Ultrafast Excited-State Deactivation of the Bacterial Pigment Violacein Published as part of The Journal of Physical Chemistry virtual special issue “Veronica Vaida Festschrift”. Ashley A. Beckstead, † Yuyuan Zhang, ‡ Jonathan K. Hilmer, †,⊥ Heidi J. Smith, § Emily Bermel, §,# Christine M. Foreman,* ,§,∥ and Bern Kohler* ,†,‡ ‡ Department of Chemistry and Biochemistry, The Ohio State University, 100 West 18th Avenue, Columbus, Ohio 43210, United States † Department of Chemistry and Biochemistry, § Center for Biofilm Engineering, and ∥ Chemical and Biological Engineering Department, Montana State University, Bozeman, Montana 59717, United States * S Supporting Information ABSTRACT: The photophysical properties of the natural pigment violacein extracted from an Antarctic organism adapted to high exposure levels of UV radiation were measured in a combined steady-state and time-resolved spectroscopic study for the first time. In the low-viscosity solvents methanol and acetone, violacein exhibits low fluorescence quantum yields on the order of 1 × 10 −4 , and femtosecond transient absorption measurements reveal excited-state lifetimes of 3.2 ± 0.2 and 4.5 ± 0.2 ps in methanol and acetone, respectively. As solvent viscosity is increased, both the fluorescence quantum yield and excited-state lifetime of this intensely colored pigment increase dramatically, and stimulated emission decays 30-fold more slowly in glycerol than in methanol at room temperature. Excited-state deactivation is suggested to occur via a molecular-rotor mechanism in which torsion about an interring bond leads to a conical intersection with the ground state. ■ INTRODUCTION Ultraviolet (UV) radiation from the sun is harmful to living organisms because of photochemical damage to critical biomolecules that can lead to mutation and cell death. Sun- exposed organisms must therefore adopt strategies to avoid photodamage. Some phototrophic microorganisms produce UV-screening molecules as a defense mechanism against solar irradiation. 1 For example, high solar irradiances induce the production of UV-absorbing compounds such as scytonemin in cyanobacteria 2,3 and mycosporine-like amino acids in coral algae and phytoplankton. 4,5 In addition, it has been shown that organisms equipped with carotenoid pigments are more resistant to photodamage. 6 Bacteria in Antarctica particularly benefit from photo- protective pigments due to the presence of continuous sunlight through the Austral summer and increased UV radiation due to ozone depletion in the polar region. 7,8 Supraglacial streams are short-lived and seasonally subject to continuous solar irradiation. Studying bacterial populations in the harsh environmental conditions of supraglacial streams provides insight into the requirements necessary for survival at life’s limits and potentially extraterrestrially. 9 Janthinobacterium sp. CG23_2, an aerobic psychrotolerant, rod-shaped organism isolated from the Cotton Glacier supraglacial stream in the Antarctic Dry Valleys (77° 07S, 161° 50E), produces the bisindole, purple pigment violacein. 10 This pigment has a structure consisting of two indole rings (Figure 1) that is uncommon among natural pigments. 11 Violacein production increases in response to UV irradiation in a closely related Antarctic bacterium, resulting in increased survival. 12 This protection could arise from the antioxidative properties of violacein, 13,14 which could mitigate cellular oxidative stress by scavenging reactive oxygen species generated by UV radiation. In addition, violacein absorbs strongly across Received: June 12, 2017 Revised: July 19, 2017 Published: July 28, 2017 Figure 1. Chemical structure of violacein. The compound consists of (from left to right) 5-hydroxyindole, 2-pyrrolidone, and oxindole heterocycles. Article pubs.acs.org/JPCB © 2017 American Chemical Society 7855 DOI: 10.1021/acs.jpcb.7b05769 J. Phys. Chem. B 2017, 121, 7855−7861