Abstract
3-methyl dihydroflavin (3-me-FlredH2) and ethylenediaminetetraacetic acid (EDTA) aggregate spontaneously in aqueous solution to form clusters in a cooperative manner (Hill coefficient n H =2.0), exhibiting a typical critical micelle concentration (cmc). These clusters can be detected either by Rayleigh-scattering, or by a newly formed broad absorption band extending beyond 1,700 nm. Based on measurements of the temperature dependency, the free energy of cluster formation was determined to be as low as δG =−48 KJ · mol−1, suggesting the presence of 5–10 hydrogen bonds per cluster. The aggregation requires pH's below 6.5, which is assumed to reflect the N1-pK of 6.5 of free flavins: the anionic form of dihydroflavin prevents clustering. In addition, cluster formation is observed only, if a hydrophobic (methyl-)group is bound to the N3 of the flavin nucleus. This feature has not been observed previously and — as a model reaction — might bear some significance for the coenzyme-apoprotein interaction in flavoprotein catalysis in general.
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