Are there other sequences that can bind to nickel more tightly than 6xHis-tagged proteins and how can they be eluted.
Yes, 7xHis-tagged proteins, proteins naturally high in histidine, and other combinations of His and other amino acids will bind. To elute them, you have to increase the concentration of imidazole. Generally these peptides will not contaminate your fraction since they remain on the column. However after multiple uses of the same column, these peptides may reduce the binding capacity of the column.