Membrane proteins constitute > 30% of the proteins in an average cell, and yet 
the number of currently known structures of unique membrane proteins is < 300. 
To develop new concepts for membrane protein structure determination, we have 
explored the serial nanocrystallography method, in which fully hydrated protein 
nanocrystals are delivered to an x-ray beam within a liquid jet at room 
temperature. As a model system, we have collected x-ray powder diffraction data 
from the integral membrane protein Photosystem I, which consists of 36 subunits 
and 381 cofactors. Data were collected from crystals ranging in size from 100 nm 
to 2 μm. The results demonstrate that there are membrane protein crystals that 
contain < 100 unit cells (200 total molecules) and that 3D crystals of membrane 
proteins, which contain < 200 molecules, may be suitable for structural 
investigation. Serial nanocrystallography overcomes the problem of x-ray damage, 
which is currently one of the major limitations for x-ray structure 
determination of small crystals. By combining serial nanocrystallography with 
x-ray free-electron laser sources in the future, it may be possible to produce 
molecular-resolution electron-density maps using membrane protein crystals that 
contain only a few hundred or thousand unit cells.