Proteins associated with diseases show enhanced sequence correlation   between charged residues
  Function of proteins or a network of interacting proteins often involves communication between residues that are well separated in sequence. The classic example is the participation of distant residues in allosteric regulation. Bioinformatic and structural analysis methods have been introduced to infer residues that are correlated. Recently, increasing attention has been paid to obtain the sequence properties that determine the tendency of disease related proteins (Abeta peptides, prion proteins, transthyretin etc.) to aggregate and form fibrils. Motivated in part by the need to identify sequence characteristics that indicate a tendency to aggregate, we introduce a general method that probes covariations in charged residues along the sequence in a given protein family. The method, which involves computing the Sequence Correlation Entropy (SCE) using the quenched probability Psk(i,j) of finding a residue pair at a given sequence separation sk, allows us to classify protein families in terms of their SCE. Our general approach may be a useful way in obtaining evolutionary covariations of amino acid residues on a genome wide level. 
Ruxandra I. Dima and D. Thirumalai