Lignin linkage dimer names

beta-O-4

Aryl glycerol B-aryl ether

beta-1

1,2-Diaryl propane

beta-5

Phenylcoumaran

beta-beta

pinoresinol

4-O-5

Diaryl ether

5-5

Biphenyl

alpha-O-4 WITH B-5

Phenylcoumarin

Free alpha-O-4

Noncyclic benzyl aryl ether; Only linkage is at alpha-O-4

alpha-o-4 with 8 member ring

Linkage with biphenyl (two) structures 5-5 at alpha and beta

p-Coumaryl alcohol

Monolignol with no methoxyl groups, forming p-hydroxyphenyl (H) units

Coniferyl alcohol

Monolignol with one methoxyl group, forming guaiacyl (G) units

Sinapyl alcohol

Monolignol with two methoxyl groups, forming syringyl (S) units

Phenoxy radical

Reactive intermediate formed by enzyme-mediated one-electron oxidation of monolignols

Guaiacyl (G) unit

Lignin building block with one methoxyl group, predominant in softwoods

Syringyl (S) unit

Lignin building block with two methoxyl groups, predominant in hardwood

p-Hydroxyphenyl (H) unit

Lignin building block with no methoxyl groups, common in grasses

Phenolic hydroxyl group

Aromatic -OH group contributing to lignin reactivity and cleavage behavior at Carbon-4

Methoxyl group

-OCH₃ substituent that defines lignin type (H, G, or S unit)

Carbonyl group

C=O functionality (conjugated or non-conjugated) important in lignin reactions

Coniferaldehyde

Conjugated carbonyl structure derived from coniferyl alcohol oxidation

Glyceraldehyde-β-aryl ether

Non-conjugated carbonyl structure formed via β-1 coupling

Softwood lignin

Rich in guaiacyl units, higher β-5 and 5-5 linkages

Hardwood lignin

Rich in syringyl units, higher β-O-4 linkages and fewer condensed bonds

Grasses lignin

Contains H, G, and S units with more esterified structures