J Chromatogr A 1653, 462442 (2021). Samples were peptides obtained through tryptic hydrolysis of the 5 most abundant milk proteins: α-lactalbumin (α-LA), β-lactoglobulin (β-LG), α-, β- and κ-casein (CA). As standards, synthetic whey and pea (Pisum sativum, Fabaceae) peptides (selected based on the in silico tryptic digest of α-LA, β-LG, legumin A, and vicilin with one or zero miscleavages) were only used in the last assay for prediction of the RF values of peptides with known amino-acid (AA) sequences. Two-dimensional HPTLC on silica gel (pre-washed with methanol and activated 10 min at 100°), first with basic mobile phase sec-butanol – pyridine – ammonia – water 39:34:10:26, and (after 12h drying) in the orthogonal direction with acidic mobile phase sec-butanol – pyridine – acetic acid – water 11:8:2:5. Derivatization for peptides and proteins by immersion into fluorescamine (0.05 % in acetone); visualization under UV 254 nm and 365 nm. Computer-assisted determination of the x- and y-coordinates of the derivatized zones. Repeatability (n=8) of the 2D-HPTLC was statistically tested with the Kolmogorov-Smirnov test for normal distribution and with Dixon’s Q test for outliers. Relative standard deviation (RSD) for the RF values was 12.9 % for the first dimension (y-coordinates) and 16.5 % for the second dimension (x-coordinates). According to their higher intensity and sharpness, 15 – 20 detected zones from each protein hydrolyzate were selected, manually scraped from the derivatized layer, dissolved in formic acid solution (0.1 % in acetonitrile – water 3:2), mixed with an equal volume of matrix (dihydroxybenzoic acid 2 % in acetonitrile – water 3:7), crystallized on air on a ground steel target, before being desorbed by the laser beam of the MALDI-TOF-MS/MS (matrix-assisted laser desorption/ionization time-of-flight tandem mass spectrometry). Direct hyphenation of HPTLC to MS was not performed, to avoid zone diffusion during plate coating with the matrix and to circumvent the stronger binding of polar peptides on the layer.  The MS spectra were acquired in positive reflector mode in m/z range 340 – 4000 (10 – 2500 for fragments), using an external peptide as calibration standard. Identification of 51 from the 85 selected peptides according to AA sequences was performed, using software programs allowing m/z calculation of protein fragments and estimation of cleavage sites. Correlation of the retention behaviour of the peptides with their properties (molecular weight MW, isoelectric point IEP, charges, polarity) was tested with Student’s two-sided t-test after calculation of Pearson’s correlation coefficients. The correlation was significant with IEP, percentages of anionic AA and of non-polar AA; but not with the following properties: MW, percentages of cationic AA and of uncharged polar AA. Finally, based on the correlation results, regression formulas were found to calculate the x- and y-coordinates of any known peptide from the percentage of non-polar AA (or vice-versa). The prediction power of these formulas was verified by repeating the complete 2D-HPTLC-MS experiment with the standard peptides of whey and of peas, and measuring the absolute and relative deviations between the actual x- and y-coordinates and the predicted values. The absolute deviations were higher in the lower RF zones. The average, relative RF value deviations (range 22.1 – 25.7 %) were not different between whey and pea peptides.

Food Chem. 133784 (2022). Review of enzymatic protein hydrolysis, including standard and conventional techniques and their applications and insights into new strategies of detection and characterization of bioactive peptides. The paper described HPTLC methods coupled with bioassays in effect-directed analysis (EDA) to detect the bioactivities of peptides.

