Adsorption kinetics and dilatational rheological studies for the soluble and anchored forms of alkaline phosphatase at the air/water interface

Abstract

Este trabalho apresenta aspectos de equilíbrio e dinâmicos da adsorção na interface ar/líquido de duas formas de fosfatase alcalina de placa óssea de ratos: DSAP, solubilizada com tensoativo não-iônico (C 12 E 9 ), contendo uma âncora de glicosilfosfatidilinositol (GPI), e PLSAP, com a porção hidrofóbica da âncora clivada por fosfolipase-C. A tensão superficial dinâmica, γ dyn , e o módulo de elasticidade superficial dilatacional, ε, foram determinados para soluções de PLSAP, DSAP e C 12 E 9 pelo método de oscilação harmônica e análise do formato da gota eixo-simétrica. Cinéticas de adsorção revelaram que DSAP adsorve trinta vezes mais rapidamente que PLSAP, apresentando um mínimo, e, para PLSAP, a tensão superficial cai continuamente. Para o sistema DSAP/C 12 E 9 , ε atinge um máximo na concentração crítica de agregação (CAC), mas para PLSAP, ε diminui continuamente com a concentração. Soluções de C 12 E 9 apresentam ε mais elevados, decrescentes com a concentração. Um modelo, baseado na influência da âncora GPI, é proposto para explicar os resultados obtidos.This work presents equilibrium and dynamic aspects for the adsorption at the air/liquid interface of two rat osseous plate alkaline phosphatase forms: DSAP, solubilized by a surfactant, C 12 E 9 , and containing a glycosylphosphatidylinositol (GPI) anchor; and PLSAP, resulting from phospholipase-C cleavage of the hydrophobic portion of the GPI anchor. Dynamic surface tension, γ dyn , and surface elasticity modulus, ε, were determined for PLSAP, DSAP and pure C 12 E 9 solutions using harmonic oscillation and axisymmetric drop shape analysis Adsorption kinetics studies revealed that DSAP adsorbs thirty times faster than PLSAP, presenting a minimum in the curve. For DSAP/ C 12 E 9 mixed system, ε increases with concentration and a maximum appears at the critical aggregation concentration (CAC). For PLSAP, a continuous decreasing with concentration for γ dyn and ε was observed. For pure C 12 E 9 solution , the elasticity modulus increases with concentration and ε values are higher when compared to the mixed system. A model based on the influence of the GPI anchor is proposed.Keywords: dynamic surface tension, adsorption kinetics, dilatational surface elasticity, alkaline phosphatase, axisymmetric drop shape analysis IntroductionSurface elasticity is associated with the ability of a system to establish a new surface tension value due to a timely area variation. Considering that it is a nonequilibrium phenomenon, changes in surface tension occur either as a consequence of a sudden compression/ expansion of the interface, or due to a local concentration variation. Since elasticity values are associated with diffusion processes, adsorption/desorption, as well as the rearrangement of molecules at the interface, they depend on the time scale (or frequency) in which the perturbation is applied. In addition, these processes are influenced by molecular weight, shape and molecular interactions of the species present at the interface.Surface elasticity is directly relate...

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