Polypeptide amphoteric/polyanionic

The insertion of the polymers into monolayers was also studied by comparing the compression isotherms. Lipid monolayers were formed on 0. IM sodium acetate buffer (pH 7.4) subphase in the presence or absence of polymers in a rectangular Teflon cuvette of 28.5 cm x 17.5 cm. After 10 min stabilisation period, a Teflon barrier compressed the monolayer at a speed of 4.2 cm/min and surface pressure vs. area (tc vs. A) isotherms were recorded. Data are summarised in Table 11. We observed no changes in the shape of area vs. pressure curves obtained in the presence of polymers in the subphase (data not shown), but polymers induced an expansion of the monolayer. These changes were detected at various surface pressures (10, 20, 30, 40 and 50 mN/m) and are expressed as area/molecule of phospholipid values (Table 2). These values indicate significant differences in the interaction of polycationic and amphoteric/polyanionic polypeptides. Marked expansion of DPPC monolayer occurred in the presence of SAK or AK (AA=0.23-0.51), while EAK and Ac-EAK initiated only moderate changes in this parameter (AA=0.01-0.13). The effect of polylysine and OAK was negligible (AA=0.01-0.03). 

Branched polypeptides developed in our laboratory with the general formula poly[Lys(Xi-DL-Ala )] (XAK), where i < I, m approx 3, and X represents the side-chain terminal residue (6-8) will be used (Fig. 1) to illustrate the methods utilized for different conjugation procedures. Depending on the nature of the amino acidX, polypeptides exhibit polycationic (e.g., poly[Lys(DL-Ala )], poly[Lys(Ser,-DL-Ala )], amphoteric (e.g., poly[Lys(Glu,-DL-Ala, or polyanionic (e.g., poly[Lys(Ac-Glu,-DL-Ala )]) character under physiological conditions (pH 7.3 in 0.15 M NaCl). 

Schematic structure of branched chain polymeric polypeptides is shown in Figure 1. Polylysine with free s-amino groups, AK or poly [Lys(SerrDL-Alam)], (SAK) containing a-amino groups and poly[Lys(Omi-DL-Ala d], (OAK) possessing both a- and e-amino groups can be considered as polycations. Side chains of poly [Lys(Glui-DL-Alam)], (EAK) contains glutamic acid at the end of the branches. Therefore this polymer has not only free a-amino, but also free y-carboxyl group in the side chain, consequently this compound has amphoteric character. Acetylation of EAK resulted in a polyanionic derivative poly [Lys(Ac-Glui-DL-Alam)], (Ac-EAK).

For liposomes of DPPC/PG (95/5 mol/mol) in Ttemperature range small increase in polarisation (AP 0.05) was observed in the presence of polylysine and OAK (Fig.6a), but in the T>Tc domain only OAK built up from double positive charged monomers induced changes. These data indicate that polylysine interacts with phospholipid membrane mainly in gel phase. In case of other polycations with single a-amino group at the end of the branches (AK, SAK) and of amphoteric (EAK) or polyanionic (Ac-EAK) polypeptides no changes in polarisation was observed. 

Polylysine had the most pronounced rigidifying effect (AP 0.05) (Fig.7b). Polycationic branched polypeptides (AK, SAK, OAK) had moderate (AP=0.01-0.04), while amphoteric (EAK) and polyanionic (Ac-EAK) derivatives had no influence on polarisation. Presence of polycationic polypeptides increased the transition temperature (ATc +0.5-1.3 C) and widened the temperature range of the phase transition, indicating that penetration of polymeric polypeptides into phospholipid bilayers resulted in increased distance of phospholipid molecules. 

The surface properties at the air/water interface and the interaction of polycationic (polylysine, AK, SAK, OAK), amphoteric (EAK) and polyanionic (Ac-EAK) polypeptides with mono- and bilayers composed of DPPC or DPPC/PG was investigated. 

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