L-Proline (100 equiv.) was added to an aqueous solution of betalamic acid (0.5–1.0 mL, pH 10), and the resulting solution was stirred at room temperature (RT). The reaction was spectrophotometrically monitored by the depletion of the HBt absorption band at 430 nm and the concomitant appearance of the BtP band at 485 nm. After completion (ca. 30 min), the solution was cooled (0°C), and HOAc (conc.) was slowly added until the solution reached pH 5. The resulting solution was then stirred at RT for 1 h. The product was purified by reversed-phase column chromatography (C18 silica gel, water as eluent), and the fractions containing BtP were combined and lyophilized. The resulting orange powder was stored at −20°C, and the purity of the product was assessed by RP-HPLC analysis prior to use. In a typical run, 0.5 mg (2.5 µmol) of betalamic acid yielded 0.7 mg (2 µmol, 80%) of BtP.
1H-NMR (400 MHz, D2O/TMSP-d4, according to Stintzing et al. ): δ 8.34 (d, 1H, 3J7′,8′ = 12.4 Hz, H-7′), 8.24 (d, 1H, 3J7,8 = 12.1 Hz, H-7), 6.20 (s, 1H, H-14), 6.16 (s, 1H, H-14), 6.08 (d, 1H, 3J7,8 = 12.1 Hz, H-8), 5.80 (d, 1H, 3J7′,8′ = 12.4 Hz, H-8′), 4.47 (t, 2H, 3J10,11 = 6.6 Hz, overlapped H-11 and H-11′), 3.76–3.71 (m, 2H, H-5), 3.33 (dd, 2H, 2J10a,10b = 17.2 Hz, 3J10a,11= 6.6 Hz, H-10a), 3.11 (dd, 2H, 2J10a,10b = 17.2 Hz, 3J10b,11 = 6.6 Hz, H-10b), 2.20–2.27 (m, 2H, overlapped H-3 and H-3′), 2.27–2.37 (m, 1H, H-3′), 2.43–2.54 (m, 1H, H-3′), 2.04–2.20 (m, 1H, H-4), 2.00–2.15 (m, 1H, H-4′).
HRMS (m/z): [M+H]+ calculated for C14H17N2O6+: 309.1087; observed: 309.1082 (dif.: 1.62 ppm).
LC-ESI(+)-MS: Rt = 6.2 min, (m/z) [M+H]+ observed: 309.1.
Fluorescence: λmaxfl 520 nm (water, λexc 510 nm), ΦFl = 4.6×10–3 (water, pH = 6) vs.fluorescein.