Fig. 5: Alkyl diazirine preferentially cross-links polar residues in the 9 test protein.

a Relative abundance of cross-linking pairs for polar residues (red shaded) and non-polar residues (blue shaded), by SDA (first chemically cross-linked to Lys) upon irradiation at an optical power density \(\left[{hv}\right]\) of 35 mW/cm² (low-power, left columns, light-colored) for 10 min or 282 mW/cm² (high-power, right columns, dark colored) for 2 min. The dashed line indicates the natural abundance of polar residues in each protein, with D (Asp), E (Glu), and Y (Tyr) labeled. The details of the test proteins, 1–9, are given in “Methods”. b Fragmentation of the cross-link between Lys and Glu residues (segment of Glu denoted as α) corroborates the assignment. c Relative abundance of the loop-links, for non-polar and polar, resides in the test proteins upon low- or high-power irradiation. d Normalized Cα-Cα distance distribution (in 4-Å bins) between the cross-linked Lys residue and polar/non-polar residues. Normalization is performed by subtracting the maximally allowed distance (Supplementary Fig. 13) from the calculated Cα-Cα distance. e A carved-in structure figure of BSA, illustrating the solvent exposure of residues K245 and K297. f Distance mapping from the PXL data involving polar residues onto the BSA structure.