Fluorescence studies with potato carboxypeptidase inhibitor

Potato carboxypeptidase inhibitor (CPI) is a 39-residue globular protein whose X-ray structure is known. The protein's two tryptophan residues (W22 and W28) appear to be on the surface in the crystal structure. The fluorescence spectrum of CPI has a maximum at 344 nm. Acrylamide solute quenching yields an upward curving Stern-Volmer plot with KSV approximately 9 M-1. KI quenching yields a linear plot with KSV approximately 5.5 M-1. These studies indicate that emission occurs from a solvent exposed residue(s). Fluorescence lifetime measurements were fitted to a biexponential decay law with tau 1 = 0.9 ns. f1 = 0.22 and tau 2 = 3.9 ns. Anisotropy decay data were described by a single rotational correlation time of 1.2 ns. This value is somewhat small for the global rotation of a protein of this size. The low temperature phosphorescence of CPI shows a biexponential decay, with a rapidly decaying 0.5-1.0 second component. The triplet state of at least one of the two tryptophan residues must be perturbed by interaction with nearby disulfide bonds.