Volume changes in the formation of internal complexes of flavinyltryptophan peptides

The effect of pressure, up to 10 kbar, on the fluorescence yield and lifetime of two flavinyltryptophan peptides was investigated. These peptides differed only in the number of methylene groups, respectively three and five, separating the chromophores. At atmospheric pressure the closed nonfluorescent form predominated in both compounds constitutin 94% of the total in the short-linked peptide and 80% in the long-linked one. The fluorescence of both peptides decreased at high pressure and the volume change upon formation of the nonfluorescent complex in the short peptide (--1.8 mL/mol) was less than half of the change in the long peptide (--4.8 mL/mol) or the value for FAD (--4.3 mL/mol). The much smaller compressibility of the short peptides is attributed to the mechanical constraint to the approach of the interacting rings, imposed by the short link. Mechanical constraints of similar nature may be expected to be operative in proteins. Their importance in pressure denaturation is discussed.