Are Lignins Optically Active?

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Abstract
The accepted derivation of lignins from non(enzymatically)-controlled radical coupling reactions has been recently challenged, and it is relevant to ascertain unequivocally whether lignins are or are not (as normally assumed) optically active. Two approaches were used. First, DFRC (derivatization followed by reductive cleavage) dimers derived from β-5- and β-β-units in pine lignins, which certainly retain unaltered chiral centers (as well as β-1- and β-O-4-units where the intactness may be debated), were shown to be optically inactive by circular dichroism (CD) and chiral high-performance liquid chromatography. CD of β-5-derived dimers following enantiomeric separation readily demonstrated the sensitivity of the method. Second, no optical activity could be detected (above 250 nm to avoid carbohydrate contributions) by CD of lignin isolates from pine, kenaf, maize, or a CAD-deficient pine mutant. Representative lignins are therefore not, within limits of detection by these methods, optically active. Keywords: Lignin; lignin dimer; DFRC; pine, kenaf; maize; CAD-deficient pine mutant; circular dichroism; optical activity; chirality; racemic