We have measured the kinetics of adsorption of small (12.5-nm radius) unilamellar vesicles onto SiO2, oxidized gold, and a self-assembled monolayer of methyl-terminated thiols, using a quartz crystal microbalance (QCM). Simultaneous measurements of the shift in resonant frequency and the change in energy dissipation as a function of time provide a simple way of characterizing the adsorption process. The measured parameters correspond, respectively, to adsorbed mass and to the mechanical properties of the adsorbed layer as it is formed. The adsorption kinetics are surface specific; different surfaces cause monolayer, bilayer, and intact vesicle adsorption. The formation of a lipid bilayer on SiO2 is a two-phase process in which adsorption of a layer of intact vesicles precedes the formation of the bilayer. This is, to our knowledge, the first direct evidence of intact vesicles as a precursor to bilayer formation on a planar substrate. On an oxidized gold surface, the vesicles adsorb intact. The intact adsorption of such small vesicles has not previously been demonstrated. Based on these results, we discuss the capacity of QCM measurements to provide information about the kinetics of formation and the properties of adsorbed layers.

No abstract

Using a combination of the quartz crystal microbalance and surface plasmon resonance techniques, we have studied the spontaneous formation of supported lipid bilayers from small (approximately 25 nm) unilamellar vesicles. Together these experimental methods measure the amount of lipid adsorbed on the surface and the amount of water trapped by the lipid. With this approach, we have, for the first time, been able to observe in detail the progression from the adsorption of intact vesicles to rupture and bilayer formation. Monte Carlo simulations reproduce the data.

Experiments have shown that the folding rate constants of two dozen structurally unrelated, small, single-domain proteins can be expressed in terms of one quantity (the contact order) that depends exclusively on the topology of the folded state. Such dependence is unique in chemical kinetics. Here we investigate its physical origin and derive the approximate formula ln(k) ‫؍‬ ln(N) ؉ a ؉ bN, were N is the number of contacts in the folded state, and a and b are constants whose physical meaning is understood. This formula fits well the experimentally determined folding rate constants of the 24 proteins, with single values for a and b. Proteins spontaneously fold to a unique structure, more rapidly than would be possible were folding an exhaustive, random search of conformational space (1). This observation has led to numerous attempts to explain how naturally occurring proteins reach their native structures within a biologically relevant time-scale (reviewed in refs. 2-6).Small, single-domain proteins often fold according to the first-order rate lawHere, N (t) is the number of unfolded proteins at time t, and k eff is an effective rate constant. The folding rates of these simple proteins satisfy the empirical relationship (7) ln͓k eff ͔ ϭ a ϩ bO. [2]Here, O is the contact order, a quantity calculated from the structure of the folded protein according to a well-defined recipe (7). The constants a and b have the same values for all the proteins in the data set. Their values were determined by fitting the experimental data. No physical significance was assigned to them. Since the publication of ref. 7, the folding rates of three dozen single-domain proteins have been measured (8, 9), all of which exhibit first-order kinetics with effective rate constants that satisfy Eq. 2. This equation is rather intriguing. We know of no other example in chemical kinetics where the rate constants for dozens of distinct chemical compounds, involved in the same type of unimolecular reaction, depend only on the molecular structure of the products. This regularity is made more striking by the fact that the proteins in the data pool were selected on the basis of their structural dissimilarity (8). Despite the interest generated by this empirical result (10, 11), however, there has been only limited success in understanding its physical basis (12)(13)(14)(15)(16).In this article, we present a kinetic model that leads to an expression for the rate constant that is similar in spirit to Eq. 2. In our analysis, we found it necessary to replace contact order with a different topological parameter: the number of native contacts (N). We say that two residues in the folded protein are in contact if the straight-line distance between their C ␣ atoms is less than d, and if there are more than C residues between them along the chain. Two residues separated by a distance (along the chain) smaller than the persistence length are not counted as being in contact. This excludes, for example, contacts occurring in ␣-helixes. Such contacts are not exclude...

Employing the Monte Carlo technique, we study the kinetics of vesicle adsorption at a solid–liquid interface. The proposed model combines a treatment of the surface kinetics, including spontaneous, adsorption- and lipid-membrane-induced decomposition of adsorbed vesicles, and limitations of the adsorption rate by vesicle diffusion in the solution. With this model, we demonstrate different kinetic cases, corresponding to various scenarios of vesicle adsorption and decomposition. The general results are employed to simulate recent experimental kinetic data for adsorption of small phospholipid vesicles at a SiO2 surface.