Apparent relative molecular mass

As mentioned above, the profiles in Figure 8.4 also indicate the presence of an additional low-sulfur component (arrow) in the alcohol-fed animals. This additional component has an apparent relative molecular mass of 70,000. 

IgA is associated mainly with seromucous secretions such as saliva, tears, nasal fluids, etc., and is secreted as a dimer with both a J chain and a secretor piece (relative molecular mass 70 000), the latter apparently to prevent damage to the molecule by proteolytic enzymes. Its major role appears to be the protection of mucous membranes and its presence in blood, mainly as the monomer, may be as a result of absorption of the degraded dimer. 

GSHPx from several tissues of a number of different species has been purified and the relative molecular mass was found to be in the range of 76000-lOOOOOdaltons [9]. The enzyme consists of four subunits which are apparently identical, with molecular weights ranging from 18 000 to 24 000 daltons, and there are 4 g-atoms of selenium per mole of enzyme. The enzyme contains... 

Above all other parameters, it is the relative molecular mass of a polymer which determines the apparent shear viscosity (7.N.3). Figure 7.14 shows the stress dependence at 170°C of four BPE polymers with relative molecular masses ranging from extremely high (MFI 0.2) to extremely low (MFI200). Note that the shape of the curves (the shear thinning characteristics) is little changed by variations in relative molecular mass. 

High relative molecular mass substrates are the standards for collagenase, hyaluronidase, lysozyme, ribonuclease, streptokinase, and urokinase. It should be emphasized that small variations in the quality of high relative molecular mass standards or conditions can result in sharp changes in the apparent composition of reaction products and can lead to nonreproducible results. 

Dependence oj Apparent I iscosity on Temperature and Relative Molecular Mass... 

Fig. 7.14. Stress dependence of the apparent viscosity /i, at 170 C for four branched polyethylene resins ranging from a very high relative molecular mass, MFI — 0.20. to a very low relative molecular mass, MFI = 200. Measurements were made by capillary flow. Note the thousandfold change in /i, produced by changes in relative molecular mass (after Cogswell).

The reaction center is built up from four polypeptide chains, three of which are called L, M, and H because they were thought to have light, medium, and heavy molecular masses as deduced from their electrophoretic mobility on SDS-PAGE. Subsequent amino acid sequence determinations showed, however, that the H chain is in fact the smallest with 258 amino acids, followed by the L chain with 273 amino acids. The M chain is the largest polypeptide with 323 amino acids. This discrepancy between apparent relative masses and real molecular weights illustrates the uncertainty in deducing molecular masses of membrane-bound proteins from their mobility in electrophoretic gels. 

Note that, apart from the filler particle shape and size, the molecular mass of the base polymer may also have a marked effect on the viscosity of molten composites [182,183]. The higher the MM of the matrix the less apparent are the variations of relative viscosity with varying filler content. In Fig. 2, borrowed from [183], one can see that the effect of the matrix MM on the viscosity of filled systems decreases with the increasing filler activity. In the quoted reference it has also been shown that the lg r 0 — lg (MM)W relationships for filled and unfilled systems may intersect. The more branches the polymer has, the stronger is the filler effect on its viscosity. The data for filled high- (HDPE) and low-density polyethylene (LDPE) [164,182] may serve as an example the decrease of the molecular mass of LDPE causes a more rapid increase of the relative viscosity of filled systems than in case of HDPE. When the values (MM)W and (MM)W (MM) 1 are close, the increased degree of branching results in increase of the relative viscosity of filled system [184]. 

Until recently, the possibility that H,K-ATPase consists not only of a catalytic a subunit but also of other subunits was not examined. This was mainly due to the fact that SDS-PAGE of purified gastric H,K-ATPase preparations principally gave one protein band with an apparent molecular mass of about 100 kDa, which was reported to comprise 75% or more of the total amount of protein [6,66,67]. This mass is lower than the mass deduced from its cloned cDNA [40], but may be due to the higher electrophoretic mobility of membrane-bound proteins, as consequence of having relatively high contents of hydrophobic amino acid residues [68]. 

Fig. 6. Under severe starvation conditions the epsi-APase and several other excreted proteins were present in the medium at very high levels relative to unstressed controls. Proteins excreted by cells growing 8 days -Pi or +Pi were separated via SDS-PAGE and immunoblotted using AP3 (A) or silver stained to show total protein (B). The psi enhancement of the epsi-APase (53.6 kDa) was clearly shown. The apparent molecular masses of several proteins selected for further study are indicated. Significant psi enhancement was shown for several of these proteins (Goldstein et al., 1989b).

Fig. 3. Monitoring of the variant formation in a continuous culture of Bacillus stearothermo-philus PV72 (as in Fig. 2) by SDS-PAGE. The organisms differed in the S- (surface) layer proteins. The wild type formed an S-layer protein of 130 kDa apparent molecular mass, the variant S-layer protein appears at 97 kDa molecular mass. The wild-type S-layer protein was quantified relative to the band of the altered protein by densitometry. Numbers on the curve represent samples harvested at distinct points of time during variant formation. The decrease of the wild-type S-layer protein followed the theoretical washout curve in a stirred tank reactor at the set dilution rate of 0.3 h 1 (Reprinted from J. Biotechnol. 54, K. C. Schuster et al, p. 20,1997, with permission from Elsevier Science)...

The apparent molecular mass of D-Ser toxin was dramatically increased by the addition of guanidine hydrochloride to the elution buffer, although that of the L-Ser toxin was not altered by the denaturing reagent. In the presence of 5.2 M guanidine hydrochloride, the D-form toxin was eluted at the same position as the L-form toxin and the apparent molecular masses of the two toxins were estimated as 6 kDa based on calibration with the standard proteins. CD and fluorescence spectroscopic analyses revealed that the two toxins were unfolded and lost their secondary and tertiary structure in 5.2 M guanidine hydrochloride at pH 7.4, as described below. It, therefore, appears that the D-Ser toxin forms a compact folded structure, whereas the L-Ser toxin has a relatively unfolded or extended structure. 

When a pure elemental gas, such as neon, was analyzed by a mass spectrometer, multiple peaks (two in the case of neon) were observed (see Fig. 1.11). Apparently, several kinds of atoms of the same element exist, differing only by their relative masses. Experiments on radioactive decay showed no differences in the chemical properties of these different forms of each element, so they all occupy the same place in the periodic table of the elements (see Chapter 3). Thus the different forms were named isotopes. Isotopes are identified by the chemical symbol for the element with a numerical superscript on the left side to specify the measured relative mass, for example °Ne and Ne. Although the existence of isotopes of the elements had been inferred from studies of the radioactive decay paths of uranium and other heavy elements, mass spectrometry provided confirmation of their existence and their physical characterization. Later, we discuss the properties of the elementary particles that account for the mass differences of isotopes. Here, we discuss mass spectrometry as a tool for measuring atomic and molecular masses and the development of the modern atomic mass scale. 

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