The simple and widely distributed tripeptide glutathione (first entry in the following table), is interesting because the side-chain carboxyl function of the N-terminal glutamic acid is used for the peptide bond. Natural peptides of varying complexity are abundant. Simple statistical probability indicates that the decapeptides made up from all possible combinations of these amino acids would total 20 10! When all twenty of the natural amino acids are possible components of a peptide, the possible combinations are enormous. Neither of the component amino acids is sweet (Phe is actually bitter), and derivatives of the other dipeptide (Phe-Asp) are not sweet.Ī tripeptide composed of three different amino acids can be made in 6 different constitutions, and the tetrapeptide shown above (composed of four different amino acids) would have 24 constitutional isomers. The methyl ester of the first dipeptide (structure on the right) is the artificial sweetener aspartame, which is nearly 200 times sweeter than sucrose. Thus, aspartic acid (Asp) and phenylalanine (Phe) may be combined to make Asp-Phe or Phe-Asp, remember that the amino acid on the left is the N-terminus. For example, a dipeptide made from two different amino acids may have two different structures. This aspect of peptide structure is an important factor influencing the conformations adopted by proteins and large peptides.īecause the N-terminus of a peptide chain is distinct from the C-terminus, a small peptide composed of different aminoacids may have a several constitutional isomers. The color shaded rectangles in the lower structure define these regions, and identify the relatively facile rotations that may take place where the corners meet (i.e. This keeps the peptide links relatively planar and resistant to conformational change. As shown in the following diagram, nitrogen electron pair delocalization into the carbonyl group results in significant double bond character between the carbonyl carbon and the nitrogen. This restriction is due to the rigid nature of the amide (peptide) bond. The conformational flexibility of peptide chains is limited chiefly to rotations about the bonds leading to the alpha-carbon atoms. The " Show Structure" button displays some bond angles and lengths that are characteristic of these compounds.
#COLOR CODE JMOL HELIX SEET FREE#
As expected, the free amine and carboxylic acid functions on a peptide chain form a zwitterionic structure at their isoelectric pH.īy clicking the " Grow Peptide" button, an animation showing the assembly of this peptide will be displayed. By convention, the amino acid component retaining a free amine group is drawn at the left end (the N-terminus) of the peptide chain, and the amino acid retaining a free carboxylic acid is drawn on the right (the C-terminus). A simple tetrapeptide structure is shown in the following diagram. If the amine and carboxylic acid functional groups in amino acids join together to form amide bonds, a chain of amino acid units, called a peptide, is formed.
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