Anna Gakamsky1, Rory R. Duncan2, Nicola M. Howarth2, Baljean Dhillon3, Kim K. Buttenschön4, Daniel J. Daly4 & Dmitry Gakamsky2

The chemical nature of the non-tryptophan (non-Trp) fluorescence of porcine and human eye lens proteins was identified by Mass Spectrometry (MS) and Fluorescence Steady-State and Lifetime spectroscopy as post-translational modifications (PTM) of Trp and Arg amino acid residues. Fluorescence intensity profiles measured along the optical axis of human eye lenses with age-related nuclear cataract showed increasing concentration of fluorescent PTM towards the lens centre in accord with the increased optical density in the lens nucleolus. Significant differences between fluorescence lifetimes of “free” Trp derivatives hydroxytryptophan (OH-Trp), N-formylkynurenine (NFK), kynurenine (Kyn), hydroxykynurenine (OH-Kyn) and their residues were observed. Notably, the lifetime constants of these residues in a model peptide were considerably greater than those of their “free” counterparts. Fluorescence of Trp, its derivatives and argpyrimidine (ArgP) can be excited at the red edge of the Trp absorption band which allows normalisation of the emission spectra of these PTMs to the fluorescence intensity of Trp, to determine semi-quantitatively their concentration. We show that the cumulative fraction of OH-Trp, NFK and ArgP emission dominates the total fluorescence spectrum in both emulsified post-surgical human cataract protein samples, as well as in whole lenses and that this correlates strongly with cataract grade and age.

Nadia Anikeeva1, Dimitry Gakamsky2, Jørgen Schøller3, Yuri Sykulev1

Noncognate or self peptide-MHC (pMHC) ligands productively interact with T-cell receptor (TCR) and are always in a large access over the cognate pMHC on the surface of antigen presenting cells. We assembled soluble cognate and noncognate pMHC class I (pMHC-I) ligands at designated ratios on various scaffolds into oligomers that mimic pMHC clustering and examined how multivalency and density of the pMHCs in model clusters influences the binding to live CD8 T cells and the kinetics of TCR signaling. Our data demonstrate that the density of self pMHC-I proteins promotes their interaction with CD8 co-receptor, which plays a critical role in recognition of a small number of cognate pMHC-I ligands. This suggests that MHC clustering on live target cells could be utilized as a sensitive mechanism to regulate T cell responsiveness.

Dmitry M. Gakamsky1, Bal Dhillon2,5, John Babraj3, Matthew Shelton4, S. Desmond Smith4,5

A novel route for early cataract diagnostics is investigated based on the excitation of tryptophan fluorescence (TF) at the red edge of its absorption band at 317 nm. This allows penetration through the cornea and aqueous humour to provide excitation of the ocular lens. The steepness of the red edge gives the potential of depth control of the lens excitation. Such wavelength selection targets the population of tryptophan residues, side chains of which are exposed to the polar aqueous environment. The TF emissions around 350 nm of a series of UV-irradiated as well as control lenses were observed. TF spectra of the UV cases were red-shifted and the intensity decreased with the radiation dose. In contrast, intensity of non-tryptophan emission with maximum at 435 nm exhibited an increase suggesting photochemical conversion of the tryptophan population to 435 nm emitting molecules. We demonstrate that the ratio of intensities at 435 nm to that around 350 nm can be used as a measure of early structural changes caused by UV irradiation in the lens by comparison with images from a conventional slit-lamp, which can only detect defects of optical wavelength size. Such diagnostics at a molecular level could aid research on cataract risk investigation and possible pharmacological research as well as assisting surgical lens replacement decisions.

Dmitry M. Gakamsky, Richard B. Dennis, S. Desmond Smith

This article describes novel data analysis of fluorescence lifetime-based protein kinase assays to identify and correct for compound interference in several practical cases. This ability, together with inherent advantages of fluorescence lifetime technology (FLT) as a homogeneous, antibody-free format independent of sample concentration, volume, excitation intensity, and geometry, makes fluorescence lifetime a practical alternative to the established “gold standards” of radiometric and mobility shift (Caliper) assays. The analysis is based on a photochemical model that sets constraints on the values of fluorescence lifetimes in the time responses of the assay. The addition of an exponential component with free floating lifetime to the constrained model, in which the lifetimes are constants predetermined from control measurements and the preexponential coefficients are “floating” parameters, allows the relative concentration of phosphorylated and nonphosphorylated substrates to be calculated even in the presence of compound fluorescence. The method is exemplified using both simulated data and experimental results measured from mixtures of dye-labeled phosphorylated and nonphosphorylated kinase substrates. A change of the fluorescence lifetime is achieved by the phosphorylated substrate-specific interaction with a bifunctional ligand, where one binding site interacts with the phosphate group and the other interacts with the dye.