Molecular mass, monoisotopic

Compound Molecular mass (monoisotopic) Found mass (M + H)+ Found mass M+... 

Figure 8.1 Heme (iron protoporphyrin IX) structure. The most frequently observed cleavages in LDMS of the intact species are denoted. Heme elemental composition is C34H32N404Fe monoisotopic molecular mass is 616.176 Da average molecular mass is 616.487 Da.

Dalton (Da) A non-SI unit of mass (symbol Da) that is equal to the unified atomic mass unit. See also related entries on accurate mass average mass molar mass molecular weight monoisotopic mass nominal mass unified atomic mass unit. 

Mass See entries on accurate mass average mass dalton molar mass molecular weight monoisotopic mass nominal mass unified atomic mass unit. 

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

Note The calculation of relative molecular mass, Mr, of organic molecules exceeding 2000 u is significantly influenced by the basis it is performed on. Both the atomic weights of the constituent elements and the natural variations in isotopic abundance contribute to the differences between monoisotopic- and relative atomic mass-based values. In addition, they tend to characteristically differ between major classes of biomolecules. This is primarily because of molar carbon content, e.g., the difference between polypeptides and nucleic acids is about 4 u at Mr = 25,000 u. Considering terrestrial sources alone, variations in the isotopic abundance of carbon lead to differences of about 10-25 ppm in Mr which is significant with respect to mass measurement accuracy in the region up to several 10 u. [41]... 

Zubarev, R.A. Hakansson, P. Sundqvist, B. Accuracy Requirements for Peptide Characterization by Monoisotopic Molecular Mass Measurements. Anal. Chem. 1996, 65,4060-4063. 

Appropriate standard peptides for calibration with monoisotopic molecular masses covering, for example, the mass range 500-3000 Da (27). Glu-fibrinopeptide-B is also used as external calibrant for MS" experiments (28). 

Matrix-assisted laser desorption-ionization ionizes molecules with molecular masses of 100-1,000,000 Da for analysis by MS and provides high sensitivity, high throughput, and simplicity of operation. MALDI combined with enzymatic reactions and protein chemistry can provide very useful information on molecular masses, peptide maps, and primary structure.15 MALDI-TOF MS can quickly and accurately determine unfractionated mixtures at concentrations below 100 fmol per liter. The obtained data are then calibrated with internal standards, monoisotopic masses are assigned for all prominent peaks, and the peptide list thus generated is used to identify the protein by using a suitable database.16... 

Answer 9.2 It vapourizes at low temperatures, so that little energy is used for that it has a low molecular mass, so that, since separation factors are proportional to the difference in mass between the and U-containing molecules, easier separations are achieved since fluorine is monoisotopic ( F), only molecules of two different masses are involved, minimizing overlap between and U-containing species (this would not be the case if, say. Cl or Br were involved). 

The accurate mass measurement was performed on a Micromass AutoSpec SE mass spectrometer using electrospray ionization on the triply charged ion of a cofactor-containing peptide (sample 2). Doubly charged ions of bradykinin (m/z 530.7885) and gramicidin S (m/z 571.3608) were used as calibration standards. Leu-enkeph in (MH+ at m/z 556.2771) was also included to verify the accuracy of the mass measurement. The monoisotopic molecular mass of this peptide was established by the mean [M-i-3H]3+ value obtained from four separate injections. Elemental compositions of the crosslinked residue were obtained by computer calculations. 

Monoisotopic molecular mass Calculated from the elemental composition using the monoisotopic atomic masses of the most abundant isotopes in nature (Table 2.2). Depending on the mass resolution, it is expressed as a nominal mass, or as an exact mass, based on exact monoisotopic atomic masses. 

Figure 10 Number of possible amino acid compositions (peptide combinations) as a function of the peptide monoisotopic molecular mass for [M] = 1000 Da ([M] is the nominal molecular mass, i.e., lower integer mass value). The histogram is built with a 10-mDa step. The top density of the distribution is 230 peptide compositions per mDa or per ppm. (Reprinted from Ref. 100.)...

RA Zubarev, P Hakansson, B Sundqvist. Accuracy requirements for peptide characterization by monoisotopic molecular mass measurements. Anal Chem 68 4060-4063, 1996. 

A commonly employed convention is to characterize the molecular mass of the protein by using the lightest isotopes. This is reasonable because for lighter atoms, such as C, H, O, N, S, and P, the stable isotopes of lowest atomic mass are also the most abundant isotopes. For the 69-amino acid protein of formula C398H8,8N,ooO,32S3, the lowest molecular mass is derived from the combined masses of the lightest nuclides of each element and equals 9,006.4 or 9,006 Da (9.006 KDa). This species is calculated to account for only 0.5 percent of the distribution noted above. Experimentally, it becomes an issue of sensitivity to accurately determine the first (lowest mass) peak in the series of istopic peaks the lowest monoisotopic molecular mass. 

A potential drawback for top-down analyses is that, as the molecular weight of a protein increases, it becomes less likely that the monoisotopic peak will be observed. For proteins > 15 kDa, the monoisotopic peak is 1% abundance and cannot be detected [131,132], It has been shown also that natural variation in C versus abundance can shift the most abundant isotopic peak of carbonic anhy-drase by 1 Da [131]. These phenomena place a limit on the accurate determination of protein molecular mass and, consequently, PTM assignment for example, is a 2 Da shift due to either abundance and deamination, or disulfide cleavage ... 

The monoisotopic mass is meaningful for low-mass compounds because the elemental composition can be determined from a well-defined isotopic pattern of the molecular ion. The nominal and monoisotopic masses can both be correlated with the most abundant peak in the isotopic cluster. As the mass of a compound, however, increases, the isotopic pattern becomes more synunetrical and extends over many masses [21]. Also, the monoisotopic peak becomes difficult to identify. For high-mass compounds (e.g., proteins and oligonucleotides), the molecular ion profile measured coalesces and becomes a single asymmetric peak. For such compounds, the average mass value is accepted as the molecular mass. 

Example 13.1 Calculate the monoisotopic molecular mass of an oligonucleotide 5 -[Pg.456]

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