有关N ,O-羧甲基壳聚糖以及O-羧甲基壳聚糖溶液带电性相反的问题？

小弟最近读了一篇文献，里面提到Zeta potential values for the CS, O-CMC and N,O-CMC nanoparticle systems were found to be +54.2 mV, 35.12 mV and+46.74 mV, respectively. All these values lie in the stable range, indicating that these nanoparticle systems prepared were stable. These results also indicated that CS and N,O-CMC nanoparticles possess positive surface charge and O-CMC nanoparticles possess negative surface charge.出自Synthesis, characterization, cytotoxicity and antibacterial studies of chitosan, O-carboxymethyl and N,O-carboxymethyl chitosan nanoparticlesCS带有正电荷能够理解，请问各位大侠，为什么N,O-CMC 的取代度比O-CMC高（也就是N,O-CMC 的羧基含量应该更高） ，反而带有正电荷？理论上不是羧基更多，可以解离出的氢离子更多,那为什么N,O-CMC还要带有正电荷？另外，是什么官能团导致其带有正电荷？谢谢各位，请大家多多赐教。