Type 1 copper (T1Cu) proteins play important roles in electron transfer in biology, largely due to the unique structure of the T1Cu center, which is reflected by its spectroscopic properties. Previous reports have suggested a correlation between a high ratio of electronic absorbance at similar to 450 nm to that at similar to 600 nm (R = A(450)/A(600)) and a large copper(II) hyperfine coupling in the z direction (A(z)) in electron paramagnetic resonance (EPR). However, this correlation does not have a clear physical meaning, nor does it hold for many proteins with a perturbed T1Cu center. To address this issue, a new parameter of R' [A(450)/(A(450) + A(600))] with a better physical meaning of a fractional S-Cys pseudo-sigma to Cu(II) charge transfer transition intensity is defined and a quadratic relationship between R' and A(z) is found on the basis of a comprehensive analysis of ultraviolet-visible absorption, EPR, and structural parameters of T1Cu proteins. We are able to find good correlations between R' and the displacement of copper from the trigonal plane defined by the His(2)Cys ligands and the angle between the N-His1-Cu-N-His2 plane and the S-Cys-Cu-axial ligand plane, providing a structural basis for the observed correlation. These findings and analyses provide a new framework for a deeper understanding of the spectroscopic and electronic properties of T1Cu proteins, which may allow better design and applications of this important class of proteins for redox and electron transfer functions.