Peptides are short chains of amino acids joined by peptide bonds — the same fundamental building blocks and linkages found in proteins, but in shorter sequences that confer distinct chemical and biological properties. They sit at a productive intersection of chemistry and biology that has made them one of the most intensively studied classes of bioactive compound in modern pharmacological research.
Amino acids: the building blocks
There are 20 standard amino acids used in the synthesis of proteins and peptides in biological systems. Each consists of a central carbon atom (the alpha-carbon) bonded to: an amino group (-NH2), a carboxyl group (-COOH), a hydrogen atom, and a side chain (R group) that is unique to each amino acid and determines its chemical properties. The R group may be hydrophobic (e.g., valine, leucine, isoleucine), hydrophilic (e.g., serine, threonine), charged (e.g., aspartic acid, lysine), or aromatic (e.g., phenylalanine, tryptophan).
The sequence of amino acids in a peptide or protein — the primary structure — determines all subsequent structural and functional properties. A single amino acid change can alter receptor binding affinity, metabolic stability, solubility, and immunogenicity.
How peptides differ from proteins: structural implications
Proteins are distinguished from peptides not just by length but by structural complexity. A protein folds into a defined three-dimensional structure — the native conformation — driven by hydrophobic interactions, hydrogen bonds, disulfide bridges between cysteine residues, and electrostatic interactions. This folded structure is essential for protein function: enzymes, receptors, antibodies, and structural proteins all depend on precise three-dimensional architecture.
Peptides are generally too short to adopt stable secondary or tertiary structures in isolation. They are largely flexible and linear in solution. This has several practical implications. Peptides are easier to synthesise chemically (solid-phase peptide synthesis is well-established), more amenable to modification (individual amino acids can be substituted without disrupting a required fold), and often more stable to heat and denaturants than proteins (because they do not rely on a folded structure). They are, however, typically more susceptible to proteolytic degradation by enzymes that cleave peptide bonds.
Receptor interactions and selectivity
Many endogenous peptides function as signalling molecules, interacting with specific receptors to trigger cellular responses. The receptor-peptide interaction is determined by the three-dimensional complementarity between the peptide's shape and charge distribution and the receptor's binding site. Because peptides are structurally defined by their amino acid sequence, changes to the sequence can significantly alter receptor affinity, selectivity, and functional outcome.
This structural specificity is a key reason peptides attract research interest. A well-designed peptide can interact with a specific receptor target with high selectivity, avoiding off-target interactions that complicate interpretation. This is more difficult to achieve with small molecules, which interact primarily through hydrophobic and electrostatic contacts without the directional hydrogen bonding patterns that define peptide-receptor interactions.
Synthetic vs naturally occurring peptides
The research peptides supplied by Nexapep are synthetic — produced by solid-phase peptide synthesis (SPPS) rather than extracted from biological sources. SPPS builds the peptide chain sequentially from the C-terminus to the N-terminus on a solid resin support, adding one amino acid at a time using coupling chemistry. After synthesis, the peptide is cleaved from the resin, deprotected (protecting groups added during synthesis are removed), and purified by preparative HPLC.
Synthesis offers several advantages for research applications: exact sequence specification, high purity (relative to extraction from biological material), batch-to-batch consistency, and the ability to incorporate non-natural amino acids, modifications, or isotopic labels that are not accessible through biological production.
Why lyophilisation for research peptides
Peptides in aqueous solution are subject to multiple degradation pathways: hydrolysis of the peptide bond (particularly at aspartyl-prolyl and aspartyl-glycyl sequences); oxidation of methionine, cysteine, and tryptophan residues; deamidation of asparagine and glutamine; aggregation driven by hydrophobic interactions; and microbial growth in non-sterile solutions. Collectively, these mechanisms mean that a peptide dissolved in water and stored at room temperature may have a usable research window of days to weeks.
Lyophilisation addresses this by removing the water that enables most of these degradation pathways. The residual water content of a well-lyophilised peptide is typically below 3%, which is sufficient to inhibit most chemical and biological degradation. Under these conditions, and with appropriate cold storage (-20°C), a lyophilised peptide is stable for 2 years or more. All Nexapep products are supplied in lyophilised form.
Molecular weight and why it matters for quality
Every peptide has a theoretical molecular weight calculable from its amino acid sequence. For BPC-157 (Gly-Glu-Pro-Pro-Pro-Gly-Lys-Pro-Ala-Asp-Asp-Ala-Gly-Leu-Val), this is 1419.53 g/mol. Mass spectrometry measures the actual molecular weight of the compound in a sample. If the observed and theoretical values match (within instrument tolerance, typically <0.5 Da), this confirms that the compound is the correct sequence. If they differ, the compound is not what is claimed. This is why molecular weight data is an essential component of complete quality documentation.