| The charge and polarity characteristics of a peptide chain
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| MASS SPECTROMETRY AND MALDI-TOF MS |
| Mass spectrometry is used to measure the molecular weight of proteins and to detect posttranslational modification of proteins and peptides and in proteome research (see below). Matrix Assisted Laser Desorption Ionization Time-of-Flight Mass Spectrometry (MALDI-TOF MS) is a particularly sensitive and powerful technique for the characterization of proteins, peptides, and various biomolecules as shown in Figure 2.7. Another powerful method, electrospray ionization mass spectrometry (ESI MS), permits the measurement of the molecular weight of a protein directly from an aqueous solution of a protein. Two researchers, Drs Koichi Tanaka and John B Fenn were awarded the 2002 Nobel Prize in chemistry for their contribution to the development of the soft desorption ionisation methods for mass spectrometry. |
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The amino acid composition of a peptide chain has a profound effect on its physical and chemical properties. Proteins
rich in aliphatic or aromatic amino groups are relatively insoluble in water and more soluble in cell membranes. Proteins rich in polar amino acids are more water-soluble. Amides are neutral compounds so that the amide backbone of a protein, including the α-amino and α-carboxyl groups from which it is formed, does not contribute to the charge of the protein. Instead, the charge on the protein is dependent on the functional side chain groups of amino acids. Amino acids with side-chain acidic (Glu, Asp) or basic (Lys, His, Arg) groups will confer charge and buffering capacity to a protein. The balance between acidic and basic side chains in a protein determines its isoelectric point (pI) and net charge in solution. Proteins rich in lysine and arginine are basic in solution and have a positive charge at neutral pH, while acidic proteins, rich in aspartate and glutamate residues, are acidic and have a negative charge. Because of their side-chain functional groups, all proteins become more positively charged at acidic pH and more negatively charged at basic pH. Proteins are an important part of the buffering capacity of blood cells and biological fluids.
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