Principles of Biochemistry (2+1)

A compound which causes rotation of plane polarized light to the right is said to be dextrorotatory (+) isomer, and  'd' is used to designate the fact.  Rotation of the plane polarised light to the left is called levorotatory isomer and designated by a (-) sign and 'l' is used to designate to the fact. The naturally occurring form of fructose is the D (-) isomer. When equal amounts of D and L isomers are present, the resulting mixture has no optical activity, since the activities of each isomer cancel one another.  Such a mixture is said to be a racemic or DL mixture. Two important monosaccharides whose isoforms are biologically active –L fucose and L iduronic acid.

Stereoisomerism and optical activity are independent properties. Thus D (+), D (-), L (-), or L (+), indicate structural relationship to D or L glyceraldehyde and the optical activity exhibited.. Synthetically produced compounds are racemic.The optical rotation of glucose in solution is dextorotation hence, the alternative name of dextrose.