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The balance between cell quiescence and proliferation is fundamental to tissue physiology and homeostasis. Recent studies have shown that quiescence is not a passive and homogeneous state but actively maintained and heterogeneous. These cellular characteristics associated with quiescence were observed primarily in cultured cells under a static medium. However, cells in vivo face different microenvironmental conditions, particularly, under interstitial fluid flows distributed through extracellular matrices. Interstitial fluid flow exerts shear stress on cells and matrix strain, and results in continuous replacement of extracellular factors. In this study, we analyzed individual cells under varying fluid flow rates in microfluidic devices. We found quiescence characteristics previously identified under conventional static medium, including serum signal-dependant quiescence entry and exit and time-dependant quiescence deepening, are also present under continuous fluid flow. Furthermore, increasing the flow rate drives cells to shallower quiescence and become more likely to reenter the cell cycle upon growth stimulation. This effect is due to flow-induced physical and biochemical cues. Specifically, increasing shear stress or extracellular factor replacement individually, without altering other parameters, results in shallow quiescence. We show our experimental results can be quantitatively explained by a mathematical model connecting extracellular fluid flow to an Rb-E2f bistable switch that regulates the quiescence-to-proliferation transition. Our findings uncover a previously unappreciated mechanism that likely underlies the heterogeneous responses of quiescent cells for tissue repair and regeneration in different physiological tissue microenvironments.more » « less
null (Ed.)Biliverdin is a bile pigment that has a very low fluorescence quantum yield in solution, but serves as a chromophore in far-red fluorescent proteins being developed for bio-imaging. In this work, excited-state dynamics of biliverdin dimethyl ether (BVE) in solvents were investigated using femtosecond (fs) and picosecond (ps) time-resolved absorption and fluorescence spectroscopy. This study is the first fs timescale investigation of BVE in solvents, and therefore revealed numerous dynamics that were not resolved in previous, 200 ps time resolution measurements. Viscosity- and isotope-dependent experiments were performed to identify the contributions of isomerization and proton transfer to the excited-state dynamics. In aprotic solvents, a ∼2 ps non-radiative decay accounts for 95% of the excited-state population loss. In addition, a minor ∼30 ps emissive decay pathway is likely associated with an incomplete isomerization process around the C15C16 double bond that results in a flip of the D-ring. In protic solvents, the dynamics are more complex due to hydrogen bond interactions between solute and solvent. In this case, the ∼2 ps decay pathway is a minor channel (15%), whereas ∼70% of the excited-state population decays through an 800 fs emissive pathway. The ∼30 ps timescale associated with isomerization is also observed in protic solvents. The most significant difference in protic solvents is the presence of a >300 ps timescale in which BVE can decay through an emissive state, in parallel with excited-state proton transfer to the solvent. Interestingly, a small fraction of a luminous species, which we designate lumin-BVE (LBVE), is present in protic solvents.more » « less
Fluorescent proteins (FPs) have become fundamental tools for live cell imaging. Most FPs currently used are members of the green fluorescent protein super-family, but new fluorophores such as bilin-FPs are being developed and optimized. In particular, the UnaG FP incorporates bilirubin (BR) as a chromophore, enhancing its fluorescence quantum yield by three orders of magnitude relative to that in solution. To investigate the mechanism of this dramatic enhancement and provide a basis for further engineering of UnaG and other tetrapyrrole-based fluorophores, we performed picosecond fluorescence and femtosecond transient absorption measurements of BR bound to UnaG and its N57A site-directed mutant. The dynamics of wt-UnaG, which has a fluorescence QY of 0.51, are largely homogeneous, showing an excited state relaxation of ∼200 ps, and a 2.2 ns excited-state lifetime decay with a kinetic isotope effect (KIE) of 1.1 for D 2 O vs. H 2 O buffer. In contrast, for UnaG N57A (fluorescence QY 0.01) the results show a large spectral inhomogeneity with excited state decay timescales of 47 and 200 ps and a KIE of 1.4. The non-radiative deactivation of the excited state is limited by proton transfer. The loss of direct hydrogen bonds to the endo -vinyl dipyrrinone moiety of BR leads to high flexibility and structural heterogeneity of UnaG N57A, as seen in the X-ray crystal structure.more » « less
2’‐Deoxy‐5‐formylcytidine (5fdCyd), a naturally occurring nucleoside found in mammalian DNA and mitochondrial RNA, exhibits important epigenetic functionality in biological processes. Because it efficiently generates triplet excited states, it is an endogenous photosensitizer capable of damaging DNA, but the intersystem crossing (ISC) mechanism responsible for ultrafast triplet state generation is poorly understood. In this study, time‐resolved mid‐IR spectroscopy and quantum mechanical calculations reveal the distinct ultrafast ISC mechanisms of 5fdCyd in water versus acetonitrile. Our experiment indicates that in water, ISC to triplet states occurs within 1 ps after 285 nm excitation. PCM‐TD‐DFT computations suggest that this ultrafast ISC is mediated by a singlet state with significant cytosine‐to‐formyl charge‐transfer (CT) character. In contrast, ISC in acetonitrile proceeds via a dark1nπ* state with a lifetime of ∼3 ps. CT‐induced ISC is not favored in acetonitrile because reaching the minimum of the gateway CT state is hampered by intramolecular hydrogen bonding, which enforces planarity between the aldehyde group and the aromatic group. Our study provides a comprehensive picture of the non‐radiative decay of 5fdCyd in solution and new insights into the factors governing ISC in biomolecules. We propose that the intramolecular CT state observed here is a key to the excited‐state dynamics of epigenetic nucleosides with modified exocyclic functional groups, paving the way to study their effects in DNA strands.