Rapid and efficient preparation of free amino acids from strong acid salts on columns of crosslinked poly-4-vinylpyridine.

Frequently, procedures for synthesizing or deblocking amino acids employ strong acids, which must usually be removed for subsequent steps. Blomquist et al. ( 1) discussed shortcomings in available methods and developed a new procedure for recovery of amino acids from their hydrohalide salts by neutralization with triethylamine and crystallization of the free amino acids. Recoveries were in the range of 72-9896. Methods involving crystallization are usually unsatisfactory for analytical samples, which involve small quantities, mixtures, and compounds of unknown solubility. A recent method avoids the need for recrystallization by converting hydrochloride salts to free amino acids by formation and subsequent solvolysis of trimethylsilyl derivatives (2,3). Procedures using ethylene oxide or propylene oxide have also been reported recently. (4,5). We have obtained amino acids, in most cases in greater than 99% yield, by simply passing aqueous solutions of strong acid salts of the amino acids through columns of crosslinked poly-4-vinylpyridine. Recrystallization is unnecessary provided that a trace of contamination by the salt can be tolerated in the product (Table 1). The method works not only for hydrohalide salts but for all strong acid salts tested (Table 2). Further, a small amount of amino acid may be recovered from a large excess of strong acid solution. For efficient removal of the strong acid more than 1 eq of the resin was required (Table 3). For the hydrophobic amino acids the column and outlet line were maintained at 98°C in a water bath to avoid premature crystallization of the product. Basic amino acids and glycine ethyl ester eluted mainly as the monoacid salts instead of the desired free bases (Table 1). However, it was possible to recover ornithine and histidine as the trifluoroacetate salts in submicromolar amounts from lOOOand lO,OOO-fold excess, respectively, of trifluoroacetic acid (Table 4). While we have applied the above procedure to the amino acids because they offer a case of special difficulty and wide appli’ To whom correspondence should be addressed at: cability, clearly the method should be useful Nuclear Medicine, Box 021, University Hospital, Ann for the recovery of many kinds of water-solArbor, Mich. 48109. uble molecules from strong acids.