Abstract
In vivo optical imaging must contend with the limitations imposed by the optical window of tissue (600-1000 nm). Although a wide array of fluorophores are available that are visualized in the red and near-IR region of the spectrum, with the exception of proteases, there are few long wavelength probes for enzymes. This situation poses a particular challenge for studying the intracellular biochemistry of erythrocytes, the high hemoglobin content of which optically obscures subcellular monitoring at wavelengths less than 600 nm. To address this, tunable fluorescent reporters for protein kinase activity were developed. The probing wavelength is preprogrammed by using readily available fluorophores, thereby enabling detection within the optical window of tissue, specifically in the far-red and near-IR region. These agents were used to monitor endogenous cAMP-dependent protein kinase activity in erythrocyte lysates and in intact erythrocytes when using a light-activatable reporter.