Physical properties of amino acids

For all other amino acids, with the exception of glycine (Gly), the a-carbon is bonded to four different groups, and the two stereoisomers are mirror images that cannot be superimposed. Eukaryotic proteins are always composed of L-amino acids although D-amino acids are found in certain peptide antibiotics and some peptides of bacterial cell walls. ° ° The physical properties of amino acids are influenced by the degree of ionization at different pH values. 

PolyPhen-2 (http //genetics.bwh.harvard.edu/pph) is a tool which predicts the possible impact of an amino acid substitution on the structure and function of human proteins and has been successfully used to predict GPCR variant function [350]. SIFT (http // sift.bii.a-star.edu.sg/) uses sequence homology and physical properties of amino acids to predict the effect of non-synonymous polymorphisms and missense mutations on protein function. 

Chapter 23 starts by looking at the physical properties of amino acids. Then you will see how amino acids are linked to form peptides and proteins. You will also see how proteins are made in the laboratory, and you will be able to compare this with how they are made in nature when you read Chapter 27. The stmcture of proteins will be examined to prepare you for understanding how enzymes catalyze chemical reactions, which is covered in Chapter 24. 

The analytically pure amino acids required for the measurement of physical properties of amino acids, the determination of amino acids by colorimetric and microbiolo cal methods and the study of the metabolic functions of amino acids, may be prepared by reciystallisation from water, dilute HCl, or an aqueous solution of methanol, ethanol or other alcohol. An amino acid is recrystallised und conditions such as those outlined in Table II until inorganic impurities have been removed, and the value found for a physical property (such as specific rotation or solubility) or a constituent (such as nitrogen) is in close agreement with the standard or theoretical figure. If no standard figure few a physical property is available the product is recrystallized until a constant value is obtained. 

W. E. Steinmetz, Quant. Struct.-Act. Relat., 14, 19 (1995). A CoMFA Analysis of Selected Physical Properties of Amino Acids in Water. 

In many cases only the racemic mixtures of a-amino acids can be obtained through chemical synthesis. Therefore, optical resolution (42) is indispensable to get the optically active L- or D-forms in the production of expensive or uncommon amino acids. The optical resolution of amino acids can be done in two general ways physical or chemical methods which apply the stereospecific properties of amino acids, and biological or enzymatic methods which are based on the characteristic behavior of amino acids in living cells in the presence of enzymes. 

A number of useful computational tools have been developed for predicting the identity of unknown proteins based on the physical and chemical properties of amino acids and vice versa. Many of these tools are available through the Expert Protein Analysis System (ExPASy) at http //www.expasy.ch (Appel et al., 1994) and other servers. 

A mammalian cell may contain as few as 2000 different proteins and as many as 50,000 at any given time. Each of these is uniquely suited to the function it performs, and this in turn depends on its size, shape, solubility in aqueous media, acid-base properties, propensity to form fibers, and numerous other physical and chemical properties. The component amino acids are largely responsible for the ability of proteins to perform their biologic roles. The properties of amino acids are therefore of paramount importance in determining how proteins work. 

II. PHYSICAL-CHEMICAL PROPERTIES OF AMINO ACID SOLUTIONS... 

Interpretation of spectrometric features of amino acids and peptides and their derivatives has its routine aspects, but the spectra of these compounds also incorporate unique detail that provides specific information about the behaviour of amino acids and peptides in solution. These methods reveal the ways in which groupings within a peptide in solution relate to each other and these details are of major importance in determining the physical and physiological properties of amino acids and peptides. Conformational studies (Chapter 2) and structure determinations for peptides have been the priority targets for investigations by physicochemical methods. 

Sjdstrdm, M. and Wold, S. (1985) A multivariate study of the relationship between the genetic code and the physical-chemical properties of amino acids. J. Mol. Evol, 22, 272-277. 

Physical and Chemical Properties of Amino Acids Peder Olesen Larsen... 

A. Some Physical Properties of Bile Acids and Their Amino Acid Conjugates... 

Larsen P O 1980 Physical and chemical properties of amino acids. In Stumpf P K, Conn E E (eds) Biochemistry of plants. Academic Press New York, 225-269... 

Once separated, the amino acids must then be quantified. The physical and chemical properties of amino acids, with the exception of tyrosine, tryptophan, and phenylalanine, limit detection to universal detectors such as refractive index (RI) or light scattering. The drawback of these detectors is that they are universal and any non-amino acid components will be detected, often interfering with the detection of the amino acids themselves. Derivatization coupled with either UV/visible or fluorescent detection is much more specific, eliminating the detection of most potentially... 

The a-amtno acids have no net charge. However, their properties resemble those of salts rather than uncharged molecules. Amino acids have low solubilities in organic solvents but are moderately soluble in water, unlike most organic compounds of comparable molecular weight. The physical states of amino acids also differ from those of comparable carboxylic acids and amines. For example, ethyl amine is a gas, and acetic acid is a liquid at room temperature. In contrast, glycine is a solid. 

The physical properties of a typical amino acid such as glycine suggest that it is a very polar substance much more polar than would be expected on the basis of its formula tion as H2NCH2CO2H Glycine is a crystalline solid it does not melt but on being heated It eventually decomposes at 233°C It is very soluble m water but practically insoluble m nonpolar organic solvents These properties are attributed to the fact that the stable form of glycine is a zwittenon, or inner salt... 

Peptides and proteins are formed by linking successive amino acids into chains or rings. The order (sequence) and types of amino acids determine the chemical and physical properties of peptides and proteins. 

Table 3 gives the corresponding physical properties of some commercially important substituted pyridines having halogen, carboxyHc acid, ester, carboxamide, nitrile, carbiaol, aminomethyl, amino, thiol, and hydroxyl substituents. 

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