Epitope analysis of SmD1 and SmD3. Carboxyl-terminal regions of (a) SmD1 and (b) SmD3 were synthesized as peptide arrays (15 mers; two amino acids offset) and probed with patient sera. Each arginine containing peptide was synthesized as three variants, one with natural arginine (R), one with symmetrical dimethylarginine (sDMA) and one with asymmetrical dimethylarginine (asDMA) at the respective positions. In addition, a highly reactive SmD3 peptide was synthesized with certain combinations of natural arginine and sDMA. A significant effect of dimethylation of arginine residues on the antigenicity of SmD derived peptides was observed (black squares indicate strong reactivity; white indicate no reactivity). Binding of an anti-Sm negative serum sample (Varelisa® Sm) that contained anti-centromere antibodies (ACA) could be observed with SmD1 but not with SmD3 peptides. Thus, the immunoreactive peptide no. 77 was further tested in (c) a replacement experiment. The SmD3 peptide exhibited exclusive reactivity with the Sm-positive sera.