 acidic sugars), bulkiness (width 2.5–3 nm, compared to toluidine blue ~0.7 x1.1 nm) makes its diffusion very slow in less permeable parts of the tissue and thus prevent it from staining highly negative yet compact structures such as chromatin and nissl substance.However prolonged staining (few days at 25 °C) or DNA denaturing conditions may allow Alcian blue to also stain the nucleus.The isolation of the positive charge from the aromatic electron cloud by the intervening methylene bridges makes the localized positive charged regions "hard" ions in contrast to soft ions where the charge is delocalized over the whole aromatic pi cloud.