Low-Temperature Study of Photoinduced Energy Transfer from Tryptophan Residues of Escherichia coli Alkaline Phosphatase to Bound Terbium

Abstract

Low-temperature phosphorescence (LTP) decay measurements have been carried out in wild-type Escherichia coli alkaline phosphatase (AP), metal-depleted AP (apoAP), and terbium-substituted AP (TbAP) at 77, 4.2, and 1.2 K. Over this temperature range, Tb emission monitored at 542.4 nm decays with two apparent lifetime components of 16−20 ms and 130−170 ms when TbAP is excited at 280 nm where Tb absorbs negligibly. These lifetimes are orders of magnitude greater than exhibited by TbCl3 at low temperature and by TbAP at room temperature; we attribute this Tb luminescence in TbAP largely to energy transfer from the triplet state of initially excited tryptophan. LTP and ODMR at 1.2 K suggest that Trp109 and Trp220 have similar environments in AP and in apoAP. Trp109 is found to occupy two distinct sites in both AP and apo AP that exhibit resolved ODMR transitions. Comparison of the phosphorescence spectra of AP and TbAP shows clearly that the sensitized Tb emission observed in TbAP is due selectively to energy transfer (ET) from Trp109 to Tb and not significantly from Trp220. Evaluation of the results also suggests that ET takes place through space by Dexter's exchange mechanism, where the lowest triplet state of Trp acts as donor and the 7F6 ground state of Tb acts as acceptor. The ratio of the two ET rate constants suggests that two sites of Trp109 (possibly those resolved by ODMR) act as donors to either or both metal-binding sites, M2 or M3, rather than one Trp109 site transferring energy to Tb at either site M2 or M3.

Time resolved studies of dual emission and photoinduced energy transfer in a Tris methoxy coumarin derivative of a cryptand and its complex with Tb(NO3)3

“…Thus through bond ET mechanism is not suitable in this case rather the ET is best understood in terms of a through-space exchange mechanism. In our study of Tb-AP [19] we also emphasized that through-space ET mechanism is operative.…”

“…Evaluation of the results also suggests that ET takes place through space by Dexter's exchange mechanism, where the lowest triplet state of Trp acts as donor and the 7 F 6 ground state of the Tb acts as acceptor. The ratio of the two ET rate constants observed in this case suggests that two sites of Trp 109 (possibly those resolved by ODMR) acts as donors to either or both metal binding sites, M1 or M3, rather than one Trp 109 site transferring energy to Tb at either site M2 or M3 [19].…”

“…Our recent study [19] of low temperature phosphorescence decay measurements in wild type Escherichia coli alkaline phosphatase (AP) which contains three metal binding sites M1, M2, M3 being occupied by two Zn 2+ ions and one Mg 2+ ion, metal depleted AP and terbium substituted AP [TbAP, where all three metal sites substituted by Tb 3+ ] at 77 K, 4.2 K and at 1.2 K suggest that the sensitized Tb emission observed in TbAP is due selectively to energy transfer (ET) from Trp 109 to Tb and not significantly from Trp 220 . Evaluation of the results also suggests that ET takes place through space by Dexter's exchange mechanism, where the lowest triplet state of Trp acts as donor and the 7 F 6 ground state of the Tb acts as acceptor.…”

“…Moreover, although coumarins are highly fluorescent, 3-ketocoumarins have been shown to display triplet-triplet energy transfer [15] which may be applied to excite lanthanide ions. The energy transfer (ET) from donor ligands to acceptor lanthanide ions in macrocyclic polyether complexes of lanthanides have been of great interest in view of their importance as model systems to study ion transport phenomena [16] and to understand metal ion induced conformational changes in proteins where metal ions may be replaced by lanthanide ions [17][18][19]. Such ET could take place either through the singlet [20] or the lowest triplet state [21][22][23] of an aromatic unit acting as ligand.…”

Comparative photophysical behaviour of naphthalene-linked crown ethers and aza crown ethers of varying cavity dimensions

“…[5][6][7][8][9][10][11][12] These systems are also of great interest in view of their importance as model systems to study ion transport phenomena, 13 to understand electric field-induced conformational changes in proteins 14 and to find photoinduced energy transfer pathways in proteins. 15 We report here the steady-state and time-resolved fluorescence of 2,3-naphtho-17-crown-5 (I), 2,3-naphtho-20-crown-6 (II), 1,8-naphtho-21-crown-6 (III), 1,5-naphtho-22-crown-6 (IV), 1,4-naphtho-22-crown-6 (V) (figure 1) and the alkali metal ion complexes of I to V in EtOH at room temperature (298 K). We use Na + ion for crown-5 ethers and K + ion for crown-6 ethers.…”