Characterization of the aggregated pattern of CHAPS using solvent paramagnetic relaxation enhancements

Abstract

CHAPS is a typical zwitterionic detergent widely used in protein purification. It has two peculiar aggregation states, whose aggregation patterns are still controversial and uncertain. In this paper, we analyzed the aggregation pattern of CHAPS by 1H solvent Paramagnetic Relaxation Enhancement (sPRE) experiment. It was found that the 1H sPREs of head groups of CHAPS are much smaller than those of the tail groups, and all of them become smaller simultaneously when the concentration of CHAPS is above CMC2. In addition, the sPREs of those protons in the steroid ring were found to be very similar to each other in both micellar states (M1 and M2). These results indicated that the micellar core of CHAPS is mainly formed by its steroid head which aggregate disorderly. On the other hand, a transition from single to double layer micelle is most likely to happen when the concentration of CHAPS is above CMC2. The micelle in M2 state is formed by the aggregation of the partially disordered primary ones as well. Furthermore, the STD (Saturation Transfer Difference) experiment showed that the apparent coherence transfer rates between water and those OH near protons at the steroid head are slower in M2 state, revealing that the CHAPS aggregate loosely in M1 state, but more tightly in M2 state, which may be driven by the increased hydrophobic interactions between the steroid groups of nearby CHAPS molecules, as well as the hydrogen bond between OH in the hydrophobic side and SO3−1, CO, or HN in the hydrophilic side of different molecules, which consequently extrude the water and stabilize the micelle.