Alternative methods for estimating common descriptors for QSAR studies of dyes and fluorescent probes using molecular modeling software. 2. Correlations between log P and the hydrophilic/lipophilic index, and new methods for estimating degrees of amphiphilicity

The log P descriptor, despite its usefulness, can be difficult to use, especially for researchers lacking skills in physical chemistry. Moreover this classic measure has been determined in numerous ways, which can result in inconsistant estimates of log P values, especially for relatively complex molecules such as fluorescent probes. Novel measures of hydrophilicity/ lipophilicity (the Hydrophilic/Lipophilic Index, HLI) and amphiphilicity (hydrophilic/lipophilic indices for the head group and tail, HLI T and HLI HG, respectively) therefore have been devised. We compare these descriptors with measures based on log P, the standard method for quantitative structure activity relationships (QSAR) studies. HLI can be determined using widely available molecular modeling software, coupled with simple arithmetic calculations. It is based on partial atomic charges and is intended to be a standalone measure of hydrophilicity/lipophilicity. Given the wide application of log P, however, we investigated the correlation between HLI and log P using a test set of 56 fluorescent probes of widely different physicochemical character. Overall correlation was poor; however, correlation of HLI and log P for probes of narrowly specified charge types, i.e., non-ionic compounds, anions, conjugated cations, or zwitterions, was excellent. Values for probes with additional nonconjugated quaternary cations, however, were less well correlated. The newly devised HLI can be divided into domain-specific descriptors, HLI T and HLI HG in amphiphilic probes. Determinations of amphiphilicity, made independently by the authors using their respective methods, showed excellent agreement. Quantifying amphiphilicity from partial log P values of the head group (head group hydrophilicity; HGH) and tail (amphiphilicity index; AI) has proved useful for undertanding fluorescent probe action. The same limitations of log P apply to HGH and AI, however. The novel descriptors, HLI T and HLI HG, offer analogous advantages to those seen with HLI over log P. The high correlation between log P and HLI, and the concordance between the two systems for assessing amphiphilicity, provide a powerful tool for QSAR studies. It is possible now to select a probe with missing fragments, and thus no log P, AI or HGH; and to estimate these important descriptors from parameters derived from HLI.