Spectra of Carbohydrate Derivatives

Mass Spectra of Carbohydrate Derivatives 1. General Remarks 

oligo-, and (certainly) poly-saccharides being thermally unstable and practically nonvolatile, mass-spectral studies on them have been performed on their more volatile derivatives, such as, methyl ethers, acetates, and alkylidene derivatives. 

Alkylidene derivatives and anhydro sugars will be discussed next, as the presence of additional rings bears strongly on the fragmentation of pyranoid and furanoid rings. 

Finally, the mass spectra of acyclic carbohydrate derivatives will be considered in many aspects, these differ essentially from those of the cyclic forms. 

In accordance with the proposal of Budzikiewicz, Djerassi, and Williams,3 we shall denote the shift of a pair of electrons as and of a single electron 

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The widespread occurrence of long-range couplings in both furanose and pyranose derivatives explains why so many of the P.M.R. spectra of carbohydrate derivatives are apparently poorly resolved, even when the resolution of the spectrometer is above reproach. For example, the Hi resonance of the 1,6-anhydro-D-glucose derivative (12) is coupled to all of the other six ring protons. A further example of the line-broadening effect follows a consideration of the spectrum of 5,6-dideoxy-5,6-epithio-l,2-0-isopropylidene-/ -L-idofuranose for which the half-height... 

It is evident from the above discussion that more-detailed studies of the P.M.R. spectra of carbohydrate derivatives will reveal an ever-increasing number of long-range couplings. Studies in this laboratory are directed towards the investigation of the occurrence of these couplings in the spectra of both furanose and pyranose derivatives. 

Unfortunately, owing to the extreme instability of the carbohydrate molecule, the molecular ion can only occasionally be traced in the mass spectra of carbohydrate derivatives. 

Experience with the p.m.r. spectra of carbohydrate derivatives has shown that most protons (or groups of protons) in similar chemical... 

It is particularly noteworthy that although the acetamido signal is at highest field in all three solvents, the axial 1-acetoxy group is not the lowest field signal when benzene is the solvent. This finding emphasizes the difficulties likely to arise if the spectra of carbohydrate derivatives that do not contain aromatic fimctions are compared. 

A number of reviews of mass spectra of carbohydrates have been published from which references to the original papers are available (4, 9, 11, 24, 26). The application of mass spectrometry to this field was initially limited by the relatively low volatility of free carbohydrates and by the complex spectra obtained from some derivatives. These limitations have been partially overcome by new inlet techniques and by pioneering studies on classes and derivatives in order to understand the characteristic fragmentations and rearrangements of the molecular ions of a wide range of carbohydrates. 

O-Isopropylidene derivatives of carbohydrates form structural isomers from carbohydrates which themselves are epimers. Since structural isomers often fragment differently whereas epimers do not, mass spectra of these derivatives may permit interpretation in terms of stereochemistry. Although molecular-ion peaks are not observed, the molecular weight can be determined readily from a relatively intense M-CH/ peak, resulting from loss of a methyl radical from a 1, 3-dioxolane ring (12). 

The value of H n.m.r. spectroscopy in determining the structures of carbohydrates is well recognized. In this Section, some of the important features observed in the 100-MHz, H n.m.r. spectra of sucrose derivatives will be discussed, and the potential of I3C nuclear magnetic resonance spectroscopy will be very briefly indicated. Horton and his colleagues162 discussed the high resolution, 4H n.m.r. spectra of octa-O-acetylsucrose (75). The chemical shifts and cou-... 

Mass spectra have been measured for ketose acetates only,16 and the most characteristic feature of these mass spectra is the formation of a new series of fragments having no analogy in the mass spectra of other types of carbohydrate derivatives. 

The study of such techniques as F.t.-i.r., computerized laser-Raman, or n.c.a., however great their degree of sophistication, should have practical utility for carbohydrate chemists and biochemists. That is why, amid the current problems elucidated by the interpretation of the vibrational spectra of carbohydrates and their derivatives, a section has been reserved for discussion of structure-properties relationships. 

