_d_-Fructose _d_-Sorbose _d_-Tagatose CH_{2}OH CH_{2}OH CH_{2}OH | | | H-C-OH HO-C-H H-C-OH | | | H-C-OH H-C-OH HO-C-H | | | HO-C-H HO-C-H HO-C-H | | | C=O C=O C=O | | | CH_{2}OH CH_{2}OH CH_{2}OH Reference will be made in subsequent paragraphs to the probable chemical constitution of the monosaccharides other than hexoses; but the above discussion of the structure of the hexoses will serve as a sufficient introduction to the study of the composition of the common carbohydrates. CHARACTERISTIC REACTIONS OF HEXOSES =Specific Rotatory Power.=--All soluble carbohydrates, since they contain asymmetric carbon atoms, with the consequent larger groups on one side of the molecule than the other, rotate the plane of polarized light when it passes through a solution of the carbohydrate in question. The amount of the rotation depends upon the nature of the carbohydrate, the concentration of the solution, and the length of the column of solution through which the ray of polarized light passes. But the same definite amount of the same sugar, dissolved in the same volume of water, and placed in a tube of the same length, will always cause the same angular deviation, or rotation, of the plane in which the polarized light which passes through it is vibrated. In other words, the same number of molecules of the optically active substance in solution will always produce the same rotatory effect. This is called the specific rotatory power of the substance in question. It is expressed as the number of degrees of angular deviation of the plane of polarized light caused by a column of the solution exactly 200 mm. in length, the concentration of the solution being 100 grams of substance in 100 cc. at a temperature of 20° C. Actual determinations of specific rotatory power are usually made with solutions more dilute than this standard, and the observed deviation multiplied by the proper factor to determine the effect which would be produced by the solution of standard concentration. If the direction of the deviation is to the right (i.e., in the direction in which the hands of the clock move) it is spoken of as "dextro" rotation and is indicated by the sign +, or the letter _d_; while if in the opposite direction, it is called "levo" rotation and indicated by the sign -, or the letter _l_. For example, the specific rotation of ordinary glucose is +52.7°; of fructose, -92°; of sucrose, +66.5°.