. Introduction == Single-molecule experiments are becoming increasingly common because they allow investigators to study the distribution of biological events that are not apparent in an ensemble measurement. When conducting single-molecule mechanical studies, there are a variety of techniques to apply and measure force [1]. Methods such as optical and magnetic tweezers typically require binding of a molecule to a surface (e.g. a glass slide, bead, etc.) [2-5]. Irrespective of the type of attachment, the lifetime of the attachment bond is strongly dependent on the applied force [6]. Many Neridronate measurements require large forces to probe the processes of interest including greater than 13pN for hairpins [7], >20 pN for polymerases [8] and for nucleosome unwinding [9]. Forces of this magnitude can translate into very short lifetimes for weaker attachment bonds [6], limiting the amount of data that can be collected from a single tether. Therefore, a surface attachment strategy that creates a strong bond [10] and which allows many individual tethers to be prepared simultaneously [11] will allow for more data to be taken during one experiment. One common way to attach DNA to a glass surface is through the non-covalent interaction of digoxigenin (dig) modified DNA with an anti-digoxigenin antibody (anti-dig) [2,12]. DNA labeled with dig at one end is bound to an anti-dig antibody that has Neridronate been non-specifically adsorbed to a glass surface. This method is straightforward and binds many individual tethers to the surface, permitting multiple different measurements to be taken simultaneously [11]. The disadvantage of this method is that the dig/anti-dig interaction is not strong [13]. Once the bond is ruptured, a new tether must be found or a new sample made. Therefore, the dig/anti-dig method, though convenient, is not ideal for experiments involving higher forces. InTable 1,we compare the lifetimes of dig/anti-dig and biotin/streptavidin bonds under stress using Bells formula [14] (Eq. (1)). == Table 1. == The average lifetimes of the biotin/streptavidin bond or the dig/anti-dig bonds calculated using Bells Formula (Eq. (1)). The values used arek0= 1.67 105s1andXB= 0.49 nm for biotin/streptavidin [15] andk0= 0.015 s1andXB= 1.15 nm for dig/anti-dig [13]. Bells formula estimates the dissociation rate at forceF, k(F), from the dissociation rate at zero force,k0, the distance from the energy minimum to the rupture barrier,XB, and thermal energy,kBT. Therefore, the average bond lifetime is 1/k(F). The values fork0andXBhave been estimated from dynamic force spectroscopy for both the biotin/streptavidin and dig/anti-dig bonds [13,15]. From these estimations, Neridronate it is clear that using dig/anti-dig is far from optimal for experiments requiring greater than 20 pN of force. Therefore an alternative DNA attachment strategy is to label each end of a single DNA molecule with a single molecule of biotin CD38 [9,16]. Because the biotinstreptavidin bond is much stronger than the dig/anti-dig interaction, having a biotin bound on each end allows the DNA to be subjected to higher forces with reasonable lifetimes [15,17]. However, a significant disadvantage of this technique is that, because the DNA has the same label on both ends, the DNA may wrap around so that both ends bind to the surface or bead. A technique to bind DNA between two streptavidin-coated beads is sometimes used, in which fluid flow keeps the DNA from being able to wrap around [18], however, this approach is time consuming, and only one DNA molecule may be attached at a time. Here we present a new method of attaching DNA to a glass surface in which DNA is covalently attached to a PEG-coated glass surface via reaction of a unique terminal primary amine group on the DNA with an N-hydroxysuccinimide (NHS) group on the PEG.Fig. 1outlines the chemistry of these reactions. This attachment strategy allows for specific binding of DNA to the surface via its amine labeled end while the other DNA end is labeled independently, permitting directionality in attachment. In the realization we present, the non-amine DNA end is labeled with biotin. The advantages of.