In the chapter on Biomolecules in my book, the following points are given regarding carbohydrates.
Numbering the C-chain with consecutive rank
The C-atom present in the aldehyde group of aldoses and the terminal C-atom nearest to the keto-group of the ketoses are numbered as the first carbon or C1.
D- and L- Configuration
i) The D- and L- symbols refer to the configuration of monosaccharides. These do not indicate the dextrorotatory or laevorotatory specific rotations.
ii) In case of a monosaccharide, if the asymmetric C-atom of the highest rank is similar to the asymmetric carbon of D-glyceraldehyde (H-atom on the right,-OH group on the left) then the monosaccharide is of D-configuration. If the H-atom is on the left and -OH group is on the right, then the monosaccharide is of L-configuration.
iii) As per convention, if in a monosaccharide unit (expressed by Fischer projection formula), the -OH group falls on the right of the highest ranked asymmetric carbon, then the monosaccharide has D-configuration and if the hydroxyl group falls on the left, it has L-configuration.
Use of + and - sign
D- and L- symbols are followed by '+' and '-' in brackets. These repesent the dextro- and laevo- specific rotation. There is no relation between optical activity and nature of rotation. Example: Both glucose and fructose have D-configuration, but their optical activities are positive (dextrorotatory) and negative (laevorotatory) respectively.
Points ii and iii seem to be contradictory to me. Can someone please shed some light on this. I noticed that point iii is talking about Fischer projection. May be point ii is talking about some about mode of representation. Can someone please explain the paragraph under Use of + and - sign