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Line 1: − + − To form [[protein]], the amino acids are linked by dehydration synthesis to form [[peptide]] bonds. The chain of amino acids is also known as a [[polypeptide]]. [http://biology.clc.uc.edu/courses/bio104/protein.htm]+ + + + + + + + + + + + + + + + + + + +
Merged with Amino Acids content
An '''amino acid''' is any [[molecule]] that contains both [[amino]] and [[carboxylic acid]] functional groups.
An '''amino acid''' is an [[organic compound]] containing an [[amine]] (-NH<sub>2</sub>) group and a [[carboxylic acid]] (-COOH) group. These two different [[functional groups]] make amino acids [[bifunctional]].
== α-amino acids ==
'''α-amino acids''' (α is the greek letter ''[[alpha]]'') have the general formula '''RCH(NH<sub>2</sub>)COOH'''. There are around 20 naturally occurring α-amino acids which [[polymer|polymerise]] by [[condensation polymerisation]] to form all known [[polypeptides]] (which form the primary structure of [[proteins]]) in biological organisms during [[gene translation]].
== Zwitterions ==
Due to the acidic carboxylic acid group and the alkaline amine group on amino acids they can form ions with themselves called '''zwitterions'''. The word zwitterion is derived from the German for [[hermaphrodite]], and in this case refers to the fact that amino acid zwitterions have both positive and negative [[charge|charges]].
=== Formation of zwitterions ===
Zwitterions from from amino acids when the hydrogen from the carboxylic acid group (-COO'''H''') dissociates from the molecule and forms a [[dative covalent bond]] with the nitrogen in the amine group (-'''N'''H<sub>2</sub>). This forms an ion with two charges: RCH(NH<sub>3</sub><sup>+</sup>)COO<sup>-</sup>.
=== Physical properties of α-amino acids due to zwitterions ===
The formation of zwitterions dramatically affects the physical properties of amino acids. The zwitterion is the predominant form in the [[solid phase]] and aqueous [[solution]]. Due to the two charges on zwitterions, there is a much stronger intermolecular attraction between the ions, raising the melting point of an amino acid far above the melting point of organic compounds with similar [[molecular mass]] and numbers of [[electron|electrons]]. For example, the amino acid [[glycine]] decomposes at 262°C (without melting) where propanoic acid melts at -21°C<ref>Chemistry 2, Ratcliff B. & Eccles H., 2001, Cambridge University Press, ISBN 0-521-79882-5</ref>.
== Optical isomerism of amino acids ==
All α-amino acids except glycine CH<sub>2</sub>(NH<sub>2</sub>)COOH have four different groups around the alpha carbon, forming a [[chiral centre]] and allowing [[optical isomerism]] to occur.
== References ==
<references/>
[[Category:Biochemistry]]
[[Category:Organic Chemistry]]