Effective Long-Range Attraction between Protein Molecules in Solutions Studied by Small Angle Neutron Scattering

Abstract

Small angle neutron scattering intensity distributions taken from cytochrome C and lysozyme protein solutions show a rising intensity at very small wave vector, Q, which can be interpreted in terms of the presence of a weak long-range attraction between protein molecules. This interaction has a range several times that of the diameter of the protein molecule, much greater than the range of the screened electrostatic repulsion. We show evidence that this long-range attraction is closely related to the type of anion present and ion concentration in the solution.PACS numbers: 87.14. Ee,61.12.Ex, 82.35.Rs The bottleneck of protein crystallography is the lack of systematic methods to obtain protein crystals. This is partly due to incomplete understanding of the physical chemistry conditions controlling the growth of protein crystals. A full comprehension of the effective protein interactions and phase behavior is therefore essential. It has been shown that the crystallization curves of some globular proteins appear to coincide with the phase diagrams of a hard sphere system interacting with a short range attraction [1,2,3]. Small angle neutron and X-ray scattering investigations of proteins suggest the presence of a short-range attractive interaction between protein molecules besides the electrostatic repulsion induced by the residual charges [4,5,6]. The DLVO potential has been successfully applied to many colloidal systems and protein solutions [3,4]. However, it does not seem to fully explain the rich behaviour of proteins [4,7,8,9], and due to the complexity of these systems (anisotropic property, irregular shape, distributed charge patches, etc.), a complete understanding of the properties of the effective interactions between protein molecules in solutions remains a challenge [8].Recent measurements of small angle neutron scattering (SANS) intensity distribution in protein solutions show interesting results [5,6,10]. Beside the normal first diffraction peak, it is present a peak (cluster peak) appearing at a much smaller scattering wave vector, Q, due to the formation of ordered clusters. The appearance of a cluster peak is explained as due to the competition of a short-range attraction and a long-range electrostatic repulsion [5,11,12]. Moreover, a rising intensity as Q approaches zero (zero-Q peak) is observed in both liquid-like and solid-like samples, which implies that the effective potential should have more features in addition to the well known short-range attraction and electrostatic repulsion. The existence of a long-range attraction between protein molecules has already been

Absence of equilibrium cluster phase in concentrated lysozyme solutions

“…1 display an intensity upturn at low q region pointing to the presence of large aggregates, as also been observed in earlier SANS studies (2,23). In agreement with ref.…”

“…The upturn near q ϭ 0 was subsequently shown to be caused by impurities in the sample (22), and this issue was further discussed in ref. 23. Indications of equilibrium clusters were also recently reported in insulin solutions, even at low protein concentrations (24).…”

“…This result was interpreted as aggregation of lysozyme molecules into small clusters with the aggregation number (up to 10) increasing with concentration such that the peak revealed the distance between the clusters, and not individual molecules. This publication prompted a series of related studies yielding interesting albeit sometimes conflicting results (2,16,22,23). SANS investigations of cytochrome c and lysozyme solutions suggested not only cluster peak but also indicated an intensity upturn near q ϭ 0 attributed to weak long-range attraction (2).…”

“…SANS investigations of cytochrome c and lysozyme solutions suggested not only cluster peak but also indicated an intensity upturn near q ϭ 0 attributed to weak long-range attraction (2). The corresponding static structure factor was modeled by an effective potential involving two Yukawa terms pertaining to intermediate range repulsion and a long-range weak attraction (2,16). The upturn near q ϭ 0 was subsequently shown to be caused by impurities in the sample (22), and this issue was further discussed in ref.…”

“…T he arrested dynamics of colloidal systems and protein solutions interacting via short-range interactions have been actively studied both theoretically and experimentally in recent years (1)(2)(3)(4). The mode coupling theory and molecular dynamics (MD) simulations have successfully unified seemingly dissimilar dynamical arrest scenarios in colloidal systems (4,5).…”