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A new optical strategy to determine the binding modes (intercalation vs groove binding) of small fluorescent organic molecules with calf thymus DNA was developed using two-photon absorption (TPA) spectroscopy. Two-photon excited emission was utilized to investigate a series of fluorescent nuclear dyes. The results show that TPA cross-sections are able to differentiate the fine details between the DNA binding modes. Groove binding molecules exhibit an enhanced TPA cross-section due to the DNA electric field induced enhancement of the transition dipole moment, while intercalative binding molecules exhibit a decrease in the TPA cross-section. Remarkably, the TPA cross-section of 4,6-bis(4-(4-methylpiperazin-1-yl)phenyl) pyrimidine is significantly enhanced (13.6-fold) upon binding with DNA. The sensitivity of our TPA methodology is compared to circular dichroism spectroscopy. TPA demonstrates superior sensitivity by more than an order of magnitude at low DNA concentrations. This methodology can be utilized to probe DNA interactions with other external molecules such as proteins, enzymes, and drugs.

Abstract: In this article, we reported here an effective and selective sensing of hemoglobin (Hb) with a stilbazolium dye, HNEP+. The nature of the HNEP+/Hb interaction was examined using a combination of various spectroscopic techniques. Complexation induced selective fluorescence quenching with a large Stern–Volmer constant, K sv, the origin of which is attributed to the neutral isoelectric point (pI) of the proteins. The stoichiometry of the DNEP+:Hb was determined to be 1:2 by a Job's plot analysis, and the induced circular dichroism measurements revealed a loss of the α-helix and the unfolding of the proteins in the complex. The addition of cyclodextrin extended fluorescence lifetime, which ensured the accuracy and repeatability of the measurements. Finally, by taking advantage of a subtle change in pI and subsequently K sv of the glycated Hb, HbA1c, we suggested the possibility of a new sensing concept for HbA1c. The current investigation demonstrates that a selective optical response of a stilbazolium dye can find significant usefulness in bio-sensing applications. [Copyright &y& Elsevier]

The photophysical properties of 4-[2-(6-hydroxy-2-naphthalenyl)-ethenyl]-1-methyl-pyridinium(HNEP+) and its deprotonatedform (NEP), a benzofused derivative of Brooker’smerocyanine (BM), were investigated through a combinedspectroscopic and computational approach. Despite their structuralsimilarities and similar pKavalues, HNEP+/NEPand BMH+/BMdiffer in the extent ofcharge delocalization in the ground and excited states. NEPexhibits the spectral characteristics of a charge transfer speciesin solvents in which BMexists in a charge-delocalizedquinoid; however, quantum chemical calculations show that the CT absorptionof NEPis not necessarily a consequence of the zwitterioniccharacter. HNEP+displays largerStokes shifts than BMH+, and NEPdemonstrates enhanced solvatochromism relative to BMas a consequence of benzofusion. [ABSTRACT FROM AUTHOR]