Physical properties of proteins

A rational understanding of the physical properties of proteins can be based on a sound knowledge of protein structure08 , and this is discussed in Appendix 5.1. The more important properties are outlined below. 

By definition, a base is a proton acceptor while an acid is a proton donor and the general equation for any acid-base reaction can be written  

The strength of an acid is determined by its ability to give up protons while the strength of a base is determined by its ability to take up protons. This strength is indicated by the dissociation or equilibrium constant, (pKfl), for the acid or base strong acids have a low affinity for protons, while weak acids have a higher affinity and only partially dissociate (e.g. HC1 (strong) and acetic acid (weak)). 

for weakly acidic groups, as found in amino-acids  

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The secondary structure is responsible for some of the physical properties of proteins. For example, structural proteins such as a-keratins in skin and hair are fibrous in nature, and have good elastic... 

Proteins are important from the nutritional and technological points of view. Proteins affect every property that characterizes a living organism, and they play different roles in the human body. Proteins are also very important in food technology and are responsible for many food properties. The physical properties of proteins and their interactions with other components contribute significantly to the functional behavior and quality of several food products, such as cheese, bread, and meat products (9). An overview of the functional roles of proteins in different food systems is presented in Table 2. Food preferences by human beings are based not on nutritional quality but on sensory attributes to the food, such as appearance, color, flavor, texture, and... 

The complete hydrolysis of proteins produces 20 a-amino acids that also occur as free metabolic intermediates. As free acids, they exist mostly as dipolar ions (zwitterions). Except for glycine, they contain chiral a-carbons and therefore exist in the d- and L-enantiomeric pair, of which the L-isomers are the monomeric units of proteins. They are differentiated structurally by their side-chain groups with varying chemical reactivities that determine many of the chemical and physical properties of proteins. These side-chain groups include ... 

The draglines of N. edulis have a good (even better) performance at low temperature. This "abnormal" property not only indicates the possibility of such silk to be used as "super-fiber" under severe conditions, but also supplies a model for researchers to investigate the contribution of inter- or intramolecular hydrogen bonds to the physical properties of protein materials (Yang et al., 2005). 

Deterioration of the physical properties of proteins during food processing or food storage can be ascribed primarily to an irreversible insolubilization of proteins. However, a deteriorative change for one purpose can be a favorable one for another purpose. In Japan, for instance, the irreversible insolubilization of soybean proteins has been utilized effectively for production of soybean protein foods, such as tofu, kori-tofu, and yuba. 

Modification refers to the intentional alteration of the physical properties of proteins to improve functional properties (2). The modifying procedure employed may be dictated by the required functionality and ideally it should not result in... 

Separation methods exploiting different physical properties of proteins have been combined with varying degrees of success. The ultimate goal is a rapid separation strategy that can be coupled with detection methods, such as MS, to provide... 

Physical Properties of Proteins.—Most proteins are colorless, amorphous substances. They have no definite melting points or boiling points, but carbonize on heating and give off gases. The proteins differ widely from one another as to solubility in water, in solutions of salt, and in dilute alcohol. 

This review is concerned with the chemical and physical properties of proteins and enzymes containing three distinct and unique forms of Cu The "blue center or, in the nomenclature proposed by Vdnng rd, Type 1 Cu2+ the colorless or Type 2 Cu2+, common to all multi-copper oxidases which reduce molecular oxygen to two molecules of water and the Cu associated with the 330 nm absorption band, again common to the oxidases. The purposes of the review are to assemble chemical and physical data related to the indicated types of Cu binding sites, to offer some interpretations (and occasionally re-interpretations) of experimental results concerned with structure-function relationships, and to generalize some of the information available as it concerns the structures of these unique Cu-co-ordination complexes. Special emphasis will be placed on the kinetic and mechanistic work which has been carried out on the multi-copper oxidases while the physiological roles of the various protein systems will not be of particular importance. 

The phenomenon of fluorescence can provide information about the physical properties of proteins and other macromolecules. The information content results from the sensitivity of the spectral properties to the average and dynamic properties of the environment surrounding the fluorescent residues. In general, more detailed information is obtainable from time-resolved data than from steady-state measurements. However, the steady-state measurements are considerably easier to perform. At present, the ability to recover time-resolved spectral data is rapidly improving, primarily because of advances in instrument design. The newer instruments may possess resolution adequate to correlate experimental data with the structural or dynamic properties of macromolecules. 

The quality and utility of each RM will be greatly enhanced by the availability of a set of amide exchange rate coefficients derived from measurements by an orthogonal method, such as HX-NMR. The physical properties of proteins and their behaviors during proteolysis vary considerably. The optimum set of reference proteins should reflect these varied behaviors. This will enable researchers to pair the measurement difficulties of their commercial biotherapeutic product with a primary RM. The chosen primary RM can then be used to measure HX-MS laboratory performance over time and location. 

It is our opinion that the pol)ipeptide chain structure of proteins, with hydrogen bonds and other interatomic forces (weaker than those corresponding to covalent bond formation) acting between polypeptide chains, parts of chains, and side-chains, is compatible not only with the chemical and physical properties of proteins but also with the detailed information about molecular structure in general which has been provided by the experimental and theoretical researches of the last decade. Some of the evidence substantiating this opinion is mentioned in Section 6 of this paper. 

Foegeding, E. A. Luck, P. J. Davis, J. P. Factors determining the physical properties of protein foams. Food Hydrocolloids, 2006, 20,284-292. 

Improving some physical properties of proteins (e. g., texture, foam stability, whippabU-ity, solubility)... 

The measurement of bulk modulus of macroscopic fibers provided the fundamental knowledge to understand the physical properties of protein based structures, such as wool, tendon, bone, connective tissues, silk, etc. in relation to their chemical composition and eventually opened a variety of ways to improve their advantageous properties or mimic them in partially or totally synthetic materials [18]. 

In addition, a recent interest in /3 -amyloid like proteins with considerable thermodynamic and mechanical stability revived interest in the mechanical measurement of the physical properties of proteins [57]. 

Table 1 Physical properties of proteins and their applicability as HPLC CSPs (data from [17,93])...

Table 8 Physical properties of proteins (Reproduced with permission from American chemical society [86, 87])...

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