gopla

I have a question about the length of typical alpha-helix transmembrane domain of membrane proteins. It is known that normally transmembrane alpha-helixes consist of 20-25 aminoacids and have the length about 3,5 nm to perforate the membrane. But what happens if the transmembrane domain is longer than 25 aa? I can't find any information about this problem, maybe here I'll find someone to help me?