Photophysical studies on a photoactive yellow protein fluorophore analog with the 4-Hydroxy group replaced by 4-Dimethylamino group

Abstract

Abstract Although several fluorescent analogs of the PYP Fluorophore have been studied so far, the focus has been overwhelmingly on the effect of the electron-withdrawing carbonyl end substituent of the fluorophore. Here, we have introduced for the first time, a PYP Fluorophore analog named pDMACT where the electron-donating phenolate has been replaced instead, by another electron-donating substituent: the N,N-dimethylamino group. Spectral properties of pDMACT were found to be extremely sensitive to the polarity of the solvents, as measured by parameters like ET(30), F(e,n) or p*. In fact, for an increase of ET(30) from 31.1 (in n-heptane) to 45.6 (in acetonitrile), pDMACT registers a fluorescence Stokes shift of 75 nm, as against ∼70 nm reported with the well known polarity probe PRODAN for a comparable increase in ET(30) (Ref. [25]). The high sensitivity of pDMACT is attributed to a strong push-pull charge-separated structure which is further enhanced in the excited state. This enables the molecule to also serve as a probe for ultrafast solvation dynamics. The neutral character of pDMACT further ensures that it can be employed as a fluorescent probe for highly non-polar domains which are inaccessible to the original, anionic PYP fluorophore.