Acknowledgment. This work was supported in part by NSF grant DMR-7823958 (MG) and by BRSG S07RR07054-17 awarded by the Biomedical Research Grant Program, Division of Research Resources, National Institutes of Health (DH). Funds were also provided to DH by a Camille and Henry Dreyfus Foundation Grant for Newly Appointed Young Faculty in Chemistry and to JWB by the Deutsche Forshungsgemeinschaft. ... Registry No. Pt"(TPP), 141 87-14-5; ...