purity peptide FAQ

'ND' (non-detectable) without a stated detection limit is functionally meaningless. A LAL assay with 10 EU/mg sensitivity that reports 'ND' could still contain 9 EU/mg of endotoxin. Far above acceptable research thresholds. Contact the supplier and request the exact LAL method used, the assay sensitivity (limit of detection), and a numeric result. If they can't provide it, assume the test wasn't performed. For LL-37 used in immune or infection models, unquantified endotoxin contamination is a deal-breaker. The peptide's own immunomodulatory effects overlap entirely with LPS-triggered responses.

For research-grade applications, 96.5% falls below the 98% threshold that ensures minimal interference from synthesis byproducts. Request a chromatogram and inspect the impurity profile. If satellite peaks represent simple acetate or TFA counterions, the impact may be negligible. If they represent deletion sequences or oxidized residues, those impurities alter the peptide's α-helical propensity and membrane interaction, directly affecting antimicrobial activity. Our standard is to reject batches below 97.5% unless the impurity profile is explicitly characterized and deemed inert.

A +1.8 Da shift suggests oxidation of a residue that shouldn't oxidize in LL-37's sequence, or the presence of a sodium or potassium adduct from the ionization process rather than true mass variance. Check whether the COA reports the deconvoluted neutral mass or an adduct ion. ESI-MS often displays [M+Na]⁺ or [M+K]⁺ peaks. If the reported mass is the true monoisotopic mass after deconvolution and it's still 4495 Da, that batch contains a structural error. Likely an extra oxygen atom from oxidation during synthesis or storage. Do not use it in experiments where LL-37's precise structure matters.

Compare peptide content percentages rather than HPLC purity. Two peptides both labeled 99% pure can differ by 15–20% in actual peptide mass per vial depending on counterion load and purification method. Request amino acid analysis or quantitative NMR data if not provided on the standard COA. The vendor with higher peptide content delivers more usable compound per dollar even if the nominal purity numbers are identical. Additionally, check whether both vendors provide mass spectrometry confirmation. Sequence accuracy matters more than an extra 0.5% chromatographic purity for most research applications.

Check the peptide content percentage on the certificate of analysis. If it's below 80%, you're underdosing by 20% or more because counterion salts and bound water comprise a significant portion of vial mass. Recalculate your working concentration using peptide content rather than nominal purity: if your vial is labeled 10mg at 99% purity but peptide content is 75%, your actual peptide mass is 7.5mg. This concentration mismatch is the most common cause of 'low activity' reports that turn out to be dosing errors rather than peptide degradation.

Request the mass spectrum before using the peptide in critical experiments. HPLC purity alone cannot confirm sequence accuracy or detect deletion sequences with similar retention times. A peptide showing 99% chromatographic purity may contain 3–5% deletion sequences that mass spec would reveal as satellite peaks below the target molecular weight. Those deletion sequences can act as competitive inhibitors or partial agonists depending on the pathway, introducing experimental artifacts that appear as protocol failures rather than compound quality issues.