[Fzang]

We measured the ability of monovalent cations to increase/decrease the effectivity of the enzyme FBPase through a 340 nm NADP/NADPH assay, and at very high concentrations (200 mM) of cesium Cs⁺ we saw that the absorbance rose extremely fast, at an exponential rate.

Since this is definitely not the enzyme activating turbo mode, what could cause the extreme rise in absorbance at 340? We also checked K, Na and Li, and none of these had a crazy effect like this. Does cesium react with something in the mixture? Free phosphates from the enzyme catalyzed reaction?

[cheese (MSW)]

Run a blank: does Cs^+(aq) exhibit such an absorption?  Could find nothing on the UV spectra of Group 1 ions in aqueous soln so what follows are some suggestions.  It may be that the 6s AO in [Cs(H2O)6]^+ is sufficiently low in energy that you can have LMCT  O n→σ Cs^+ that absorbs at 340 nm, but the absorption is at higher energy (shorter wavelength) for Rb^+, K^+.  If this is the case [Cs(H2O)6]^+ should have a yellow tinge, but there is no mention in the texts about this although the oxides M2O gradually deepen in color such that Cs2O is orange [1].
Another possibility is that Cs^+ forms a stronger ion pair with a phosphate species [1] and again LMCT can occur.  So run a blank of Cs^+(aq) with NaADP.  Running blanks is a fundamental technique when something unusual happens.  And serendipitous discoveries are what chemistry is all about!
[1] N. N. Greenwood, A. Earnshaw, Chemistry of the Elements 2nd ed. (1997) p 84; p 90: sulfate forms a slightly stronger ion pair than those for the other Group 1 M^+.

[Fzang]

Probably should have run a blank with high concentration Cs. We were in a hurry, and uh... I can't explain why, but we never got to do a water-Cs-only control.

/blush

I can however say that the solution is as clear as water, to the naked eye.

I guess that the lack of a blank control means we'll probably never know.

[cheese (MSW)]

But you have learned an important lesson. As you probably know many
of the most important discoveries were made by accident or serendipity
(X-rays, penicillin, Teflon, etc.). Who was it that said the mind has to be
prepared (Pasteur, Pauling?)  So when you expect A but get X it could
be something exciting.  So systematically remove one of the variables
and repeat the expt until you track it down.  It will probably end up being
a known phenomenon, but if you do enough expts one day the Gods
might smile down upon you.   Best wishes.

[cheese (MSW)]

Another quality of a good researcher is perseverance.  What would it take for you to carry out
a UV-vis spectrum of 200mM Cs^+ and then run again with 200mM [HPO4]^2- added?
Do you understand what I mean by a LMCT transition?  In your case absorption of the UV may causes an e⁻ in
the HOMO, the nonbonding lone pair on O (n), to the LUMO presumed to be a Cs^+ AO a charge transfer occurs
from the ligand to metal.
The reason I am posting is that the LUMO may not be the 6s AO but the 5d AO.
Such p→d transitions are strongly allowed and have huge molar absorptivities (up to ~50,000).
This type of transition is believed to occur in [MnO4]^- (Mn(VII) is d^0) that is an intense
purple.