emphasis You can use an iota or one iota to emphasize a negative statement. Not an iota or not one iota means not even to a small extent or degree. Our credit standards haven't changed one iota. = jot. not one/an iota not even a small amount (name of the fifth and smallest letter of the Greek alphabet)
emphasis If you say that there is not an iota or not one iota of something, you are emphasizing that there is not even a very small amount of it. He's never shown an iota of interest in any kind of work. = jot