Are lignins optically active?

The accepted derivation of lignins from non(enzymatically)-controlled radic al coupling reactions has been recently challenged, and it is relevant to a scertain unequivocally whether lignins are or are not (as normally assumed) optically active. Two approaches were used. First, DFRC (derivatization fo llowed by reductive cleavage) dimers derived from beta-5- and beta-beta-uni ts in pine lignins, which certainly retain unaltered chiral centers (as wel l as beta-1- and beta-O-4-units where the intactness may be debated), were shown to be optically inactive by circular dichroism (CD) and chiral highpe rformance liquid chromatography. CD of beta-5-derived dimers following enan tiomeric separation readily demonstrated the sensitivity of the method. Sec ond, no optical activity could be detected (above 250 nm to avoid carbohydr ate contributions) by CD of lignin isolates from pine, kenaf, maize, or a C AD-deficient pine mutant. Representative lignins are therefore not, within limits of detection by these methods, optically active.