title: Preparation and some properties of 6-substituted flavins as active site probes for flavin enzymes.
abstract: 6-Azidoflavins, 6-thiocyanatoflavins, and 6-mercaptoflavins at the lumiflavin, riboflavin, FMN, and FAD level were prepared from the corresponding 6-aminoflavins and some of their properties investigated. They are bound tightly by apoflavin enzymes which bind either riboflavin, FMN, or FAD. 6-Azidoflavins undergo facile photolysis. One major product was identified as 6-aminoflavin. A further product, which was formed also during acid decomposition of the azide, results from opening of the flavin benzene ring and is proposed to have a lumazine structure. 6-Thiocyanatoflavins are easily converted by dithiothreitol to 6-mercaptoflavins. The latter are stabilized against dimerization in the presence of reducing thiols. 6-Mercaptoflavins have a pK of 5.9, which corresponds to ionization of the 6-SH function. The neutral form is yellow, while the anion is green, due to a long-wavelength band (lambda max approximately 600 nm) extending beyond 700 nm. These properties suggest the use of these 6-substituted flavins for probing the active site of flavin enzymes. Because their reactive substituents are in close proximity to the flavin N(5)-position, these 6-substituted derivatives should also serve as useful probes of the environment around the flavin N(5), a position known to be involved in all flavin-mediated redox processes.
mesh_terms: D001665:Binding Sites; D005182:Flavin-Adenine Dinucleotide / Q000031:analogs & derivatives; D005415:Flavins / Q000138:chemical synthesis* / Q000494:pharmacology; D005420:Flavoproteins / Q000378:metabolism*; D007202:Indicators and Reagents; D009682:Magnetic Resonance Spectroscopy; D013058:Mass Spectrometry; D010088:Oxidoreductases / Q000378:metabolism; D011485:Protein Binding; D012256:Riboflavin / Q000031:analogs & derivatives; D013053:Spectrophotometry; D013329:Structure-Activity Relationship
chemical_list: D005415:Flavins; D005420:Flavoproteins; D007202:Indicators and Reagents; D005182:Flavin-Adenine Dinucleotide; D010088:Oxidoreductases; D012256:Riboflavin