How the substitution of hydrophillic glutamic acid with hydrophobic valine cause sickle-cell anaemia?

The 6th amino acid of the resulting beta globin polypeptide becomes the hydrophobic amino acid valine instead of hydrophilic glutamic acid. The mutant protein giving rise to a defective variant of haemoglobin, called haemoglobin S (HbS) which does not fold up correctly. The variant is suceptible to polymerisation in cooler or more hypoxic conditions, the precipitated haemoglobin distorts the red corpuscle causing it to resemble a sickle and ultimately damages the erythrocyte causing haemolysis.