Involved in pterocarpan phytoalexin biosynthesis (PubMed:28394400). Catalyzes the last step in the biosynthesis of the phytoalexin medicarpin, and thereby contributes to plant defense reactions (PubMed:28394400). Dirigent proteins impart stereoselectivity on the phenoxy radical-coupling reaction, yielding optically active lignans from two molecules of coniferyl alcohol in the biosynthesis of lignans, flavonolignans, and alkaloids and thus plays a central role in plant secondary metabolism (By similarity). a (4R)-4,2'-dihydroxyisoflavan = a pterocarpan + H2O. (3R,4R)-7,2'-dihydroxy-4'-methoxyisoflavanol = (-)-medicarpin + H2O (3S,4R)-7,2'-dihydroxy-4'-methoxyisoflavanol = (+)-medicarpin + H2O (3R,4R)-3-(6-hydroxy-1,3-benzodioxol-5-yl)-3,4-dihydro-2H-chromene-4,7-diol = (-)-maackiain + H2O (3R,4R)-7,2',4'-trihydroxyisoflavanol = (6aR,11aR)-3,9-dihydroxypterocarpan + H2O Optimum pH is 6.5-7.5. Homodimer. Belongs to the plant dirigent protein family.