Negative slope in Scatchard plot indicates
Because the Scatchard slope equals –1/Kd, the negative sign is an intrinsic feature of the plot for any saturable binding interaction. The absolute magnitude of the slope is the reciprocal of Kd and therefore reports affinity. A larger absolute value of the negative slope indicates higher affinity. The sign itself does not convey additional information beyond the inverse relationship with the dissociation constant.
Ref: NCERT Biology Class 11–12 Alberts et al Molecular Biology of the Cell Lodish et al, Molecular Cell Biology Cooper & Hausman, The Cell Abbas et al., Cellular and Molecular Immunology (for immunology sections)