Effects of hydrophobicity and electrostatic charge on complement activation by amino groups

Some studies have demonstrated that amino groups, acting as nucleophiles, are potent activators of the complement system, but others not. To clarify these contradictory results, we examined complement activation on two series of NH(2)/CH(3) and NH(2)/COOH mixed self-assembled monolayers (SAMs). NH(2)/CH(3) mixed SAMs were not potent activators of the complement system regardless of the ratio of NH(2)/CH(3) in mixed SAMs. Numerous serum proteins, such as albumin, were adsorbed onto those SAMs and formed a protein layer which inhibited access of C3b to amino groups. In contrast, much C3b and/or C3bBb were deposited on NH(2)/COOH mixed SAMs with approximately 50-60% NH(2) density on the surface and SC5b-9 was found in serum exposed to this SAM, indicating activation of the complement system. These results suggest that C3b can easily access nucleophilic NH(2) groups because of the decrease in electrostatic interaction between negatively charged proteins and the NH(2) SAM surface.