Protein Exposed Hydrophobicity Reduces the Kinetic Barrier for Adsorption of Ovalbumin to the Air−Water Interface

Abstract

Using native and caprylated ovalbumin, the role of exposed hydrophobicity on the kinetics of protein adsorption to the air-water interface is studied. First, changes in the chemical properties of the protein upon caprylation were characterized followed by measurement of the changes in adsorption kinetics. No change in the molecular structure of ovalbumin was observed upon caprylation. However, aggregation of the protein was observed when more than three capryl chains were coupled per protein. A batch of caprylated ovalbumin with an average coupling of four capryl chains per protein was separated into a monomeric and an aggregated protein fraction. The exposed hydrophobicity of the monomeric and the aggregated species was measured using 8-anilino-1-naphthalenesulfonic acid fluorescence. The exposed hydrophobicity of the monomeric fraction was significantly higher than that of the nonmodified protein. The changes in adsorption kinetics were studied by measuring the increase in surface load (Γ) and in surface pressure (Π) as a function of time (t) using an ellipsometer and a Wilhelmy plate, respectively. It was found that the increase of surface load in time (even at low surface coverage) is much lower than the value that was calculated from diffusional transport. This shows that the adsorption of native ovalbumin is barrier limited. The adsorption kinetics of the caprylated protein follow the calculations from diffusional transport more closely, which shows that the energy barrier for adsorption of caprylated ovalbumin is much lower than for the native protein. The surface pressure at a certain surface load (Π-Γ) was not affected by the modification, indicating that the effect of increased hydrophobicity is limited to the adsorption process.

Application Potential of Food Protein Modification

“…This effect is further amplified by the finding that local protein concentrations at the air-water interface can reach up to 150 to 300 mg/ml (Meinders et al, 2001). The rate of absorption to the air-water interface has been reported to depend largely on the hydrophobic nature of the protein under investigation (Kudryashova et al, 2003;Wierenga et al, 2003). Increasing the exposed hydrophobicity of proteins by means of conjugation with lipid chains was shown to increase the adsorption rate to the air-water interface .…”

“…Increased exposed hydrophobicity of proteins has for example been related to an improved capacity to form and stabilize emulsions and foams which is the result of improved potential to interact with hydrophobic surfaces, both the air-water and oil-water interface, and including (model)membranes (Nakai, 1983;Wierenga et al, 2003;reviewed in Wilde, 2000;Wilde et al, 2004). Various saturated and unsaturated fatty acids have been employed to induce lipophilization of proteins including caproic acid (Liu et al, 2000), capric acid (Aewsiri et al, 2010;Liu et al, 2000), lauric acid (Aewsiri et al, 2010), myristic acid (Aewsiri et al, 2010;Ibrahim et al, 1993;Liu et al, 2000), palmitic acid (Haque et al, 1982;Haque & Kito, 1983a, 1983bIbrahim et al, 1991), stearic acid (Djagny et al, 2001;Ibrahim et al, 1993), and oxidized forms of linoleic acid (Aewsiri et al, 2011a(Aewsiri et al, , 2011b, and the efficiency of the lipophilization reaction was found to be inversely proportional to the length of the lipid chains used (Liu et al, 2000).…”

“…The OPA assay was further developed to evaluate the degree of proteolysis of dairy proteins by determining the number of -amino groups released upon hydrolysis (Church et al, 1983). The assay can also be used to quantify all types of reactions involving the modification of lysine and, hence, includes lipidation of ovalbumin Wierenga et al, 2003), lysozyme (Liu et al, 2000), and gelatin (Aewsiri et al, 2010(Aewsiri et al, , 2011a(Aewsiri et al, , 2011b. Other modifications which can be quantified using the OPA assay are succinylation Wierenga et al, 2005), methylation , glycosylation (Broersen et al, 2004;, and thiolation of proteins (Broersen et al, 2006).…”

Sugar beet leaves: from biorefinery to techno-functionality

“…For example, for kale it was shown that the protein yield (g/m 2 ) increased from 11.5 to 26.2% when a combination of nitrogen-phosphorus-potassium (NPK) fertilizer was used (4,36). At the same time the ratio of protein nitrogen to total nitrogen in the extracted juice increased from 64.6 to 67.7%.…”

“…At the same time the ratio of protein nitrogen to total nitrogen in the extracted juice increased from 64.6 to 67.7%. Critical parameters regarding fertilization are the type and amount of fertilizer and the application time (36). Weather conditions during plant growth also play a key role in the amount of extractable leaf proteins.…”