Molecular weight determination by mass spectrometry

The previously determined (38) molecular formula was corrected to C16H22N4O3 by quantitative elemental analysis and a molecular weight determination by mass spectrometry. 

An unknown substance, X, was isolated from rabbit muscle. Its structure was determined from the following observations and experiments. Qualitative analysis showed that X was composed entirely of C, H, and 0. A weighed sample of X was completely oxidized, and the H20 and C02 produced were measured this quantitative analysis revealed that X contained 40.00% C, 6.71% H, and 53.29% O by weight. The molecular mass of X, determined by mass spectrometry, was 90.00 u (atomic mass units see Box 1-1). Infrared spectroscopy showed that X contained one double bond. X dissolved readily in water to give an acidic solution the solution demonstrated optical activity when tested in a polarimeter. 

The molecular weights of the fused ring systems (XVII) and (XVIII) have been determined by mass spectrometry (55). 

Hexafluorobut-2-yne and Ru3(CO) 12 give the cyclopentadienone complex (CXVIII), and the molecular weight was determined by mass spectrometry... 

The molecular weight of evonine,68 composed of a base and a polyhydroxy moiety of still unknown structure, could not be determined by mass spectrometry, because the alcohol component was too easily eliminated. Nevertheless, important parts of the structure of the base can be deduced from the mass spectrum of the LiAlH4-reduction product of evonine (137) which exhibits a significant peak at mass 137, indicating the presence of a substituted pyridine of type [138], (137)—>[138]— -[139] ... 

Mass spectrometry can determine the molecular weights of peptides and proteins with mass accuracies orders of magnitude better than the molecular weights determined by gel electrophoresis. It is important to note that in determining molecular... 

The molecular formula of villalstonine, earlier presumed to be C40H50N4O4, was subsequently corrected to C41H48N4O4 on the basis of the molecular weight (660) determined by mass spectrometry (28). 

At one time, the molecular weight of a compound was determined by its vapor density or its freezing-point depression, and molecular formulas were determined by elemental analysis, a technique that determined the relative proportions of the elements present in the compound. These were long and tedious techniques that required relatively large amounts of a very pure sample. Today, molecular weights and molecular formulas can be rapidly determined by mass spectrometry from a very small amount of a sample. 

The heat-denatured P40 migrated with an apparent molecular weight of 36 kDa, which agrees with the molecular mass determined by mass spectrometry and deduced from amino acid sequence, whereas the non denatured form of the protein migrated with a lower apparent molecular mass of approximately 31 kDa. This phenomenon of heat-modifiability has been well described for E. coli OmpA (Heller, 1978 Dommair et al., 1990) and is a common property to bacterial outer membrane proteins with a P-barrel membrane domain. Boiling such proteins in the presence of SDS extends their P structure to a helices and thereby causes reduction of SDS binding and electrophoretic mobility. 

Now that you have a basic understanding of the information that is available in an IR absorption spectmm, it is possible, with some additional information, to work out the likely structure of an unknown for small molecules (MW < 300). For unknowns where the molecular weight has been determined by mass spectrometry the analyst generally takes the following approach. 

The exact molecular weight of pimaricin 73 (Scheme 73) (as the iV-acetyl derivative) was determined by mass spectrometry of its tri-methylsilyl derivative, which was sufficiently volatile and thermally stable. A revised structure for the antibiotic is based on such studies (107). The mass spectra of 0-trimethylsilyl pimaricin and several of its derivatives correspond to a molecular formula of C88H47N07 for pi-... 

The structures of long-chain bases can best be determined by mass spectrometry (Karlsson, 1970b). The dinitrophenylamines with their free hydroxyl groups methylated give characteristic fragmentation patterns for determination of their molecular weights and the primary structure of the base. Double-bond positions can be found if they are first hydroxylated and derivatized to TMS (tri-methylsilyl) ethers. Mass spectra can also be obtained (after GLC) of the A -acetyl, O-TMS ether derivatives of the base (Polito et aL, 1969). trans-Douhlc bonds can be identified by infrared absorption spectroscopy. 

I One of the first steps in determining molecular structure is establishing the molecular formula. In the past, this task was most commonly done by elemental analysis, combustion analysis to determine percent composition, molecular weight determination, and so forth. More commonly today, molecular weight and molecular formula are determined by mass spectrometry (Chapter 14). In the examples that follow, we assume that the molecular formula of any unknown compound has already been determined, and we proceed from that point using spectral analysis to determine a structural formula. 

One way to determine the number of acidic hydrogens in a molecule is to treat the compound with NaOD in D20, isolate the product, and determine its molecular weight by mass spectrometry. For example, if cyclohexanone is treated with NaOD in DzO, the product has MW = 102. Explain how this method works. 

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