Proline is an organic acid classed as a proteinogenic amino acid, although it does not contain the amino group -NH ₂ but is rather a secondary amine. The secondary amine nitrogen is in the protonated form under biological conditions, while the carboxyl group is deprotonated at physiological pH. Proline has molecular formula C5H9NO2 and a nominal molar mass of 115.13 g mol 1. Its side chain forms a five membered pyrrolidine ring that tethers back to the alpha carbon, constraining backbone phi angles and promoting turns and kinks in polypeptide chains. Typical pKa values are about 1.95 for the carboxyl group and about 10.6 for the secondary amine, giving an isoelectric point near 6.3. The unique cyclic structure increases the statistical occurrence of cis peptide bonds compared with other residues and stabilizes type I and II beta turns. Common analytical methods used for identification and quantification include amino acid analysis, reversed phase HPLC, LC MS and NMR; chiral HPLC or polarimetry are used to determine enantiomeric purity.
Parent: Proline
Typical quality specifications vary by grade and supplier; consult the certificate of analysis for lot specific numbers. Representative target assay for high quality L proline is ≥ 98.5% on a dry basis with enantiomeric excess such that D proline is usually ≤ 0.2%. Typical limits for individual related compounds are in the subpercent range, for example hydroxyproline and other amino acid contaminants commonly ≤ 0.5% each, pyroglutamic acid and N acetylated derivatives often ≤ 0.3% each, and total related compounds generally controlled to ≤ 1.0%. Moisture content is commonly specified below 0.5 to 1.0% and inorganic residue or ash below 0.2%. Heavy metals are typically controlled to single digit parts per million levels, for example ≤ 10 ppm, depending on application. These impurities are quantified by HPLC, LC MS, capillary electrophoresis and elemental analysis and monitored to meet application specific thresholds.
Proline is used as a structural determinant in proteins where it induces turns and limits backbone flexibility, as a major component of collagen and gelatin based materials, as an osmoprotectant and stress protectant in some microbes and plants, and as a chiral building block and stereocenter in organic synthesis; it is also used as a stabilizer and excipient in certain pharmaceutical formulations.
Foods high in proline are typically collagen rich or protein dense items such as gelatin, bone broth, connective tissue from meat and poultry, pork, beef, chicken, dairy products like cottage cheese and milk protein concentrates, eggs, and plant sources including soy and certain legumes; collagen supplements are concentrated sources.
Proline is a proteinogenic, non essential amino acid classified as a cyclic imino acid with a secondary amine in its side chain; it is considered non polar and aliphatic and is notable for imposing conformational constraints on peptide backbones.
Proline and lysine are both proteinogenic amino acids but with distinct chemistries: proline is a cyclic secondary amine that influences protein conformation and promotes turns, while lysine is a basic, positively charged amino acid with a primary epsilon amino group that participates in electrostatic interactions and post translational modifications such as acetylation and methylation; both play specific structural and functional roles in proteins.
