Dynamic solvation and coupling of the hydration shell of Zn(II)-substituted cytochrome c in the presence of guanidinium ions.
The fluorescence Stokes shift (FSS) response of Zn(II)-substituted cytochrome c (ZnCytc) is transformed from a monotonic red-shifting response in water to a bidirectional response with much slower time constants in the presence of low concentrations of guanidinium (Gdm(+)) ions. The FSS response in water observed over the 100 ps to 10 ns range has two exponential components with time constants of 135 ps and 1.6 ns that account for a total shift of 30 cm(-1), about one-half of the solvation reorganization energy. In contrast, in the presence of only 0.25 M Gdm(+), the FSS response initially shifts 21 cm(-1) to the blue with a 820 ps time constant and then shifts 60 cm(-1) back to the red with a 3.5 ns time constant. The effect of Gdm(+) on the FSS response effectively saturates at 1.0 M, well below the 1.75 M midpoint of the two-state unfolding transition. These results establish that the FSS response in ZnCytc includes a significant contribution from the surrounding hydration shell, which assumes a perturbed hydrogen-bonding network owing to the binding of Gdm(+) ions to the protein surface. The blue-shifting part of the FSS response arises from a light-induced conformational change that expands the protein- and solvent-derived cavity around the excited-state Zn(II) porphyrin. This non-polar part of the solvation response is enhanced in the presence of Gdm(+) because the protein/solvent surroundings of the Zn(II) porphyrin are effectively more flexible than in water. The enhanced flexibility in the presence of Gdm(+) increases the amplitudes and accordingly lengthens the correlation time scales for the protein and hydration-shell fluctuations that contribute to the FSS response.
Department of Chemistry, Michigan State University , East Lansing, Michigan 48824, United States.
Jagnyaseni
J
Tripathy
Jenny Jo
JJ
Mueller
Nolan C
NC
Shepherd
Warren F
WF
Beck
D000818:Animals; D045304:Cytochromes c / Q000737:chemistry*; D019791:Guanidine / Q000737:chemistry*; D006736:Horses; D006860:Hydrogen Bonding; D007477:Ions / Q000737:chemistry; D008958:Models, Molecular; D011487:Protein Conformation; D013050:Spectrometry, Fluorescence; D015032:Zinc / Q000737:chemistry*
D007477:Ions; D045304:Cytochromes c; D015032:Zinc; D019791:Guanidine