With the mvGlu-AB series in hand, we began by determining the experimental octanol-water partition coefficients at pH 7.4 (logD7.4). The measured logD7.4 values in ascending order (most to least water soluble) are: mvGlu-AB2 (-1.12 ± 0.16), mvGlu-AB3 (-0.71 ± 0.11), mvGlu-AB1 (0.75 ± 0.02), and AB-OMe2 (2.51 ± 0.92) (Figure ). These results indicate the installation of a single mvGlu unit already has a dramatic effect on aqueous solubility and the inclusion of a second mvGlu group can further shift dye distribution from octanol to favor water. Next, we obtained the absorbance spectrum of each dye (3:7 v/v acetonitrile:PBS). The peak appearance of a dye is an excellent indicator of its aggregation status (Figure ). For instance, owing to the broad spectrum of AB-OMe2, it is likely to be aggregated and thus, we would anticipate spectral unmixing to be sub-optimal and the corresponding PA signal to be weak. However, when two mvGlu groups are present (regardless of the position), the peak shape is sharp, well defined, and resembles that of AB-OMe2 in a pure organic solvent such as DMSO. In addition to improving spectral unmixing, a sharp peak is also associated with a larger extinction coefficient (ε) value. According to PA Brightness Factor (PABF) calculations, a large ε is the most influential property that determines the strength of a PA signal. Because the ε for mvGlu-AB2 (2.6 ×