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Multiphasic Kinetics of Myoglobin/Sodium Dodecyl Sulfate Complex Formation

Alessandro Feis ^*, Luca Tofani ^*, Giampiero De Sanctis , Massimo Coletta  and Giulietta Smulevich ^*

^* Department of Chemistry, Università di Firenze, I-50019 Sesto Fiorentino (FI), Italy; Department of Molecular, Cellular, and Animal Biology, Università di Camerino, I-62032 Camerino (MC), Italy; and Department of Experimental Medicine and Biochemical Sciences, Università di Roma Tor Vergata, I-00133 Roma, Italy

Correspondence: Address reprint requests to Giulietta Smulevich, Tel.: 39-055-4573083; Fax: 39-055-4573077; E-mail: giulietta.smulevich{at}unifi.it.

We have carried out a kinetic analysis of the conformational changes that myoglobin (Mb) undergoes in the presence of the anionic surfactant sodium dodecyl sulfate (SDS). The time-resolved results have been combined with steady-state circular dichroism (CD) and resonance Raman (RR) spectroscopy. Time-resolved absorption spectra indicate that SDS induces changes in the heme coordination with the formation of three different Mb species, depending on SDS concentration. The formation of the Mb/SDS complex involves three or four phases, depending on surfactant concentration. The kinetic data are analyzed assuming two modes of interaction according to whether SDS is monomeric or micellar. The two pathways are separated but interconnected through free Mb. At the lowest concentrations a six-coordinated, low-spin form dominates. Two distinct five-coordinated species are formed at higher SDS concentrations: one is a protein-free heme and the other reequilibrates slowly with the six-coordinated, low-spin form. The resulting complexes have been characterized by CD and RR. In addition, CD spectra show that the local changes in the heme environment are coupled to changes in the protein structure.