YADDA HARAFIN DNA KE TSIRA DAGA BUGUN HASKEN ULTRAVIOLET

Tushen DNA (bases) suna shan hasken ultraviolet da ƙarfi, amma duk da haka suna da kwanciyar hankali abin mamaki: suna kawar da kuzarin cikin ɗan lokaci kaɗan, ba tare da fitar da haske ba. Wannan iyawa tana kare kayan gado. Thymine yana da muhimmanci musamman, in ji takardar, saboda tashin hankalinsa da hasken ultraviolet zai iya fara mafi yawan nau’in lalacewar UV ga DNA, da aka sani da cyclobutane pyrimidine dimer. Wannan lalacewa za ta iya samuwa cikin ɗan lokaci kaɗan, ko daga baya, ta hanyar yanayin triplet masu tsawon rai, wani tsari na daban na juyawar (spin) electrons.

To ina kuzarin thymine ke tafiya? Idan aka yi nazarinsa a cikin ruwa, kusan kashi 80% na ƙwayoyin da aka tayar suna komawa kai tsaye zuwa yanayin hutunsu cikin ƙasa da femtosecond 100 (miliyan ɗaya na biliyan ɗaya na daƙiƙa). Idan aka keɓe shi a cikin iskar gas, thymine ba ya bin wannan gajeriyar hanya: dukan kuzarin yana shiga wani yanayi na tashin hankali. Bayan picosecond 100, wani “yanayin duhu” mai tsawon rai yana bayyana, wanda galibi ake danganta shi da triplet — mafi yawa bisa tsawon lokacin da yake ɗauka.

Atom biyar a matsayin ’yan rahoto biyar

Xiaojun Wang, Markus Gühr da abokan aikinsu a dakin bincike na DESY a Hamburg, tare da masana ka’ida a Koriya ta Kudu (Jami’ar Ƙasa ta Kyungpook) da Faransa (Jami’ar Aix-Marseille da CNRS), sun yi amfani da wata dabara da suke kira time-resolved multi-center X-ray photoelectron spectroscopy.

Ƙa’idar: walƙiyar ultraviolet (nanomita 263, tsawonta kusan femtosecond 80) tana tayar da ƙwayoyin thymine da aka turara daga murhu a 180 °C. Gajeren bugun X-ray masu laushi daga laser na electron mai ’yanci FLASH sannan yana fitar da electron na ciki daga atom na carbon. Kuzarin wannan electron ya dogara da yanayin electrons da ke kewaye da atom ɗin, don haka kowane ɗaya daga cikin atom ɗin carbon biyar na thymine — huɗu a cikin zoben, ɗaya a cikin ƙaramar ƙungiyar methyl ɗinsa — yana ba da rahoton abin da ke faruwa a kewayensa. Jinkirin ya kasance daga femtoseconds kaɗan zuwa nanosecond ɗaya, kusan matakai shida na girma, kuma yawan bugun ya ba da bayanai masu tsabta cikin sa’o’i biyu kawai. Lissafin kimiyyar quantum na spectra da ake tsammani ga kowane yanayi sun zama mabuɗin karanta su.

Hanyar, mataki-mataki

Daidaitawa da yanayi huɗu masu bin juna yana ba da:

1. Yanayin tashin hankali na farko yana ɗaukar femtosecond 92 ± 18.
2. Yanayin singlet na biyu, “mai duhu”, yana kama ƙwayar tsawon picosecond 6.3 ± 0.2. Lissafi suna nuna dalili: hanyar komawa yanayin hutu daga can tana kan tudu na kusan electronvolt 1.6.
3. Sannan yanayin duhu mai tsawon rai ya zo — kuma a nan ne abin mamaki. Tsawon farkon picosecond 240 ko makamancin haka, alamarsa ba triplet tsantsa ba ce, sai dai gaurayen wani yanayin singlet da ya ƙara sakin jiki da triplet.
4. Sai bayan haka ne yawan ƙwayoyin ya zama triplet tsantsa, wanda ke ɗorewa fiye da tagar nanosecond ɗaya. Shinge na electronvolt 1.33 yana hana shi komawa yanayin hutu.

Lissafin kuma sun nuna cewa tsalle daga singlet zuwa triplet zai iya zuwa kai tsaye zuwa mafi ƙasƙancin triplet, ta wurin ketarewa da ke kusan electronvolt 0.5 kawai sama da singlet ɗin da aka kama, ba tare da tsayawa a wani triplet na tsakiya ba. Gaurayen yana bayyana wani asiri, in ji marubutan: wani bincike na baya ya auna yawan triplet na kusan kashi 86%, kuma rabon da ya ɓace ya dace da reshen singlet da suka gano.

Zoben da ke numfashi

A cikin femtosecond 200 na farko, alamar tana girgiza a kusan 730 cm⁻¹ — girgiza mai lokacin femtosecond 46, “numfashin” zoben thymine. Tana tsira daga tsallen farko tsakanin yanayin tashin hankali, kuma kwaikwayo sun nuna cewa girgizar tana ci gaba da tafiya daidai har zuwa kusan femtosecond 750. Saboda X-rays suna kallon atom da yawa, ƙungiyar ta iya ganin cewa atom daban-daban suna amsawa a saɓanin juna: idan zoben ya faɗaɗa, alamar wani carbon tana ƙaruwa yayin da ta wasu biyu ke raunana.

Ayyuka biyu na ƙungiyar methyl

Binciken da ya fi ban mamaki ya shafi ƙungiyar methyl, carbon mai ɗauke da hydrogen uku da aka haɗa da zoben. A lokacin matakan farko na singlet, ba ta shiga harkar electrons ko kaɗan: ita “’yar kallo ce mai nauyi” (inertia spectator) wadda nauyinta ke rage saurin wani motsin murɗewar zoben da in ba haka ba zai bar ƙwayar ta koma kai tsaye zuwa yanayin hutunta. Da zarar triplet ya samu, rawarta tana juyawa. Electrons ɗinta suna haɗuwa da zoben ta wani tasiri da ake kira hyperconjugation, alamar X-ray ɗinta tana matsawa da kusan electronvolt 2, kuma ta zama alama mafi saurin ji cewa triplet ya bayyana.

Iskar gas, ba ƙwayar halitta ba

An yi waɗannan ma’auni ne a kan ƙwayoyin thymine keɓantattu a cikin iskar gas, waɗanda halinsu ya bambanta da na thymine a cikin ruwa ko a cikin DNA. Ainihin yanayin singlet na tsakiyar har yanzu ba a warware shi ba; marubutan sun ba da shawarar ma’aunin X-ray a atom ɗin oxygen a tsawon jinkiri mai tsawo don keɓe shi. Sun kuma bayyana cewa an yi amfani da AI don inganta sauƙin karantawa da harshen rubutun.