HBTU (0.52 g, 1.37 mmol) and HOBt (0.18 g, 1.37 mmol) and the solution was left for stirring at RT for 15 min.
HBTU (1.0 g, 1.95 mmol) and HOBt (1.0 g, 1.95 mmol), and the solution was left for stirring at RT for 15 min.
Double couplings were systematically performed with a 5-fold excess of Fmoc-amino acid or [O.sub.2]Oc in the presence of HBTU (5 eq) and DIEA (10 eq) with DMF as solvent for 2 x 30 min.
Hydroxyl-protected PMM 10 (190.4 mg, 200 [micro]moL) was coupled to the Asp [beta]-carboxyl group of 4 in the presence of HBTU (75.8 mg, 200 [micro]moL) and DIEA (70 pL, 400 [micro]moL) in DMF for 2 h, followed by extensive DMF and C[H.sub.2][Cl.sub.2] washes.
Abbreviation Ac-P1 to Ac-P4: N-Acetyl-pentapeptides [IC.sub.50]: Half maximal inhibitory concentration HQ: Dihydroxybenzene Phe: Phenylalanine AC: Acetyl group S: Serine R: Arginine F: Phenylalanine K: Lysine L-DOPA: L-3,4-Dihydroxyphenylalanine DMSO: Dimethyl sulfoxide MTT: 3-(4,5-Dimethylthiazol-2-yl)-2,5- diphenyltetrazolium bromide [alpha]-MSH: Alpha-melanocyte-stimulating hormone PBS: Phosphate buffer solution TIS: Triisopropylsilane TFA: Trifluoroacetic acid Fmoc: 9-Fluorenylmethyloxycarbonyl- (Fmoc-) protected amino acids HBTU: 2-(1H-Benzotriazol-1-yl)-1,1,3,3- tetramethyluronium hexafluorophosphate HOBt: 1-Hydroxybenzotriazole SPPS: Solid phase peptide synthesis DMEM: Dulbecco's modified Eagle's medium PS: Proline-serine VS: Valine-serine MCR-1: Melanocortin 1 receptor.
Noble, "HBTU activation for automated Fmoc solid-phase peptide synthesis," Peptide Research, vol.
Each coupling reaction was accomplished using a 3-fold excess of amino acid with HBTU and HOBt in the presence of DIEA (6 eq.).
HBTU: 2-(1H-benzotriazole-1-yl)-1,1,3,3-tetramethyluronium hexafluoro-phosphate
Fmoc-amino acids were attached as in (B), except that HBTU was used instead of HATU in the same concentration.
HATU, HBTU, and HCTU were employed as more efficient acylation activators, among which HATU is the most and HBTU is the least (close to DIC) efficient.
Fmoc amino acids, 1-hydroxybenzotriazol (HOBt), 2-(1H-benzotriazol-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate (HBTU), and ninhydrin test kits were obtained from AnaSpec.
The N termini were acylated by using acetic anhydride (10 mol equivalents) with HOBt (10 mol equivalents), HBTU (5 mol equivalents), and DIEA (10 mol equivalents) in DMF for 2 h.