Mass spectrometry in the biosynthetic and structural investigation of lignins

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.