On the Helix-Coil transition in grafted chains

– The helix-coil transition is modified by grafting to a surface. This modification is studied for short peptides capable of forming α-helices. Three factors are involved: (i) the grafting can induced change of the boundary free energy of the helical domain (ii) the van der Waals attraction between the helices and (iii) the crowding induced stretching of the coils. As a result the helix-coil transition acquires " all or nothing " characteristics. In addition the transition temperature is elevated and the transition itself sharpens as the grafting density increases. During the past two decades the physics of polymer brushes formed by terminally anchored chains were studied extensively[1, 2, 3]. Most of the research effort dealt with brushes of flexible, synthetic polymers, devoid of internal degrees of freedom. In contrast, this letter concerns brushes formed by biopolymers capable of undergoing a cooperative helix-coil transition. It is motivated by two experimental observations. First, the promotion of the adhesion and spreading of cells by brushes of collagen model peptides[4]. Second, membrane fusion induced by model fusogenic peptides grafted to vesicles[5]. In both cases, the function of the short peptide chains was correlated with a helical state. Furthermore, the helical state was favored by the grafting. With this in mind we present a highly simplified theoretical model for the helix-coil transition in brushes of short, laterally immobile peptides. In particular, we discuss the transition temperature T t and the width of the transition ∆T as a function of grafting density, Σ −1. We focus on the simplest situation, of short neutral homopeptides forming a single stranded α-helix. As we shall see, the grafting of the chains can lead to qualitative modifications of the helix-coil transition: (i) in marked distinction to the case of a free peptide, the helix-coil transition in an isolated grafted peptide can acquire " all or nothing " characteristics; (ii) T t is elevated with the grafting density while ∆T decreases; (iii) eventually, for high grafting density the transition may take place as a first-order phase transition. These distinctive features arise because of three factors: The lower configurational entropy of the