UV‐Dissipation Mechanisms in the Eumelanin Building Block DHICA

Abstract

The UV-dissipative mechanisms of the eumelanin building block 5,6-dihydroxyindole-2-carboxylic acid (DHICA) and the 4,7-dideutero derivative (DHICA-d(2)) in buffered H(2)O or D(2)O have been characterized by using ultrafast time-resolved fluorescence spectroscopy. Excitation of the carboxylate anion form, the dominating state at neutral pH, leads to dual fluorescence. The band peaking at lambda=378 nm is caused by emission from the excited initial geometry. The second band around lambda=450 nm is owed to a complex formed between the mono-anion and specific buffer components. In the absence of complex formation, the mono-anion solely decays non-radiatively or by emission with a lifetime of about 2.1 ns. Excitation of the neutral carboxylic acid state, which dominates at acidic pH, leads to a weak emission around lambda=427 nm with a short lifetime of 240 ps. This emission originates from the zwitterionic state, formed upon excitation of the neutral state by sub-ps excited-state intramolecular proton transfer (ESIPT) between the carboxylic acid group and the indole nitrogen. Future studies will unravel whether this also occurs in larger building blocks and ESIPT is a built-in photoprotective mechanism in epidermal eumelanin.

Excited-State Proton-Transfer Processes of DHICA Resolved: From Sub-Picoseconds to Nanoseconds

“…The result of the Gaussian band spectral deconvolution is shown in the SI ( Figure SI1A−F). At low pH (1.0), we observe a strongly redshifted fluorescence, with λ max ≈ 450 nm (and not 430 nm, as previously reported 6 ) and a lifetime of 240 ps. On the basis of extensive time-resolved fluorescence work on DHICA 6,11 and experiments on indole-2-carboxylic acid (ICA), 12 as well as recent combined time-resolved fluorescence and quantum chemistry calculations 13 on ICA, this red-shifted fluorescence was assigned to a zwitterionic species formed through an unresolved sup-picosecond ESIPT process involving the COOH and NH groups.…”

“…At low pH (1.0), we observe a strongly redshifted fluorescence, with λ max ≈ 450 nm (and not 430 nm, as previously reported 6 ) and a lifetime of 240 ps. On the basis of extensive time-resolved fluorescence work on DHICA 6,11 and experiments on indole-2-carboxylic acid (ICA), 12 as well as recent combined time-resolved fluorescence and quantum chemistry calculations 13 on ICA, this red-shifted fluorescence was assigned to a zwitterionic species formed through an unresolved sup-picosecond ESIPT process involving the COOH and NH groups. At the same time, solvent (water) molecules were shown to play an important role in the proton transfer, 13 implying that the exact nature of interaction between the proton dissociated from the COOH group and the NH group has still to be established.…”

“…For instance, at pH 7, the fluorescence spectrum has a broad red tail and can be described as a superposition of the DHICA 2− spectrum and a (dominating) band at ∼380 nm, which we assign to the DHICA − carboxylate anion, in agreement with previous reports. 6 The fluorescence spectrum of DHICA in methanol with a maximum at ∼360 nm is also shown in Figure 1.…”

“…At a pH where the carboxyl group of the molecule is fully protonated (e.g., pH 2.5), a red-shifted fluorescence band (λ max ≈ 430 nm) with a relatively short lifetime of 240 ps was observed and attributed to a zwitterionic species formed as a result of rapid excited-state intramolecular proton transfer (ESIPT) from the COOH group toward the NH group. 6 However, the actual transfer time and decay of the original excited state of the fully protonated molecule could not be resolved with the temporal resolution of the employed streak camera technique. By using femtosecond fluorescence upconversion (FU), we can now directly resolve this reaction step and show that it indeed proceeds on the sup-picosecond time scale.…”

“…6 This species was assigned to a complex between the excited DHICA − and a buffer species formed through a diffusion process. 6 On the other hand, calculations by Olsen et al 9 suggested that the 5-and 6-OH groups of DHICA should be involved in an ESIPT process or multistep excited-state proton transfer (ESPT) to the solvent. This discrepancy between experimental and theoretical results motivated us to further examine the role of the OH groups of the carboxylate anion of DHICA in proton transfer.…”