A peptide is a biologically occurring chemical compound consisting of two or more amino acids linked by peptide bonds.

The Chemistry of Peptides
Scientifically, a peptide bond is a covalent bond formed between two amino acids. This occurs when the carboxyl group (C-terminus) of one amino acid reacts with the amino group (N-terminus) of another in a process known as a condensation reaction, releasing a water molecule.

The resulting connection is a CO-NH bond, forming a peptide (or amide) molecule. Consequently, peptide bonds are also classified as amide bonds.

Origins and Function
The term "peptide" is derived from the Greek word πέσσειν, meaning "to digest." Peptides are fundamental to nature and biochemistry; thousands occur naturally within human and animal physiology.

Beyond nature, new peptides are regularly discovered and synthesized in laboratory settings. This continuous discovery and innovation in peptide research holds immense promise for the future of scientific exploration and pharmaceutical development.

Peptide formation occurs through two distinct pathways: organic biological processes and synthetic laboratory methodologies.

1. Biological Formation
The human body naturally manufactures peptides to regulate various physiological functions. These naturally occurring compounds are generally categorized as either ribosomal or non-ribosomal peptides, depending on the cellular machinery used to assemble them.

2. Chemical Synthesis
In the laboratory, modern chemistry allows for the precise construction of peptide sequences. While Liquid Phase Peptide Synthesis (LPPS) was an early method with specific advantages, Solid Phase Peptide Synthesis (SPPS) has emerged as the prevailing standard in the industry today.

Historical Milestones in Synthesis

The ability to synthesize peptides artificially is a major scientific achievement marked by two key dates:

• 1901: Emil Fischer, in collaboration with Ernest Fourneau, reported the first synthetic peptide.
• 1953: Vincent du Vigneaud achieved the synthesis of Oxytocin, marking the first time a polypeptide was successfully synthesized.

Peptides are categorized based on their size—specifically, how many amino acids are linked together in the chain.

The Breakdown

• Dipeptide: The shortest possible peptide, consisting of just two amino acids.
• Tripeptide: A peptide chain made of three amino acids.
• Oligopeptide: A term used for "short" peptides, generally containing fewer than ten amino acids.
• Polypeptide: A longer chain, typically composed of ten or more amino acids.

When Does a Peptide Become a Protein?
Once a chain grows much larger—usually exceeding 40 to 50 amino acids—it is generally classified as a protein. However, these rules aren't always set in stone.

The Grey Area
Biology often has exceptions. For instance, Amyloid Beta is a longer chain but is still classified as a peptide. Conversely, Insulin is a small protein that is frequently referred to as a peptide.

Peptides are generally categorized by their method of biosynthesis. While there are many variations, they typically fall into four primary classes:

• Ribosomal Peptides: Synthesized through the translation of mRNA, these peptides often function as hormones and signaling molecules (such as opioids, pancreatic peptides, and calcitonin). Before reaching their mature form, they frequently undergo proteolysis (breakdown). Antibiotics like microcins also fall into this category.
• Nonribosomal Peptides: Unlike their ribosomal counterparts, these are assembled by peptide-specific enzymes rather than the ribosome. They are common in plants, fungi, and unicellular organisms. Structurally, they are often cyclic and highly complex, though linear forms exist. The most well-known nonribosomal peptide is Glutathione, a vital antioxidant.
• Milk Peptides & Peptones: These are derived from the breakdown of proteins. Milk peptides are formed when digestive enzymes or lactobacilli (during fermentation) process milk proteins. Similarly, peptones are created through the proteolytic digestion of animal milk or meat and are commonly used in laboratories as nutrients for bacterial growth.
• Peptide Fragments: These are specific pieces of proteins resulting from enzymatic degradation. While they can occur naturally, they are most often identified as the products of controlled laboratory degradation.

• Amino Acids: The fundamental building blocks of all peptides. These molecules contain both amine and carboxyl functional groups. Specifically, alpha-amino acids link together to construct peptide chains.
• Peptide Bond: The chemical link that holds amino acids together. This covalent bond forms when the carboxyl group of one amino acid reacts with the amino group of another, releasing a water molecule in a process known as a condensation reaction.
• Peptide Sequence: The specific order in which amino acid residues are arranged and connected within the peptide chain.
• Cyclic Peptides: A polypeptide chain where the amino acid sequence forms a circular ring structure rather than a linear line. Notable examples often used in research include Melanotan-2 and PT-141 (Bremelanotide).
• Peptide Library: A large collection of peptides containing systematic combinations of amino acids. These are typically created using Solid Phase Peptide Synthesis (SPPS) and are crucial tools for pharmaceutical screening and biochemical profiling.
• Peptide Mimetics: Synthetic or natural molecules designed to mimic the biological action of natural hormones, cytokines, or enzyme substrates. Mimetics are often engineered to improve stability or potency compared to the natural parent molecule.
• Peptide Mapping: A method used to identify or validate a peptide’s sequence. Enzymes are used to break the protein into smaller pieces, allowing researchers to examine the specific patterns of the resulting sequences.
• Peptide Fingerprint: A unique chromatographic pattern generated by partially hydrolyzing a peptide into fragments and mapping them in two dimensions. This serves as a unique identifier for specific proteins.