J Chromatogr A, 1605, 460366 (2019). Samples were standards and complex mixtures of non-ribosomally synthesized cyclic lipopeptides (CLPs) from Bacillus strains (Bacillaceae) found in soil or in manure: B. amyloliquefaciens (SS-12.6, SS-13.1, SS-27.2, SS-38.4) and B. pumilus (SS-10.7). Two extraction methods were compared: ethyl acetate extraction (Ex1), and the acidic precipitation followed by methanol extraction (Ex2). HPTLC on silica gel with chloroform – methanol – water 65:25:4. Detection under white light, UV 254 nm and 366 nm. Absorption densitometry measured at 190 nm. Derivatization for peptides, amino acids and amino derivatives, by immersion into ninhydrin – collidine reagent (ninhydrin 0.3 %, collidine 5 %, acetic acid 5 %, in ethanol), followed by heating 5 min at 110 °C. Effect-directed analysis using automated immersion: A) for free radical (DPPH•) scavengers; B) for enzymatic inhibition (acetyl-cholinesterase, α-glucosidase); C) for activity against Gram-negative (Aliivibrio fischeri bioluminescence assay) or Gram-positive bacteria (Bacillus subtilis bioassay). Active bands were eluted with methanol through the oval elution head of a TLC-MS interface pump, into a quadrupole-Orbitrap mass spectrometer. Full scan mass spectra (m/z 200−2000) in positive and in negative ionization modes were recorded using heated electrospray ionization (HESI, spray voltage 3.5 kV, capillary temperature 270 °C). Active zones were assigned to be CLPs: iturin A, surfactin dimethyl-ester, and surfactin, fengycin and kurstakin homologues. Ex1 provided richer extracts compared to Ex2. Standards were seen to contain a free radical scavenging impurity.

J. Liq. Chromatogr. Relat. Technol. 31, 752-762 (2008). TLC and HPTLC of nine dipeptides (gly-gly, ala-gly, pro-leu, pro-asp, pro-gly, leu-pro, ala-pro, phe-pro, val-pro) on silica gel with ethanol - dichloromethane 2:1 and methanol - dichloromethane 1:1 in a horizontal chamber saturated for 20 min. Detection by spraying with sodium azide and starch solution (25 mL aqueous starch solution, containing 2.5 g starch, was added to 20 mL aqueous sodium azide solution containing 2 g sodium azide, the mixture was adjusted to pH 5.5 with 0.1 mol/L hydrochloric acid and diluted to 50 mL with water to obtain 4 % and 5 % solution for sodium azide and starch, respectively). All solutions were prepared fresh daily. The limit of detection was 2-200 pmol/spot for the iodine azide procedure, 1-100 pmol/spot for iodine, 20-2000 pmol/spot for UV 254 nm, and 40-1000 pmol/spot for spraying with ninhydrine and drying at 110 °C .

Anal. Chem. 324, 339-340 (1986). TLC of dipeptides on a) microcrystalline cellulose with pyridine - water 2:1 and 4:1 and b) on chiral plate with methanol - water - acetonitrile 5:5:20 and 5:5:3. Detection with ninhydrin reagent.

J. Chromatogr. 448, 11-30 (1988). TLC of proteinogenic and non-proteinogenic amino acids, dipeptides and a-hydroxy acids. Separation of the enantiomers, without derivatization, on a chiralplate. Quantification by densitometry. Determination of the respective enantiomers at trace levels (0,25%). Detection limits: 0.1% of the minor enantiomer. Other examples from the field of +-methyl, N-alkyl and halogenated amino acids.

J. of Medicinal Chemistry 32, 2555-2561 (1989). TLC of cyclic melanotropin analogues an silica with butanol - acetic acid - water 4:1:5 (upper phase), butanol - acetic acid - water - pyridine 15:3:12:10, butanol - pyridine - acetic acid - water 5:5:1:4. Visualization under UV and by spraying with ninhydrin.

Acta Chimica, 127, 803-812 (1990). TLC of peptides on silica with ethyl acetate - pyridine - acetic acid - water 333:20:6:11, chloroform - methanol 9:1 or 1:9, butanol - acetic acid - water 4:1:1, butanol - acetic acid - water - pyridine 30:6:24:20. ethyl acetate - pyridine - acetic acid - water 148:20:6:11 and butanol - acetic acid - water 1:1:1. Detection by spraying with a 0.3% ninhydrin solution in acetone and with starch - KI reagent.