Molecular recognition on the supported and on the air/water interface-spread protein monolayers.

Targeting of proteins at interfaces via affinity ligands or specific antibodies is important for the understanding of protein functioning in biological membranes. This review brings together a great number of research works accomplished in this field in the past decade by a variety of analytical methods. It highlights two simple in situ techniques of monitoring molecular recognition processes at interfaces recently developed in the author's laboratory. The first of these techniques is based on the measurements of surface pressure increments of a protein monolayer spread at the air/water interface at a constant area resulting from the interaction with its specific ligands injected into the aqueous subphase beneath the preformed protein monolayer. The second technique takes advantage of the feature of [(14)C]-labeled proteins that enable in situ measurements of surface density changes of adsorbed protein molecules on a solid support resulting from the interaction with its specific antibody.

Mixed monolayers of Bauhinia monandra and concanavalin A lectins with phospholipids, part II.

Isotherms of surface pressure and surface potential versus mean molecular area for dibehenoylphosphatidylcholine (DBPC), dipalmitoylphosphatidylcholine (DPPC), dipalmitoylphosphatidylethanolamine (DPPE), and dioleoylphosphatidylcholine (DOPC) monolayers were shown to be greatly modified when these lipids were cospread with either Bauhinia monandra (BmoLL) or Concanavalin A (Con A) lectins. For the binary films of DBPC, DPPC, and DPPE cospread with each of these two lectins, there was both a displacement of the Pi-A and DeltaV-A isotherms toward higher molecular areas relative to pure lipids and an increase in the maximum surface potential values relative to the DeltaV-A relationships observed for the corresponding single-lectin systems. Both effects can be understood in terms of the occurrence of an explicit interaction between the lipids and the lectins. The plots of the corresponding compressibilities versus molecular areas reveal that, for all lipids but DOPC, the extent of this interaction was always larger for BmoLL than for Con A. The DPPC and DPPE mixed films with BmoLL differed in compressibility. Owing to the small DPPE polar headgroup, the DPPE-BmoLL film was much more incompressible than the DPPC-BmoLL mixed monolayer. However, for the DOPC-BmoLL and DOPC-Con A mixed films there was no evidence that an interaction between the lectins and the lipid took place, a fact attributed to the unsaturated character in the DOPC aliphatic chains, which leads to an expanded Pi-A isotherm.

Dielectric properties of Bauhinia monandra and concanavalin A lectin monolayers, part I.

The dielectric properties of the galactose-binding lectins Bauhinia monandra (BmoLL) and Concanavalin A (Con A) were assessed by surface potential measurements of their spread monolayers on an aqueous subphase containing a monovalent electrolyte. For both lectins the curves of surface potential versus mean molecular area (DeltaV-A) and the independently recorded isotherms of surface pressure versus mean molecular area (Pi-A) were shown to be pH-dependent. As the subphase pH changed from 2 to 9, a noticeable trend to higher surface pressures accompanied the compression of the monolayers. Conversely, the surface potentials values of both monolayers decreased with increasing pH. For Con A, with the single exception of the pH 9 case, lowering the pH yielded DeltaV values higher than those for BmoLL. The contribution of the electric double layer (Psi0) to the overall DeltaV values at a given Pi (15 mN/m) was calculated using a modified Davies equation and assuming that at this surface pressure the monolayers of both studied lectins were stable. While at all studied pHs the Psi0 values for Con A exceeded those calculated for BmoLL, for both lectins they were insensitive to pH changes. This provided evidence that the reorientation of lectin molecules, during compression predominantly contributed to the alteration of the overall DeltaV values. The calculated Psi0 values made possible the evaluation of the dipole moments for BmoLL and Con A, and it has been estimated that the decrease in the pH of the subphase from 9 to 2 produced a 1.6-fold (twofold) increase in the value of for BmoLL (Con A). The differences in dielectric properties between the two film-forming lectins have been attributed to the differences in their structures. Indeed, the circular dichroism (CD) spectrum of Con A showed the predominance of beta-plated sheet structures while that of BmoLL was typically rich in alpha-helix structures.

Specific interaction of lectins with liposomes and monolayers bearing neoglycolipids.

The interaction of three lectins (wheat germ, Ulex europaeus I, and Lotus tetragonolobus agglutinins: WGA, UEA-I and LTA) with either N-acetyl-D-glucosamine or L-fucose neoglycolipids incorporated into phospholipid monolayers and liposome bilayers was studied at the air/water interface and in bulk solution. The results show that for both systems studied, synthesized neoglycolipids were capable of binding their specific lectin and that, in general, the binding of lectins increased with the increase in the molar fraction of the saccharide derivative incorporated in either the monolayers or bilayers. However, whereas for UEA-I, molecular recognition was enhanced by a strong hydrophobic interaction, for WGA and LTA successful recognition was predominantly related to the distance between neighboring sugar groups. The observed lengthy adsorption times of these lectins onto their specific ligands were attributed to interfacial conformational changes occurring in the proteins upon their adsorption at the interfaces.