Hi there,

As S-tag fusion protein purification is based on the interaction of the S tag with bead conjugated anti S tag antibody, elution has to disrupt the interaction between epitope and antibody. Lowering the pH is a way to disrupt such a strong interaction.

I agree with @Dhiraj. If you want to use smaller tags, His-tag and FLAG-tag should be good choices, and they can be easily eluted at native conditions. For example, Imidazole used to elute His-tagged proteins can be efficiently removed from protein samples by dialysis or buffer exchange.