Molar mass of proteins

The molecular weight or molar mass of proteins and nucleic acids (DNA, RNA) is identical for each specific species, e.g., the molecular weights of all casein molecules from a specific source are identical. These polymers are members of a homologous series and are said to be monodisperse or molecularly homogeneous. 

One of the earliest methods for determining the molar mass of proteins was based on chemical analysis. A hemoglobin preparation produced from red blood cells separated from the blood by the use of a centrifuge was found to contain 0.335% iron, (a) If the hemoglobin molecule contains 1 atom of iron, what is its molar mass (b) If it contains 4 atoms of iron, what is its molar mass ... 

These equations can be used for the measurement of the molar masses of proteins and of polymeric materials although strictly speaking equations (53.15) and (53.19) need to be expanded in order to apply to non-ideal situations. 

Figure 4-2S. SDS-PAGE determination of molar masses of proteins under denaturing conditions. The pattern of bands from a standard protein mix after SDS-PAGE is shown...

Mass spectrometry has assumed great importance in determinations of the molar masses of biological macromolecules, even quite large ones. This is due to developments such as electrospray ionisation (ESI) and matrix assisted laser desorption/ ionisation (MALDI), which have made it possible to determine the molar masses of biopolymers up to several 100 kDa (Pitt 1996 Kellner et al. 1999 Snyder 2000). The combination of MALDI techniques with time-of-flight mass spectrometers (MALDI-TOF) is of particular significance for determination of the molar masses of proteins with high sensitivity (typically pmol quantities, although exceptionally fmol) and precision (proteins up to 100 kDa with precision of about 0.01 %). Mass spectrometry can provide very accurate measurements of protein molar mass that can yield information about even minor structural modifications not readily accessible by other means. 

The thermodynamics of phase changes of pure materials is also important because it prepares us first for the study of mixtures and then for the study of chemical equilibria (Chapter 4). Some of the thermodynamic concepts developed in this chapter also form the basis of important experimental techniques in biochemistry, such as the measurement of molar masses of proteins and nucleic acids and the investigation of the binding of small molecules to proteins. 

In studies of the compressibilities of proteins in aqueous solution molar quantities are not usually determined. Since the molar masses of proteins vary over a wide range, it is more convenient when making comparisons of the results for one protein with those for another, to use specific rather than molar quantities. The partial specific volume of a component i in a solution is defined by... 

A biochemist isolates a new protein and determines its molar mass by osmotic pressure measurements. A 50.0-mL solution is prepared by dissolving 225 mg of the protein in water. The solution has an osmotic pressure of 4.18 mm Hg at 25°C. What is the molar mass of the new protein ... 

A 1.00-mg sample of a pure protein yielded on hydrolysis 0.0165 mg of leucine and 0.0248 mg of isoleucine. What is the minimum possible molar mass of the protein (MM leucine = MM isoleucine = 131 g/mol)... 

A sample consisting of 155 mg of a purified protein is dissolved in 10.0 mL of ethanol. This solution is placed in a device for measuring osmotic pressure and rises to a final height of 32.5 cm above the level of pure ethanol. The experiment is performed at 1.00 atm and 298 K. The density of ethanol at 298 K is 0.79 g-cm "3. What is the molar mass of the protein Assume that the density of the solution is the same as that of pure ethanol. See Exercise 8.95. 

In nature, there are 20 amino acids available for incorporation into the protein chain. They are arranged in a specific and characteristic sequence along the molecule. This sequence is generally referred to as the primary structure of the protein. Also part of the primary structure is the relative molar mass of the macromolecule. 

C03-0023. Twenty different amino acids are the essential building blocks of proteins. Calculate the molar masses of these three. 

C12-0018. When 7.50 mg of a protein is dissolved in water to give 10.00 mL of solution, the osmotic pressure is found to be 1.66 torr at 21°C. Determine the molar mass of the protein. 

C12-0019. A water-soluble protein molecule has a molar mass of 985 g/mol. Calculate the freezing point... 

Add a 25-molar excess of sulfo-NHS acetate over the amount of amines present in the sample. If the precise amount of amines is not known, adding an equal mass of reagent to the mass of protein will provide a large excess of reactivity to completely block all amines. 

