A frequently used basic polymer is polylysine, in which every amino acid unit has an amino side chain that is quite strongly basic. Its pKa is 10.53. This is the pH at which half the amino groups are protonated. The side-chain of lysine has an unusually high pKa for an amino group and consequently polylysine is positively charged even in moderately alkaline media. Polylysine is applied to slides as an aqueous solution, which is then allowed to dry by evaporation. Either D- or L-polylysine or the somewhat less expensive racemic DL- mixture may be used because the stereochemistry does not affect the ionization of the side chain. The guanidine side-chain of arginine is a strong base (pKa=12.5). Polyarginine should therefore retain its positive charge even in the most strongly alkaline conditions, but it is even more expensive than polylysine and does not seem to have been used as a section adhesive.