<P> The variation in titration curves when the amino acids can be grouped by category . With the exception of tyrosine, using titration to distinguish among hydrophobic amino acids is problematic . </P> <P> At pH values between the two pKa values, the zwitterion predominates, but coexists in dynamic equilibrium with small amounts of net negative and net positive ions . At the exact midpoint between the two pKa values, the trace amount of net negative and trace of net positive ions exactly balance, so that average net charge of all forms present is zero . This pH is known as the isoelectric point pI, so pI = 1⁄2 (pKa + pKa). The individual amino acids all have slightly different pKa values, so have different isoelectric points . For amino acids with charged side chains, the pKa of the side chain is involved . Thus for Asp, Glu with negative side chains, pI = 1⁄2 (pKa + pKa), where pKa is the side chain pKa . Cysteine also has potentially negative side chain with pKa = 8.14, so pI should be calculated as for Asp and Glu, even though the side chain is not significantly charged at neutral pH . For His, Lys, and Arg with positive side chains, pI = 1⁄2 (pKa + pKa). Amino acids have zero mobility in electrophoresis at their isoelectric point, although this behaviour is more usually exploited for peptides and proteins than single amino acids . Zwitterions have minimum solubility at their isoelectric point and some amino acids (in particular, with non-polar side chains) can be isolated by precipitation from water by adjusting the pH to the required isoelectric point . </P> <P> Amino acids are the structural units (monomers) that make up proteins . They join together to form short polymer chains called peptides or longer chains called either polypeptides or proteins . These polymers are linear and unbranched, with each amino acid within the chain attached to two neighboring amino acids . The process of making proteins encoded by DNA / RNA genetic material is called translation and involves the step - by - step addition of amino acids to a growing protein chain by a ribozyme that is called a ribosome . The order in which the amino acids are added is read through the genetic code from an mRNA template, which is an RNA copy of one of the organism's genes . </P> <P> Twenty - two amino acids are naturally incorporated into polypeptides and are called proteinogenic or natural amino acids . Of these, 20 are encoded by the universal genetic code . The remaining 2, selenocysteine and pyrrolysine, are incorporated into proteins by unique synthetic mechanisms . Selenocysteine is incorporated when the mRNA being translated includes a SECIS element, which causes the UGA codon to encode selenocysteine instead of a stop codon . Pyrrolysine is used by some methanogenic archaea in enzymes that they use to produce methane . It is coded for with the codon UAG, which is normally a stop codon in other organisms . This UAG codon is followed by a PYLIS downstream sequence . </P>

What are the chemical structure and characteristics of amino acids