Osmotic pressure contribution of albumin to colloidal interactions

Abstract

Two surfaces that come in close contact in a solution with macromolecules present experience an attractive force caused by the osmotic pressure. We present a measurement of this effect by using a micrometer-sized sphere bound to a f lat plate through a single molecular attachment in an albumin-containing solution. We obtain the osmotic part of the interaction potential with a resolution of <1 nm and a fraction of kT room . This attractive interaction is seen to have a range comparable to the size of the albumin molecule. The results are broadly in agreement with a geometric model first proposed by Asakura and Oosawa.

Single-molecule detection of DNA hybridization

“…Previously we have shown (1,25) that it is possible to attach a micrometer-size bead to a microscope slide through a singlenanometer-size molecular contact (e.g., a globular protein), while the bead is effectively separated from the surface by the potential barrier. Furthermore, from the thermal motion of the bead, one can reconstruct the potential and thus the forces on the bead and the molecular contact.…”

“…1c). The intensity of light scattered by a single bead provides a measurement of the displacement of the bead relative to the slide with subnanometer resolution (1,25). The absolute bead-slide separation is obtained by measuring the contact intensity, I c .…”

“…Using part of the time series before the hybridization event (t Ͻ 36 in Fig. 3a), we calculate the interaction potential ss : because the bead moves in a field of force, the probability of finding it at position h is P(h) ϰ exp(Ϫ(h)͞kT); P(h) is obtained from a histogram of the time series (1,25). ss reflects the interaction potential when the bead is held by the ss tether; using part of the time series after the hybridization event, we can also obtain ds, the potential for the bead tethered by the ds tether.…”

“…Thus measuring the scattered intensity yields a measurement of the distance between the bead and the interface: I ϭ I c exp(Ϫh͞␦), where I is the scattered intensity, I c the intensity at contact, h the separation between the bead and the slide, and ␦ the penetration depth of the evanescent wave (␦ ϭ 86 nm in our setup). Therefore a displacement of the bead can be measured as: h 2 -h 1 ϭ ⌬h ϭ ␦ ln (I 2 ͞I 1 ) (1,(23)(24)(25). This relation has been shown experimentally to remain valid down to contact (24).…”

The Vegetative Vacuole Proteome ofArabidopsis thalianaReveals Predicted and Unexpected Proteins[W]

“…The reason that this protein accumulates to such high levels in the vacuoles of mature Arabidopsis leaves is unknown. However, proteins that accumulate to such high levels in other systems play roles in buffering changes in osmotic pressure, such as serum albumin in animals (Singh-Zocchi et al, 1999). Perhaps this MAP plays a role in controlling drastic osmotic fluctuations within the vacuole.…”