A protein loop is any stretch of non-regular polypeptide chain connecting secondary structures.
Short loops commonly have little structure and are often found joining two adjacent beta strands . They are often referred to as reverse turns.
Long loops are often flexible and are able to adopt a number of different conformations, effectively making them invisible in x-ray crystal structures. These loops are likely to be involved in protein function and can switch from a 'open' conformation (frequently to allow access to an active site) to a 'closed' conformation (to protect reactive groups from bulk solvent).
Sibanda BL, Thornton JM
Nature316p170-4(1985 Jul 11-17)
Martin AC, Toda K, Stirk HJ, Thornton JM
Protein Eng8p1093-101(1995 Nov)
Leszczynski JF, Rose GD
Science234p849-55(1986 Nov 14)