A bridging water anchors the tethered 5-(3-aminopropyl)-2'-deoxyuridine amine in the DNA major groove proximate to the N+2 C.G base pair: implications for formation of interstrand 5'-GNC-3' cross-links by nitrogen mustards

Site-specific insertion of 5-(3-aminopropyl)-2'-deoxyuridine (Z3dU) and 7-deaza-dG into the Dickerson-Drew dodecamers 5'-d(C (1)G (2)C (3)G (4)A (5)A (6)T (7)T (8)C (9) Z (10)C (11)G (12))-3'.5'-d(C (13)G (14)C (15)G (16)A (17)A (18)T (19)T (20)C (21) Z (22)C (23)G (24))-3' (named DDD (Z10)) and 5'-d(C (1)G (2)C (3)G (4)A (5)A (6)T (7) X (8)C (9) Z (10)C (11)G (12))-3'.5'-d(C (13)G (14)C (15)G (16)A (17)A (18)T (19) X (20)C (21) Z (22)C (23)G (24))-3' (named DDD (2+Z10)) (X = Z3dU; Z = 7-deaza-dG) suggests a mechanism underlying the formation of interstrand N+2 DNA cross-links by nitrogen mustards, e.g., melphalan and mechlorethamine. Analysis of the DDD (2+Z10) duplex reveals that the tethered cations at base pairs A (5).X (20) and X (8).A (17) extend within the major groove in the 3'-direction, toward conserved Mg (2+) binding sites located adjacent to N+2 base pairs C (3).Z (22) and Z (10).C (15). Bridging waters located between the tethered amines and either Z (10) or Z (22) O (6) stabilize the tethered cations and allow interactions with the N + 2 base pairs without DNA bending. Incorporation of 7-deaza-dG into the DDD (2+Z10) duplex weakens but does not eliminate electrostatic interactions between tethered amines and Z (10) O (6) and Z (22) O (6). The results suggest a mechanism by which tethered N7-dG aziridinium ions, the active species involved in formation of interstrand 5'-GNC-3' cross-links by nitrogen mustards, modify the electrostatics of the major groove and position the aziridinium ions proximate to the major groove edge of the N+2 C.G base pair, facilitating interstrand cross-linking.