Mass spectrometry in the biosynthetic and structural investigation of lignins
Authors: Reale Samantha , Di Tullio Alessandra , Spreti Nicoletta , De Angelis Francesco 
Published: January 2004

I.
Introduction
88

 II.
Lignin Paradigm
88

A.  Occurrence and Formation of Lignin
88

B.  Structural Models of Lignin
89

C.  Isolation and Chemical–Physical Properties of Lignin
90

D.  In Vitro Biosynthetic Studies
92

E.  Structural Elucidation of Lignins
94

  III.
MS in the structure elucidation of degraded lignins
94

A. Thermal Degradation of Lignins Combined with MS
94

   1.  Analytical py‐EI‐MS in the Study of Lignins
95

   2.  Py of Lignin in Conjunction with Soft‐Ionization Mass Spectrometric Techniques
97

      a.  Chemical Ionization (CI)‐MS
97

      b.  Photoionization (PI)‐MS
97

      c.  Single‐Photon Ionization (SPI)‐Molecular‐Beam (MB)‐MS
97

      d.  Fast‐Atom Bombardment (FAB)‐MS
99

      e.  Resonance‐Enhanced Multiphoton Ionization (REMPI)‐MS
99

      f.  FI‐MS
100

   3.  Thermal Degradation of Lignins with Slow Heating Rates Combined with MS
101

   4.  Natural Burning of Wood and GC‐MS Analysis
102

B.  Chemical Degradation of Lignins Combined with MS
102

    1.  Nitrobenzene Oxidation and CuO Oxidation
102

    2.  Permanganate Oxidation
104

    3.  Hydrogenolysis
104

    4.  Acidolysis
105

    5.  Thioacidolysis
105

    6.  Derivatization Followed by Reductive Cleavage (DFRC)
107

    7.  Ozonolysis
108

    8.  Photochemically Induced Degradation
110

    9.  Singlet Oxygen Degradation
110

C.  Thermochemolysis with Tetramethylammonium Hydroxide (TMAH) of Lignins Combined with MS
110

D.  Mass Spectrometric Investigation of Lignin Degradation Products Deriving from Fungal and Microbial Activity
111

  IV.
Studies of DHPs by py‐MS
114

 V.
Soft‐ionization MS as a Powerful Tool in the Study of Undegraded Lignin Macromolecules
115

A.  MALDI‐MS
115

B.  Thermospray MS
117

C.  ESI‐MS
117

Acknowledgments
119

Abbreviations
119

References
120

Lignin, a resistant cell‐wall constituent of all vascular plants that consists of ether and carbon‐linked methoxyphenols, is still far from being structurally described in detail. The main problem in its structural elucidation is the difficulty of isolating lignin from other wood components without damaging lignin itself. Furthermore, the high number and variegated forms of linkages that occur between the monomeric units and the chemical resistance of certain ether bonds limit the extent to which analytical and degradation procedures can be used to elucidate the lignin structure. Most of our present knowledge about the molecular structure of lignin is based on the analysis of monomers, dimers or, at the most, tetramers of degraded isolated lignins. Mass spectrometry (MS), which offers advantages in terms of speed, specificity, and sensitivity, has revealed to be a very powerful technique in the structural elucidation of lignins, in combination with the great number of chemical and thermal degradation methods available in the study of lignin. Moreover, the recent development of new ionization techniques in MS—electrospray ionization (ESI)‐MS and matrix‐assisted laser desorption/ionization (MALDI)‐MS—has provided new possibilities to also analyze the undegraded lignin macromolecule. © 2003 Wiley Periodicals, Inc., Mass Spec Rev 23:87–126, 2004.