title: Excimer fluorescence of equine platelet tropomyosin labeled with N-(1-pyrenyl)iodoacetamide.
abstract: Tropomyosin from equine platelets was reacted with N-(1-pyrenyl)iodoacetamide, a sulfhydryl-specific fluorescent reagent, to give an average extent of incorporation of 1.12 pyrene (Py) groups per platelet tropomyosin (P-TM) chain. The predominant site of reaction on P-TM was the penultimate COOH-terminal residue, Cys-246. The high proportion of the total emission that is due to pyrene ecximers and the pretransition observed in thermal denaturation of Py-P-TM point to a rather loose structure for the COOH-terminal amino acid residues of P-TM. The label on Cys-246 also reports on end-to-end overlap interactions that occur between two different tropomyosin molecules. Additions to a Py-P-TM solution at low ionic strength of unlabeled P-TM, rabbit cardiac tropomyosin (C-TM), or a carboxypeptidase A treated, nonpolymerizable derivative of C-TM all reduce the extent of excimer fluorescence from the sample. Addition of salt greatly reduces the effects of the unlabeled TM species on the Py-P-TM emission spectrum. Circular dichroism measurements indicate Py-P-TM still to be greater than 95% helical. However, analysis of excimer fluorescence levels in samples that contained a constant protein concentration but different mole ratios of labeled to unlabeled P-TM suggests that the bulky pyrene group may diminish the tendency of Py-P-TM to polymerize in an end-to-end manner.
mesh_terms: D000818:Animals; D001792:Blood Platelets / Q000378:metabolism*; D005456:Fluorescent Dyes; D006736:Horses; D007460:Iodoacetamide / Q000031:analogs & derivatives*; D007461:Iodoacetates; D011489:Protein Denaturation; D013050:Spectrometry, Fluorescence; D013816:Thermodynamics; D014335:Tropomyosin / Q000097:blood*
chemical_list: D005456:Fluorescent Dyes; D007461:Iodoacetates; D014335:Tropomyosin; C029358:N-(1-pyrenyl)iodoacetamide; D007460:Iodoacetamide