The complex structure transition of Humanin peptides by sodium dodecylsulfate and trifluoroethanol

We have examined the structure of two Humanin (HN) analog peptides, HNG and AGA-(C8R)HNG17, in the presence of sodium dodecylsulfate (SDS) and trifluoroethanol (TFE) using CD and sedimentation velocity. Both HNG and AGA-(C8R)HNG17 underwent complex conformational changes with increasing concentrations of SDS and TFE, in contrast to general trend of increasing alpha-helix with their concentration. To our surprise, both peptides appear to converge into a similar structure in SDS and TFE at higher concentrations; e.g., above 0.05 % SDS or 30-40 % TFE. Sedimentation velocity analysis showed extensive aggregation of HNG at 0.1 mg/ml in PBS in the absence of SDS, but a highly homogeneous solution in 0.1 % SDS, indicating formation of a uniform structure by SDS. These two peptides also formed an intermediate structure both in SDS and TFE at lower concentrations, which appeared to be associated with extensive aggregation. It is interesting that the structure changes of these peptides occur well below the critical micelle concentration of SDS, suggesting that conformational changes are mediated through molecular, not micellar, interactions with SDS.