Amino acids in the membrane spanning regions must be hydrophobic. Positively charged amino acids cannot form stable associations with membrane lipids, and, therefore, are very uncommon. Lys, Gln, Asp, and Arg are positively charged amino acids. Ala has a hydrophobic side chain.
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They must be hydrophobic in order to interact with the hydrophobic environment of the membrane. Since, Lys, Gln, Asp, and Arg are positively charged, they are generally not found in membrane spanning regions. Since Ala has a hydrophobic side chain, it can be a part of the membrane spanning region.