S100a is a heterodimeric, acidic calcium-binding protein that interacts with calmodulin antagonists in a Ca2(+)-dependent manner. In order to study the behavior of the hydrophobic domain on S100a when bound to Ca2+, its interaction with trifluoperazine (TFP) was investigated using 16F nuclear magnetic resonance (NMR) and circular dichroism (CD) spectroscopy. The dissociation constant (Kd) values of TFP, as estimated from the chemical shifts of 19F NMR, were 191 and 29 microns in the absence and presence of Ca2+, respectively, and were similar to those previously reported for S100b. However, the TFP linewidth in the presence of Ca2(+)-bound S100a was 65 Hz greater than in the presence of Ca2(+)-bound S100b. This suggests a slower TFP exchange rate for S100a than for S100b. Thus, the TFP linewidths observed for each isoform may reflect differences in structural and modulatory properties of the Ca2(+)-dependent hydrophobic domains on S100a and S100b. Additionally, the presence of magnesium had no effect on the observed Ca2(+)-induced TFP spectral changes in S100a solutions. Circular dichroism studies indicate that Ca2+ induces a small transition from alpha-helix to random coil in S100a; in contrast, the opposite transition is reported for calmodulin (Hennessey et al., 1987). However, TFP did not significantly alter the secondary structure of Ca2(+)-bound S100a; this observation is similar to the effect of TFP on Ca2(+)-bound calmodulin and troponin C (Shimizu and Hatano, 1984; Gariépy and Hodges, 1983). It is, therefore, proposed that TFP binds to a hydrophobic domain on S100a in a fashion similar to other calcium-modulated proteins.