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A tetrapeptide is a short chain of four amino acids linked together by peptide bonds. This sequence begins with an N-terminus, which is the free amino group ( H_2N- ) of the first amino acid, and ends with a C-terminus ( -COOH ), the free carboxyl group of the last amino acid. In the tetrapeptide Ala-Pro-Phe-Gly, each amino acid plays a specific role within the structure. Alanine, proline, phenylalanine, and glycine have unique side chains that define their structural and chemical characteristics.
The order of these amino acids is crucial, as it influences the peptide's overall properties and functionality. In simplifying the concept, picture the tetrapeptide as a string where each bead represents a different amino acid. Attached tightly together through specific links, they maintain the structure and functionality of the chain.

Amino acids are the building blocks of peptides and proteins. Each amino acid consists of a central carbon atom, also known as the alpha-carbon. This carbon is bonded to:

• an amino group ( H_2N- )
• a carboxyl group ( -COOH )
• a hydrogen atom
• an R group or side chain

The side chain varies between different amino acids and determines their unique characteristics. In our tetrapeptide example, we have:

• Alanine (Ala) with a simple -CH_3 side chain
• Proline (Pro) with a unique cyclic structure
• Phenylalanine (Phe) with a benzene ring
• Glycine (Gly) with just a hydrogen as its side chain

These side chains contribute to how the amino acids interact with each other and with external environments, impacting the stability and reactivity of the peptide.

Peptide bond formation is a dehydration reaction that links amino acids together by removing a molecule of water ( H_2O ). This bond specifically occurs between the carboxyl group ( -COOH ) of one amino acid and the amino group ( H_2N- ) of the next. In doing so, a covalent bond is formed, represented as a -CONH- linkage.
The process involves removing an -OH from the carboxyl group and an -H from the amino group, facilitating the release of water. Peptide bonds are stable under physiological conditions, meaning they don’t easily break apart, which is crucial for maintaining the peptide's structural integrity. Each peptide bond contributes to the overall shape and function of the protein or peptide chain in which it resides.