As David Greer wrote, a string of 52 characters is impossible using parity as follows: The letters A, C, E... have an odd number of spaces between their two appearances, so they must appear in positions of the same parity. Similarly B, D, F... must appear in positions of opposite parity. In a string of 52 letters, there are 26 odd and 26 even positions, of which 13 of each would be taken by the letters B, D, F..., leaving 13 odd and 13 even positions, with no way to divide into pairs of equal parity.
Todd Will sent us a nice solution:
IBMPONDERTHIS wasn't possible since the two letter Is are too far apart.
But here are two other options.
And Florian Fischer sent us an even nicer one:
Matthew Charlap was the first among many who sent us the elegant solution:
By the nature of the required spacing, all the "even" letters (B, D, F, etc.) can be nested, as can the odds:
ZXVTRPNLJHFDB--BDFHJLNPRTVXZ (evens - 28 characters)
YWUSQOMKIGECA-ACEGIKMOQSUWY (odds - 27 characters)
Concatenating these gives us 55 characters - two characters too long (since three hyphens are used):
Now, all 26 letters are between the hyphens, so if the Ys are removed, we get:
which has 25 letters between the hyphens--exactly what we need to put the Ys back in. Note that since Y is the outermost letter in the odds-string, removing it does not cause any other spacings to be broken.
Putting the Ys around those middle 25 letters (replacing hyphens) results in:
This last string has the desired length of 53.
Note that if we instead remove the Zs, and place them surrounding the other 25 letters and one of the two hyphens, we get another solution:
This process can easily be generalized for any set of characters separated by 1, 2, 3, ..., n spaces.