Sam has three ionizable groups besides the sulfonium pole : the carboxyl and the aminic groups of the amino acidic chain with pk 1 . 8 and 7 . 8 respectively and the aminic group of the purine with pk 3 . 4 . in an acid environment sam is positive charged , so it could be adsorbed by weak - acid cation exchange resin Sam除具有一个带正电荷的锍外，还有三个可离子化的基团： pk _ a分别为1 . 8 、 7 . 8的氨基酸链上的羧基和氨基， pk _ a为3 . 4的嘌呤环上的氨基。
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