reagents employed were of analytical
grade. Sodium tetraborate, lithium carbonate, (+)-FLEC (18 mM in acetonitrile),
9-fluorenylmethoxycarbonyl chloride (FMOC-Cl), ammonium hydroxide
(28%, w/v), acetic acid, and sodium hydroxide were purchased from
Sigma-Aldrich (St. Louis, MO). Potassium carbonate was obtained from
Acros Organics (New Jersey). Acetonitrile (ACN) was supplied by Fluka
(Steinheim, Germany). Water was deionized and purified with a Milli-Q
purification system (Millipore, Belford, NJ).
DL- and L-forms
of tyrosine (Tyr), alanine (Ala), valine (Val), phenylalanine (Phe),
tryptophan (Trp), histidine (His), arginine (Arg), lysine (Lys), isoleucine
(Ile), methionine (Met), leucine (Leu), asparagine (Asn), serine (Ser),
glutamic acid (Glu), proline (Pro), threonine (Thr), 2-Aminoadipic
acid (Aad), and selenomethionine (SeMet), and D-glutamine (D-Gln),
L-glutamine (L-Gln), D-pipecolic acid (D-Pipe), D-ornithine (D-Orn),
and glycine (Gly) were purchased from Sigma-Aldrich (Madrid, Spain),
whereas D-citrulline (D-Cit), L-citrulline (L-Cit), L-pipecolic acid
(L-Pipe), and L-ornithine (L-Orn) were obtained from Fluka (Buchs,
Switzerland).