Peptide

A peptide is a molecule formed of several amino acids linked by peptide bonds (amide bonds between one amino acid's carboxyl group and the next one's amine group). It is the fundamental building block of the living world: all proteins are large peptides (polypeptides), and peptide chemistry underpins hormones, neurotransmitters, growth factors, cytokines, venoms, natural antibiotics and a growing share of modern pharmacopeia.

The peptide vs protein distinction is gradual rather than sharp: peptide is generally used up to 50 amino acids, polypeptide between 50 and 100, protein beyond. A short peptide (2-20 aa) is called oligopeptide, with precise subcategories (dipeptide, tripeptide, etc.). This terminology influences how the molecule is synthesized (SPPS for short peptides, recombinant expression for proteins) and analytically studied.

Peptides play major biological roles. Hormonal signaling: insulin, glucagon, GLP-1, GIP, somatostatin, oxytocin, vasopressin, PTH, calcitonin. Neurotransmission: enkephalins, endorphins, substance P, neuropeptide Y, VIP. Immune defense: defensins, cathelicidins, antimicrobial peptides. Growth regulation: IGF-1, IGF-2, EGF, GHK-Cu. Maturation factors: BPC-157, thymosins.

In RUO research, synthetic and recombinant peptides constitute irreplaceable experimental tools: selective receptor agonists and antagonists, specific enzyme inhibitors, localization probes, affinity ligands for purification. Their high selectivity (far above classical small molecules), low intrinsic toxicity and capacity to faithfully reproduce endogenous biological motifs make them a foundational pillar of contemporary molecular biology, pharmacology and therapeutic development.

Physicochemical characterisation of a peptide relies on several quantifiable parameters: monoisotopic molecular mass (ESI-HRMS or MALDI-TOF, ± 1 ppm), isoelectric point (pI) calculable from sequence, log D predictive of water/octanol solubility, dominant secondary structure (α-helix, β-sheet, random coil) estimated by circular dichroism (CD), conformational stability by DSC or DLS. In analytical RP-HPLC, each peptide shows a characteristic retention time reproducible to ± 0.1 min on C18 column with ACN/H2O/0.1% TFA gradient.

Molecular mass conditions several practical parameters: nmol/kg dosing (conversion from µg/kg = peptide mass), reconstitution volume calculation, molecular filtration (3 kDa, 10 kDa ultrafiltration), SEC-HPLC column selection. On a Certificate of Analysis (COA), complete sequence, theoretical mass and HPLC purity are the basic data to check. Conjugated peptides (PEG-MGF, fluorescein-peptide, biotin-peptide) show apparent masses higher than the sum of native masses, with impact on pharmacokinetic profile and target receptor recognition.