Rhodamine composition experiments detect tight self-association of TMD and non-random interaction of TMD:FP association. (A) The large self-quenching (i.e. low intensity) of Rhodamine is virtually unchanged in the x = 0.3–1.0 region as the labeled TMD manifests packing of TMD molecules into a tight subunit in the membrane at pH 5.0 and 7.4. In contrast, labeled FP exhibits less self-quenching, indicative of a loose association for the peptide molecules. (B) Association between TMD and FP in the bilayer is not arbitrary as FP of HIV-1 gp41 causes no change in Rho-TMD dequenching or Rho-FP of gp41 dequenching was not affected by mixing with TMD. Change in Rho-FP or Rho-TMD of HA2, in contrast, is obvious when complexed to their counterpart. Note that the smallest value of x in the measurements is 0.02 for (A) and 0.05 for (B). (C) A higher propensity of self-association for TMD than FP is revealed by SDS-PAGE. Lanes 1 and 2 show that FP has less tendency than TMD to form oligomers in SDS in either neutral or acidic buffer. In contrast, TMD formed multiple oligomeric species (lane 4) at pH 4.8 for which minimal association owing to disulfide linkage is expected. The association between TMD and FP is not strong enough to sustain the dispersing force of SDS detergent and the electric field as seen in lane 3.