In this equation, u is the osmotic pressure in atmospheres, n is the number of moles of solute, R is the ideal gas constant (0.0821 Latm/K mol), T is the Kelvin temperature, V is the volume of the solution and i is the van t Hoff factor. If one knows the moles of solute and the volume in liters, n/V may be replaced by the molarity, M. It is possible to calculate the molar mass of a solute from osmotic pressure measurements. This is especially useful in the determination of the molar mass of large molecules such as proteins. 

A solution prepared by dissolving 6.95 x 10 1 3 g of protein in 0.0300 L of water has an osmotic pressure of 0.195 torr at 25°C. Assuming the protein is a nonelectrolyte, determine the molar mass of the gene fragment. 

In this form, van t Hoff s law of osmotic pressure is also used to determine the molar masses of biological and synthetic macromolecules. When the osmotic pressure is measured for a solution of macromolecules that contains more than one species of macromolecule (for example, a synthetic pol5mer with a distribution of molar masses or a protein molecule that undergoes association or dissociation), the osmotic pressures of the various solute species II, are additive. That is, in sufficiently dilute solution... 

If proteins are in contact with some detergents as sodium do-decylsulfate (SDS) or cetyltrimethylammonium bromide (CTAB) they become denaturated, e.g., their secondary, tertiary, and quaternary structures are destroyed. They get a rod-like shape and the amount of bound detergent is proportionate to the molar mass of the proteins These protein-detergent complexes have negative charges at slightly alkaline pH if SDS is used and their size (hydrodynamic radius) is approximately proportional to their molar... 

Place the IPG strip face down onto the stacking gel, about 5 mM apart of the cathode paper bridge. Place silicone rubber sample applicators for molar mass marker proteins at one or both sides of the IPG strip. 

Fill the gel particles together with the surrounding buffer into the tube. The tube is inserted into the vertical electrophoresis apparatus (dialysis membrane down to anode), electrode buffer is poured, and electroelution is started with 10 mA per tube for 3-8 h, depending on tbe molar mass of the protein. 

Solutions of macromolecules may be concentrated by means of polymer membranes of defined pore size. Applying a pressure or centrifugal force, small molecules pass the pores, whereas large molecules retain. The nominal cutoff of an ultrafiltration membrane (MWCO) helps you to select a membrane Molecules smaller than the MWCO will pass the membrane, whereas larger molecules are held back. This separation is not sharp and depends on protein conformation and solvent composition. Complete retention is achieved when using a membrane with a MWCO 1/3 to 1/5 of the molar mass of the macromolecule of interest. Figure 3.6 illustrates the separation of proteins by ultrafiltration. 

Because of the very high molar masses of enzymically polymerized lignin sulfonates, Sephacryl S-300 is used as the gel matrix. The fractionation range is 10 000 to 1 000 000 dalton. Even in this case proteins of known molar mass can be used as calibration standards (Figs. 7 and 8). 

It is important for us to keep in mind that biopolymers are generally not monodisperse components. Proteins are typically paucidisperse — mixtures of monomers, dimers and multimers. And polysaccharides are polydisperse their chain lengths and molar masses can be represented as a continuous distribution. For this reason the virial coefficients appearing in equations (5.16) and (5.17) should be interpreted as averages. So the inverse of the number-averaged molar mass of component / is given by... 

We turn now to the other major whey protein, p-lactoglobulin. This is an acidic globular protein (p/= 5.1) with a molar mass of 18.4 kDa and a radius of about 2 nm (Aymard et al., 1999). The protein can form long semi-flexible fibrils when heated in solution at around or above its dena-turation temperature (60-80 °C) at pH = 2 and low ionic strength (Durand et al., 2002 Veerman et al., 2002, 2003a,b). (An example of the... 

Osmotic pressure measurements are particularly convenient for determination of the molar mass of macromolecules such as proteins. 

A purified cytochrome protein isolated from a bacterial preparation was found to contain 0.376% iron. What can be deduced about the molar mass of the protein ... 

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