Fmoc-N-methyl-D-alanine

A high-performance liquid chromatography (HPLC) method is described for determining subpicomole concentrations of beta-N-methylamino-L-alanine (BMAA) in plant and animal tissue. BMAA and other amino acids were reacted with 9-fluorenylmethyl chloroformate (FMOC) for 10 min under alkaline conditions to form highly fluorescent and stable derivatives. All amino acids, including BMAA, eluted from the column within 22 min. BMAA (tr = 18.02 +/- 0.07 min) was detected in Cycas circinalis L. seed and in serum, cerebrospinal fluid and brain tissue from BMAA-treated monkeys and rats. The primary amino acids glutamine, glutamic acid, aspartic acid, alanine, glycine and gamma-aminobutyric acid (GABA) could also be detected since they were well resolved from BMAA. These amino acids and BMAA were linear over the concentration range of 0.15-7.5 microM with a relative standard deviation ranging from 2.1-6.7%. This method should prove useful in studies to determine the role of BMAA in the Western Pacific amyotrophic lateral sclerosis/Parkinsonism-dementia complex for which cycad seed is the principal etiological candidate.

5-(1-Hydroxy-pyridin-2(1H)-onyl)-l-alanine (Hop) is a N-hydroxy-1,2-pyridone functionalized α-amino acid with the desired metal-chelating properties of DOPA (3,4-dihydroxy phenylalanine) but without its unwanted redox activity. The Fmoc-protected amino acid Fmoc-l-Hop(tBu)-OH (11) was synthesized from glycine phosphonate followed by enzymatic hydrolysis of the methyl ester yielding the Hop l-isomer in 96 % ee. The amino acid 11 is used in automated peptide synthesis for the assembly of a 14mer β-hairpin peptide with the sequence [dsb1, 14 ]H-CHXETGKHGHKLVC-OH (X=W, l-Hop). While the 10 π electron containing indole side chain of l-Trp in peptide 14 completes the formation of a hydrophobic cluster and results in 90 % folding, the folded fraction is significantly decreased to approximately 30 % for the 6 π electron l-Hop side chain in peptide 16. Metal chelation of Ga3+ reconstitutes the folding of 16 to above 60 % due to the formation of the Ga(16)3 trimer. The chelation process of 16 is monitored by NMR spectroscopy and the subsequent release of Ga3+ by a competitive metal chelator exemplifies the reversible oligomerization of peptide epitopes by metal chelation, bearing the opportunity to synthesize protein-sized aggregates on the basis of reversible chemistry in water.

A synthetic strategy for the preparation of two orthogonally protected methyl esters of the non-proteinogenic amino acid 2,3-l-diaminopropanoic acid (l-Dap) was developed. In these structures, the base-labile protecting group 9-fluorenylmethyloxycarbonyl (Fmoc) was paired to the p-toluensulfonyl (tosyl, Ts) or acid-labile tert-butyloxycarbonyl (Boc) moieties. The synthetic approach to protected l-Dap methyl esters uses appropriately masked 2,3-diaminopropanols, which are obtained via reductive amination of an aldehyde prepared from the commercial amino acid Nα-Fmoc-O-tert-butyl-d-serine, used as the starting material. Reductive amination is carried out with primary amines and sulfonamides, and the process is assisted by the Lewis acid Ti(OiPr)4. The required carboxyl group is installed by oxidizing the alcoholic function of 2,3-diaminopropanols bearing the tosyl or benzyl protecting group on the 3-NH2 site. The procedure can easily be applied using the crude product obtained after each step, minimizing the need for chromatographic purifications. Chirality of the carbon atom of the starting d-serine template is preserved throughout all synthetic steps.