I. Cerny, M. Budesinsky, T. Trnka, and M. Cerny, Preparation of 2-amino- 1,6-anhydro-2,3-dideoxy-jS-D-arabino-hexopyranose. H- and 13C-N.M.R. spectra of deoxy derivatives of 2-amino-l,6-anhydro-2-deoxy-D-glucose and 2-amino-l,6-anhydro-2-deoxy-D-mannose, Carbohydr. Res., 130 (1984) 103-114. 

The first two reports on carbon-13 nuclear magnetic resonance (,3C-n.m.r.) spectra of carbohydrates appeared1-2 in 1968 and 1969 since then, 13C-n.m.r. spectroscopy has become increasingly important as a tool for the characterization and structural elucidation of sugars and their derivatives. Although 13C-n.m.r. is closely related to H-n.m.r. spectroscopy, especially when both types of spectra are recorded with... 

At pH 10, diphenylborinic acid gives a tetrahedral anion that complexes with various diol systems, and thus it can be used in electrophoresis like borate.109 In a more detailed study of such complexing,110 diols were examined by 13C-n.m.r. spectroscopy, before and after addition of sodium diphenylborinate, and complexes were detected, and their spectra observed, for a variety of carbohydrate derivatives. 1,2-Diol groupings in acyclic and cis-cyclic compounds, 1,3-related diols at C-4,C-6 of hexopyranosides, the 3,5-diols of glucofuranoses, and 2,4-diols of the anomeric methyl 3,6-anhydro-D-glucopyranosides were all found to react. No interaction occurred with l,6-anhydro-/3-D-glucopyranose (compare Section V,2). 

The interpretation of the mass spectra of oligosaccharide derivatives follows the general principles that have been elaborated for simple cyclic and acyclic carbohydrates (66MI1 74MI1). 

Alternatively, chromophores can be added to carbohydrates through the formation of para-substituted benzoate esters or other functional groups containing chromophores. The high intensities of the CD spectra of these derivatives make this technique quite sensitive [452]. The orientations of the chromophores with respect to the sugar can be estimated and the resulting CD curves are very sensitive to substituent orientations. Thus, this technique is also useful for conformational studies [199,200,203,453]. 

The thio-disuccinate 13 aggregates in chloroform/cyclohexane (1 1) to form inverse micelles, which bind and solubilize carbohydrates. H-NMR spectra of nitrophenolates and ESR spectra of TEMPO derivatives indicate that the carbohydrates bind tightly to the surfactant s head group at low water con-tent ". ... 

The infrared spectra of a large number of carbohydrates have been published aU of these will be useful for future comparative work. The work of Kuhn and of Stevenson and Levine has already been mentioned. In addition, Solms, Denzler and Deuel have recently pubUshed the spectra of several derivatives of poly-D-galacturonic acid. The collection of infrared spectra of the sugar acetates and related compounds also forms a valuable source of data for comparative work. ... 

Spectra, infrared, of carbohydrates, 12,13-33 Spectrometry, mass, of carbohydrate derivatives, 21,39-93 Spectroscopy, infrared, and carbohydrate chemistry, 19,23-49 Sphingosines, conjugates with sugars, 24, 381-433 Starch,... 

In a survey " of the behavior of various classes of carbohydrate derivatives in c.i.m.s., it was found that the principal ion is the MH (isobutane) or M -I- NH (ammonia) ion, or a simple elimination product from it. The ammonia-mediated spectra feature M -I- NH f as the major ion with simple aldoses, their acetylated methyl glycosides, 1,6-anhydro-aldohexopyranoses (and their triacetates), and acetylated 1-thioaldoses. Aldose diethyl dithioacetals show M -I- NHf — EtSH as the principal ion, whereas this ion for aldehydo-aldose peracetates is MH — AcOH. 

Much of our knowledge on the fragmentation patterns of carbohydrate derivatives arises from studies on methylated monosaccharides, but in structural investigations, such compounds are seldom subjected to mass spectrometry. On the other hand, the molecular weight of a permethylated oligosaccharide is significantly less than that of the per(trimethylsilyl) derivative. For this reason, the mass spectra of disaccharides as -their permethylated alditols have been determined by Chizhov and coworkers, Karkkainen, and Krone and Beckey. Trisaccharides have also been studied by Karkkainen, either as permethylated glycosides or as alditols. ... 

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