Conformational properties and adsorbed layer structure of adsorbed Poly-L-Lysine and Poly-DL-Lysine as inferred from adsorption measurements and proton titrations

Abstract

Adsorption properties of mainly poly-L-lysine (PL-L) and poly-DL-lysine (PL-DL) and proton titrations of free and adsorbed PL-L and PL-DL are discussed. Polystyrene latex was the adsorbent used. The PL results are compared with the results of some titration experiments performed with other weak polyelectrolytes adsorbed on polystyrene latex: partially esterified sodium polymethacrylate (PMA-pe) and poly-L-glutamic acid (PGA). The adsorbed amounts of PL-L and PL-DL increase strongly with decreasing chain charge density. The secondary chain structure of PL-L does not affect the adsorbed amount, probably because electrostatic effects are still dominating also at low charge densities on the PL-L molecule. A result that appears to be general for weak polyelectrolytes with secondary structure in the bulk is that at a low and constant adsorbed amount no conformational transition occurs in the case of charge contrast (different sign) between adsorbate and adsorbent. At high adsorbed amounts, when appreciable numbers of loops and tails are present, conformational transitions are possible in an adsorbed polyelectrolyte layer, and hence adsorbed PL-L is partly helical. In the first adsorption layer the degree of dissociation of the PL ammonium groups is significantly lower than in the bulk. This deviation is largely determined by the presence of ion-pairs at the surface. The formation of a rather high fraction of ion-pairs (OSO − 3···NH − 3) and a hydrophobic interaction between styrene units and the  (CH 2) 4 − side groups of lysine cause unfolding of PL-L helices upon adsorption at low surface coverages.