Molecular mass, monoisotopic chemical

Monoisotopic vs. chemical (average) relative molecular mass... 

Monoisotopic Versus Chemical (Average) Molecular Mass... 

While this monograph is primarily concerned with the characterization of small molecules, it is useful to point out some of the differences in interpretation and thought process needed to interpret mass spectra of proteins. The shift from monoisotopic molecular masses in small molecules to chemical molecular masses in peptides and proteins is necessary when the chemical entities exceed 1500-2000 Da. Mass calibration procedmes and philosophies of instrument operation change. The concept and application of chemical molecular mass accuracy changes from 0.1 to 0.2 Da accmacy for nominal monoisotopic measurements to 0.01-0.02% for proteins (acceptable 0.01% accuracy for the BSA example used here is 7 Da). In short, the MS of proteins and other intact biopolymers deserves an entire monograph of its own. 

The isotope-coded affinity tag approach utilizes chemical labeling that allows quantitation when combined with mass spectrometry. ICAT is desirable because the chemical labeling takes advantage of the mass defects of monoisotopic stable isotopes. ICAT uses an ICAT reagent to differentially label protein samples on their cysteine residues. ICAT is advantageous because it permits the evaluation of low-abundance proteins and proteins at both extremes of molecular weights and isoelectric points.60... 

Partial DCl spectrum of poly(styrene) using argon as the reagent gas (Eq. 1.6). The solid lines are n-mer molecular ions, and the dashed lines are fragment ions. The numbers not in parentheses are the evaporation temperatures in K. The first and second numbers in parentheses are the number of monomer units and the monoisotopic mass, respectively. (Reprinted from Ref. 11 with permission of the American Chemical Society)... 

Fig. 22 Identification of sequence variants (isoforms) of human histone H2B isolated from asynchronously grown HeLa S3 cells by ESI-FT-ICR MS and MS/MS. To minimize sample heterogeneity (partial oxidation during the sample preparation), both methionine residues of H2B were quantitatively oxidized with performic acid prior to MS. a Broadband mass spectrum of the H2B isolate after RP HPLC purification revealed five main isotopic distributions, which were afterward analyzed by top-down MS. The determined oxidized monoisotopic mass is listed above each molecular ion species carrying 16 positive charges. Using BCD for direct MS/MS analysis, the H2B isoforms H2B.K and H2B.T (both 13 814.5 Da), H2B.J (13 816.5 Da), H2B.A (13 830.5 Da), H2B.Q and H2B.E (both 13 844.5 Da), H2B.B (13 860.5 Da), H2B.F (13874.5), and monoacetylated H2B.A (13 872.5 Da) were identified. The masses include oxidation of both methionine residues to their sulfones (-1-64 Da), b Identification of the isobaric H2B sequence variants H2B.Q and H2B.E by ECD MS/MS of peak 2 (13 844.6 Da), c Key fragment ions in the 625-725 m/z region reporting on the presence of H2B.E and H2B.Q. d Sequence alignment of H2B.E and H2B.Q. Sequence differences are underlined. Reproduced from [59] with permission from American Chemical Society, 2006...

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