pH-related fluorescence quenching mechanism of pterin derivatives and the effects of 6-site substituents

Abstract

2-Amino-4-hydroxypteridine (pterin) and its derivatives serve as photooxidants and exhibit strong fluorescence. When they interact with hydrogen acceptors such as acetate and phosphate, their fluorescences are significantly quenched in acidic conditions (pH 4.9–5.5) but are retained in basic conditions (pH 10.0–10.5). This pH-related fluorescence quenching mechanism of pterin and its derivatives are fully investigated by using density functional theory (DFT) and time-dependent density functional theory (TD-DFT). Pterin and its derivatives are demonstrated to show favorable excited-state proton transfer (ESPT) abilities in acidic conditions that induce the experimentally observed fluorescence quenching. In contrast, the ESPT processes are found to be retarded due to the lack of strong hydrogen-bonding interactions in basic environments, which sustain their fluorescence. Interestingly, these ESPT processes are found to show different site specificities depending on the 6-site substituents. The introduction of electron-donating substituent activates the N1 site, making it the preferred ESPT site. By contrast, the introduction of an electron-withdrawing substituent activates the N5 site, making it the favorable ESPT site. The substitutions of different functional groups are found to affect the locations of acidic centers during the excitation and relaxation processes. This further affects the hydrogen-bonding patterns and ultimately brings site specificity to the ESPT process.

Theoretical unraveling detailed excited state proton transfer mechanism for 2,5‐bis (benzoxazol‐2‐yl)thiophene‐3,4‐diol‐ethyl compound in different solvents

“…As is known, theoretical constructed PECs could be adopted to explore and illustrate molecular excited state reaction processes. [63][64][65][66][67][68][69][70] In this work, we construct the PECs of both S0 and S1 states for BBTE compound (seen in Figure 6). Depending on DFT and TDDFT methods, the constructed PECs are via restricted optimization.…”

Endogenous Photosensitizers in Human Skin

“…The presence of electron-donating substituents at C6 activates N3 toward ESPT, while electron-withdrawing substituents activate the 2-amino group. 548 Iodide ion was found to promote fast nonradiative T 1 → S 0 transitions of both pterin and lumazine with k q values of 3.0 × 10 9 L mol −1 s −1 and 1.0 × 10 10 L mol −1 s −1 , respectively. Although singlet excited states are deactivated as well, at low concentrations of I − (μmol L −1 ) the deactivation of the excited singlet state of pterin and lumazine is negligible due to the much shorter lifetimes compared to the excited triplet states.…”

“…In aqueous acidic media (pH 4.9–5.5), their interaction with hydrogen acceptors, such as acetate and phosphate ions, leads to fluorescence quenching due to the occurrence of ESPT. The presence of electron-donating substituents at C6 activates N3 toward ESPT, while electron-withdrawing substituents activate the 2-amino group . Iodide ion was found to promote fast nonradiative T 1 → S 0 transitions of both pterin and lumazine with k q values of 3.0 × 10 9 L mol –1 s –1 and 1.0 × 10 10 L mol –1 s –1 , respectively.…”

Insights into Molecular Structure of Pterins Suitable for Biomedical Applications

“…It occurs due to the excited state proton transfer (ESPT) from the amino group to one of the acetate oxygens. Liu and Sun have also studied the influence of pterin 6-substituent on the ESPT [ 187 ]. The substitution of 6-site with an electron-donating group (for example, dihydroxypropyl radical in biopterin) activates NH 2 group, which makes it the favorable ESPT site.…”