Proline
Also known as Proline, L-Proline, (S)-Pyrrolidine-2-carboxylic acid
“CIR Expert Panel says: safe as used in cosmetics.”
Proline (CAS 147-85-3; L-Proline) is the only proteinogenic amino acid with a secondary amine — its pyrrolidine ring makes it uniquely rigid and interrupts alpha-helical structures, which is why it is critical to collagen's triple-helix stability. The CIR Expert Panel assessed all 21 α-amino acids in cosmetics (Burnett et al., IJT 32(6 Suppl):41S-64S, 2013; PMID 24335967); Proline is named in the study scope and the Panel concluded these amino acids are safe as used in the present practices of use and concentration. The QRT records Proline with Finding=S (Safe), no qualifying conditions, citation IJT 32(S4):41-64, 2013. Table 4 of the CIR assessment shows 279 total uses for Proline at up to 2% concentration in cosmetics. A 2024 review (PMID 38534748) confirms proline's structural role as a principal constituent of collagen, underpinning its use in anti-aging and skin-conditioning cosmetic formulations.
Secondary amine with cyclic pyrrolidine ring — the only proteinogenic amino acid with this structure; the ring restricts backbone conformation, making proline essential to collagen triple-helix formation and stability
Collagen synthesis precursor: proline and its hydroxylated derivative hydroxyproline together account for approximately 25% of collagen's amino acid composition; marketed as a topical collagen-support ingredient
Skin-conditioning and humectant: free amino acids including proline are endogenous NMF (natural moisturizing factor) components in the stratum corneum; attracts and retains water
CIR Expert Panel concluded proline is safe as used in cosmetics in the present practices of use and concentration:41-64, 2013; QRT Finding=S); used at up to 2% in 279 cosmetic products per Table 4 usage data
Excellent tolerability profile: as an endogenous human amino acid, proline is non-reactive under normal cosmetic conditions
- · No known sensitization at cosmetic use concentrations — the CIR panel found no dermal irritation or sensitization signals for α-amino acids as a class
Marketed collagen-precursor claims (proline as a topical source for collagen synthesis) are biologically plausible but not directly demonstrated for topical application; the CIR safety scope does not assess efficacy claims
CIR Quick Reference Table (12/2017, revised 07/2018) — Proline row: Finding=S, Citation=IJT 32(S4):41-64, 2013
“Proline | S | | IJT 32(S4):41-64, 2013”— QRT-122017revised072018.pdf, p. 112