Studies of the helix–coil transition of poly‐ L ‐lysine in film and solution

Water‐insoluble films of poly‐ L ‐lysine, crosslinked with formaldehyde, were suspended in aqueous media and their relative lengths measured as a function of pH. A sharp transition of the polymer was observed in the pH range which corresponded with that observed in polylysine solutions by optical rotation or dilatometry. In NaBr and NaCl solutions the coiled form of the polylysine film shrinks with increasing salt concentration, but in NaHCO 3 solution the extent of the contraction is larger, and the coil–helix transition of polylysine occurs at lower pH when NaHCO 3 is added to the medium. If one assumes the formation of amino carbamate in this case, this phenomenon can be well explained. Urea does break up the hydrogen bonds in helical polylysine film, but not completely. This result is interesting compared with that obtained for poly( L ‐glutamic acid). After the coil–helix transition region was found by film experiments, the volume change associated with the coil‐to‐helix transition was measured and found to be about 1–l.5 ml. per amino residue after taking electrostatic interaction into consideration. This value is nearly same as that obtained for poly( L ‐glutamic acid). By contrast, the value for poly‐γ‐benzyl‐ L ‐glutamate was reported to be −0.077 ml./mole of repeating unit. So it is still necessary to determine the magnitude and direction of the volume change for various kinds of polypeptides.