Diastereoisomers

The diastereomers are a class of isomers which, together with the enantiomers, belong to the family of optical isomers. Therefore, they differ in the configuration of chiral carbons. However, in the case of diastereoisomers, the molecules must have at least two chiral carbons which means that whenever there is only one chiral carbon, the molecule do not present diastereoisomers. As I mentioned in a previous post here on the blog, the enantiomers differ in the configuration of ALL chiral carbons. The diastereomers are characterized by presenting AT LEAST ONE chiral carbon with different configuration and AT LEAST ONE chiral carbon with the same configuration.

Therefore, if a molecule with two chiral carbon presents one with the same configuration in both isomers, these are diastereoisomers.

If the molecule has 100 carbons chiral, an enantiomer has to have an opposite configuration at all of them. All the intermediate situations, where at least one configuration remains the same, these are diastereoisomers.
Interestingly, despite the difference between the diastereomers can be in the configuration of only one chiral carbon, they have distinct properties. Please see what happens in the following curious molecules ...

Both have two chiral carbons, one of which has the same configuration and other features different configuration. However, this "small" difference causes one of the diastereoisomers to have a sweet taste (molecule from left), while the other has a bitter taste (right molecule).