MASS SPECTROMETRIC DETECTION OF INDOPHENOLS FROM THE GIBBS REACTION FOR PHENOLS ANALYSIS
Phenols are ubiquitous in our surroundings including biological molecules such as L-Dopa metabolites, food components, such as whiskey and liquid smoke, etc. This dissertation describes a new method for detecting phenols, by reaction with Gibbs reagent to form indophenols, followed by mass spectrometric detection. Unlike the standard Gibbs reaction which uses a colorimetric approach, the use of mass spectrometry allows for simultaneous detection of differently substituted phenols. The procedure is demonstrated to work for a large variety of phenols without para‐substitution. With para‐substituted phenols, Gibbs products are still often observed, but the specific product depends on the substituent. For para groups with high electronegativity, such as methoxy or halogens, the reaction proceeds by displacement of the substituent. For groups with lower electronegativity, such as amino or alkyl groups, Gibbs products are observed that retain the substituent, indicating that the reaction occurs at the ortho or meta position. In mixtures of phenols, the relative intensities of the Gibbs products are proportional to the relative concentrations, and concentrations as low as 1 μmol/L can be detected. The method is applied to the qualitative analysis of commercial liquid smoke, and it is found that hickory and mesquite flavors have significantly different phenolic composition.
In the course of this study, we used this technique to quantify major phenol derivatives in commercial products such as liquid smoke (catechol, guaiacol and syringol) and whiskey (o-cresol, guaiacol and syringol) as the phenol derivatives are a significant part of the aroma of foodstuffs and alcoholic beverages. For instance, phenolic compounds are partly responsible for the taste, aroma and the smokiness in Liquid Smokes and Scotch whiskies.
In the analysis of Liquid Smokes, we have carried out an analysis of phenols in commercial liquid smoke by using the reaction with Gibbs reagent followed by analysis using electrospray ionization mass spectrometry (ESI-MS). This analysis technique allows us to avoid any separation and/or solvent extraction steps before MS analysis. With this analysis, we are able to determine and compare the phenolic compositions of hickory, mesquite, pecan and apple wood flavors of liquid smoke.
In the analysis of phenols in whiskey, we describe the detection of the Gibbs products from the phenols in four different commercial Scotch whiskies by using simple ESI-MS. In addition, by addition of an internal standard, 5,6,7,8-tetrahydro-1-napthol (THN), concentrations of the major phenols in the whiskies are readily obtained. With this analysis we are able to determine and compare the composition of phenols in them and their contribution in the taste, smokey, and aroma to the whiskies.
Another important class of phenols are found in biological samples, such as L-Dopa and its metabolites, which are neurotransmitters and play important roles in living systems. In this work, we describe the detection of Gibbs products formed from these neurotransmitters after reaction with Gibbs reagent and analysis by using simple ESI‐MS. This technique would be an alternative method for the detection and simultaneous quantification of these neurotransmitters.
Finally, in the course of this work, we found that the positive Gibbs tests are obtained for a wide range of para-substituted phenols, and that, in most cases, substitution occurs by displacement of the para-substituent. In addition, there is generally an additional unique second-phenol-addition product, which conveniently can be used from an analytical perspective to distinguish para-substituted phenols from the unsubstituted versions. In addition to using the methodology for phenol analysis, we are examining the mechanism of indophenol formation, particularly with the para-substituted phenols.
The importance of peptides to the scientific world is enormous and, therefore, their structures, properties, and reactivity are exceptionally well-characterized by mass spectrometry and electrospray ionization. In the dipeptide work, we have used mass spectrometry to examine the dissociation of dipeptides of phenylalanine (Phe), containing sulfonated tag as a charge carrier (Phe*), proline (Pro) to investigate their gas phase dissociation. The presence of sulfonated tag (SO3-) on the Phe amino acid serves as the charge carrier such that the dipeptide backbone has a canonical structure and is not protonated. Phe-Pro dipeptide and their derivatives were synthesized and analyzed by LCQ-Deca mass spectroscopy to get the fragmentation mechanism. To confirm that fragmentation path, we also synthesized dikitopeparazines and oxazolines from all combinations of the dipeptides. All these analyses were confirmed by isotopic labeling experiments and determination and optimization of structures were carried out using theoretical calculation. We have found that the fragmentation of Phe*Pro and ProPhe* dipeptides form sequence specific b2 ions. In addition, not only is the ‘mobile proton’ involved in the dissociation process, but also is the ‘backbone hydrogen’ is involved in forming b2 ions.