Can I label a protein with a single His-tag with Ni-NTA-Atto conjugates? Papers generally use this technique to label 6His-tag.
According to the unreferenced introduction to Wikipedia's article on poly-histidine tags, the number of His residues can vary from two to ten, with six being the most common. Since histidine is a naturally-occurring amino acid in many (most?) proteins, adding just a single one at either end likely won't do much of anything. In my work with His-Tags (20-something years ago), I seem to remember that 3-4XHis was about the minimum that produced any kind of differential binding in the purification system we were using (Ni2+-agarose, elution with imidazole). However, for our purposes (scaling up protein purification), the quality of the end product wasn't great, and there was a lot of our target protein in the flow-through, so we ended up going with either five or six His residues, I forget which one.
As always, your mileage may vary, so I strongly suggest testing out a range for your application, if for some reason using the full 6X-His isn't an option.