Quartz high precision cell

The CD spectra of the MS2(9x) soluble proteins diluted in ultrapure water at concentrations from 0.125 to 1 mg/ml were acquired using a Jasco J-815 (Jasco Inc. Easton MD) circular dichroism spectropolarimeter in a 0.5 mm path length quartz high precision cell (Hellma Analytics, Müllheim, Germany). Spectra were recorded as an average of 10 scans in the range of 300–185 nm with a bandwidth of 0.5 nm at a scan rate of 50 nm/min. The background CD spectrum of ultrapure water was recorded and subtracted from sample spectra. The spectra were smoothed using a Savitzky-Golay filter with a convolution width of 9 points using Spectra Analysis software (Jasco Inc. Easton MD). The secondary structure of the soluble silk proteins were deduced from the CD spectra (expressed as mean residue ellipticity value) using the DICHROWEB online server42 (link) and both CDSSTR and CONTIN analysis procedures43 (link). The graphical presentation of deconvoluted CD spectra was performed using Origin package (Microcal Software, Inc., Northampton, MA, USA).

Steady-state fluorescence emission spectra were collected upon excitation of the Trp pool of the protein at 295 nm. Excitation spectra were acquired with emission wavelength at 330 nm. All measurements were conducted on a fluorescence RTC 2000 spectrometer (Photon Technology International, Canada, Inc.347 Consortium Court London, Ontario N6E 2S8) with a T-configuration, using a 75-W Xenon arc lamp coupled to a monochromator. Samples were analyzed in quartz high precision cell with 10 cm x 2 cm of light path (Hellma Analytics) and gently shaken before each measurement. All slits were set to 5 mm.