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Transcript of Dipeptide Formation
CARBOXY GROUP AMINO GROUP E.g. CYSTEINE (an alpha-amino acid) Note: Both the carboxy and amino group are bonded to the same carbon atom. There are amino acids that have amino and carboxy groups which are NOT bonded to the same carbon atom, however these aren't called alpha-amino acids. The "alpha" represents that both the amino and carboxy are connected to the same carbon. A carboxy group can react with an amino group to form what is known as a peptide bond (also called a peptide link, or an amide link/bond). A peptide bond is written as CONH and during the formation of a peptide bond we lose two hydrogen atoms and one oxygen atom, which forms water, H2O. Note: R and R' represent the rest of the chain of the molecule. It could be any combination of atoms that is actually possible. All we are interested in here is the carboxy group on the first molecule and the amino group on the second molecule, so for simplicity we use R and R' to represent the rest of the molecules. When a peptide bond is formed between two amino acids we get what is known as a dipeptide. Side Notes:
1: If we have three amino acids (rather than 2) joined by peptide bonds this is called a tripeptide, as opposed to a dipeptide. If we have numorous amino acids joined by peptide bonds this is called a polypeptide. If more than 50 amino acids are joined by peptide bonds this is generally referred to as a protein.
2: If we have two different amino acids forming a dipeptide we can have 2 different dipeptides formed, since all amino acids have both a carboxy group AND an amino group. For example if we take cysteine and serine, the amino group in the cysteine molecule can bond with the carboxy group in the serine molecule OR the carboxy group in the cysteine molecule can bond with the amino group in the serine molecule.
3: A list of certain alpha-amino acids will be provided in the exam in your data booklet. Note 2: This is called a condensation reaction because 2 molecules combine to form a large molecule with a small molecule released. Amino Acid Amino Acid Dipeptide Amino Acid + Amino Acid
= Dipeptide + Water (For Unit 3 VCE Chemistry)